Remove prototype FastLRUCache (#10954)
Summary: This was just a stepping stone to what eventually became HyperClockCache, and is now just more code to maintain. Pull Request resolved: https://github.com/facebook/rocksdb/pull/10954 Test Plan: tests updated Reviewed By: akankshamahajan15 Differential Revision: D41310123 Pulled By: pdillinger fbshipit-source-id: 618ee148a1a0a29ee756ba8fe28359617b7cd67cmain
Peter Dillinger
2 years ago
committed by
Facebook GitHub Bot
parent
b55e70357c
commit
32520df1d9
@ -1,580 +0,0 @@ |
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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//
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#include "cache/fast_lru_cache.h" |
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#include <cassert> |
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#include <cstdint> |
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#include <cstdio> |
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#include <functional> |
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#include "monitoring/perf_context_imp.h" |
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#include "monitoring/statistics.h" |
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#include "port/lang.h" |
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#include "util/distributed_mutex.h" |
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#include "util/hash.h" |
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#include "util/math.h" |
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#include "util/random.h" |
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namespace ROCKSDB_NAMESPACE { |
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namespace fast_lru_cache { |
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LRUHandleTable::LRUHandleTable(int hash_bits) |
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: length_bits_(hash_bits), |
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length_bits_mask_((uint32_t{1} << length_bits_) - 1), |
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occupancy_(0), |
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occupancy_limit_(static_cast<uint32_t>((uint32_t{1} << length_bits_) * |
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kStrictLoadFactor)), |
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array_(new LRUHandle[size_t{1} << length_bits_]) { |
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assert(hash_bits <= 32); |
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} |
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LRUHandleTable::~LRUHandleTable() { |
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ApplyToEntriesRange([](LRUHandle* h) { h->FreeData(); }, 0, GetTableSize()); |
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} |
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LRUHandle* LRUHandleTable::Lookup(const Slice& key, uint32_t hash) { |
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int probe = 0; |
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int slot = FindVisibleElement(key, hash, probe, 0); |
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return (slot == -1) ? nullptr : &array_[slot]; |
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} |
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LRUHandle* LRUHandleTable::Insert(LRUHandle* h, LRUHandle** old) { |
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int probe = 0; |
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int slot = FindVisibleElementOrAvailableSlot(h->key(), h->hash, probe, |
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1 /*displacement*/); |
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*old = nullptr; |
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if (slot == -1) { |
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// TODO(Guido) Don't we need to roll back displacements here?
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return nullptr; |
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} |
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if (array_[slot].IsEmpty() || array_[slot].IsTombstone()) { |
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bool empty = array_[slot].IsEmpty(); |
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Assign(slot, h); |
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LRUHandle* new_entry = &array_[slot]; |
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if (empty) { |
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// This used to be an empty slot.
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return new_entry; |
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} |
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// It used to be a tombstone, so there may already be a copy of the
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// key in the table.
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slot = FindVisibleElement(h->key(), h->hash, probe, 0 /*displacement*/); |
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if (slot == -1) { |
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// No existing copy of the key.
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return new_entry; |
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} |
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*old = &array_[slot]; |
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return new_entry; |
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} else { |
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// There is an existing copy of the key.
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*old = &array_[slot]; |
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// Find an available slot for the new element.
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array_[slot].displacements++; |
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slot = FindAvailableSlot(h->key(), probe, 1 /*displacement*/); |
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if (slot == -1) { |
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// No available slots. Roll back displacements.
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probe = 0; |
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slot = FindVisibleElement(h->key(), h->hash, probe, -1); |
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array_[slot].displacements--; |
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FindAvailableSlot(h->key(), probe, -1); |
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return nullptr; |
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} |
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Assign(slot, h); |
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return &array_[slot]; |
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} |
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} |
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void LRUHandleTable::Remove(LRUHandle* h) { |
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assert(h->next == nullptr && |
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h->prev == nullptr); // Already off the LRU list.
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int probe = 0; |
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FindSlot( |
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h->key(), [&h](LRUHandle* e) { return e == h; }, probe, |
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-1 /*displacement*/); |
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h->SetIsVisible(false); |
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h->SetIsElement(false); |
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occupancy_--; |
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} |
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void LRUHandleTable::Assign(int slot, LRUHandle* h) { |
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LRUHandle* dst = &array_[slot]; |
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uint32_t disp = dst->displacements; |
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*dst = *h; |
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dst->displacements = disp; |
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dst->SetIsVisible(true); |
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dst->SetIsElement(true); |
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occupancy_++; |
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} |
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void LRUHandleTable::Exclude(LRUHandle* h) { h->SetIsVisible(false); } |
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int LRUHandleTable::FindVisibleElement(const Slice& key, uint32_t hash, |
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int& probe, int displacement) { |
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return FindSlot( |
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key, |
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[&](LRUHandle* h) { return h->Matches(key, hash) && h->IsVisible(); }, |
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probe, displacement); |
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} |
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int LRUHandleTable::FindAvailableSlot(const Slice& key, int& probe, |
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int displacement) { |
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return FindSlot( |
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key, [](LRUHandle* h) { return h->IsEmpty() || h->IsTombstone(); }, probe, |
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displacement); |
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} |
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int LRUHandleTable::FindVisibleElementOrAvailableSlot(const Slice& key, |
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uint32_t hash, int& probe, |
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int displacement) { |
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return FindSlot( |
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key, |
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[&](LRUHandle* h) { |
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return h->IsEmpty() || h->IsTombstone() || |
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(h->Matches(key, hash) && h->IsVisible()); |
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}, |
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probe, displacement); |
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} |
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inline int LRUHandleTable::FindSlot(const Slice& key, |
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std::function<bool(LRUHandle*)> cond, |
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int& probe, int displacement) { |
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uint32_t base = ModTableSize(Hash(key.data(), key.size(), kProbingSeed1)); |
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uint32_t increment = |
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ModTableSize((Hash(key.data(), key.size(), kProbingSeed2) << 1) | 1); |
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uint32_t current = ModTableSize(base + probe * increment); |
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while (true) { |
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LRUHandle* h = &array_[current]; |
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probe++; |
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if (current == base && probe > 1) { |
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// We looped back.
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return -1; |
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} |
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if (cond(h)) { |
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return current; |
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} |
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if (h->IsEmpty()) { |
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// We check emptyness after the condition, because
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// the condition may be emptyness.
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return -1; |
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} |
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h->displacements += displacement; |
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current = ModTableSize(current + increment); |
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} |
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} |
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LRUCacheShard::LRUCacheShard(size_t capacity, size_t estimated_value_size, |
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bool strict_capacity_limit, |
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CacheMetadataChargePolicy metadata_charge_policy) |
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: CacheShardBase(metadata_charge_policy), |
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capacity_(capacity), |
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strict_capacity_limit_(strict_capacity_limit), |
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table_( |
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CalcHashBits(capacity, estimated_value_size, metadata_charge_policy)), |
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usage_(0), |
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lru_usage_(0) { |
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// Make empty circular linked list.
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lru_.next = &lru_; |
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lru_.prev = &lru_; |
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lru_low_pri_ = &lru_; |
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} |
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void LRUCacheShard::EraseUnRefEntries() { |
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autovector<LRUHandle> last_reference_list; |
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{ |
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DMutexLock l(mutex_); |
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while (lru_.next != &lru_) { |
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LRUHandle* old = lru_.next; |
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// LRU list contains only elements which can be evicted.
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assert(old->IsVisible() && !old->HasRefs()); |
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LRU_Remove(old); |
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table_.Remove(old); |
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assert(usage_ >= old->total_charge); |
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usage_ -= old->total_charge; |
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last_reference_list.push_back(*old); |
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} |
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} |
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// Free the entries here outside of mutex for performance reasons.
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for (auto& h : last_reference_list) { |
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h.FreeData(); |
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} |
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} |
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void LRUCacheShard::ApplyToSomeEntries( |
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const std::function<void(const Slice& key, void* value, size_t charge, |
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DeleterFn deleter)>& callback, |
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size_t average_entries_per_lock, size_t* state) { |
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// The state is essentially going to be the starting hash, which works
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// nicely even if we resize between calls because we use upper-most
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// hash bits for table indexes.
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DMutexLock l(mutex_); |
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size_t length_bits = table_.GetLengthBits(); |
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size_t length = table_.GetTableSize(); |
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assert(average_entries_per_lock > 0); |
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// Assuming we are called with same average_entries_per_lock repeatedly,
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// this simplifies some logic (index_end will not overflow).
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assert(average_entries_per_lock < length || *state == 0); |
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size_t index_begin = *state >> (sizeof(size_t) * 8u - length_bits); |
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size_t index_end = index_begin + average_entries_per_lock; |
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if (index_end >= length) { |
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// Going to end
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index_end = length; |
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*state = SIZE_MAX; |
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} else { |
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*state = index_end << (sizeof(size_t) * 8u - length_bits); |
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} |
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table_.ApplyToEntriesRange( |
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[callback, |
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metadata_charge_policy = metadata_charge_policy_](LRUHandle* h) { |
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callback(h->key(), h->value, h->GetCharge(metadata_charge_policy), |
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h->deleter); |
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}, |
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index_begin, index_end); |
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} |
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void LRUCacheShard::LRU_Remove(LRUHandle* h) { |
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assert(h->next != nullptr); |
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assert(h->prev != nullptr); |
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h->next->prev = h->prev; |
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h->prev->next = h->next; |
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h->prev = h->next = nullptr; |
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assert(lru_usage_ >= h->total_charge); |
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lru_usage_ -= h->total_charge; |
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} |
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void LRUCacheShard::LRU_Insert(LRUHandle* h) { |
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assert(h->next == nullptr); |
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assert(h->prev == nullptr); |
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// Insert h to head of LRU list.
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h->next = &lru_; |
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h->prev = lru_.prev; |
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h->prev->next = h; |
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h->next->prev = h; |
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lru_usage_ += h->total_charge; |
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} |
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void LRUCacheShard::EvictFromLRU(size_t charge, |
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autovector<LRUHandle>* deleted) { |
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while ((usage_ + charge) > capacity_ && lru_.next != &lru_) { |
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LRUHandle* old = lru_.next; |
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// LRU list contains only elements which can be evicted.
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assert(old->IsVisible() && !old->HasRefs()); |
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LRU_Remove(old); |
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table_.Remove(old); |
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assert(usage_ >= old->total_charge); |
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usage_ -= old->total_charge; |
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deleted->push_back(*old); |
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} |
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} |
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size_t LRUCacheShard::CalcEstimatedHandleCharge( |
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size_t estimated_value_size, |
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CacheMetadataChargePolicy metadata_charge_policy) { |
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LRUHandle h; |
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h.CalcTotalCharge(estimated_value_size, metadata_charge_policy); |
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return h.total_charge; |
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} |
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int LRUCacheShard::CalcHashBits( |
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size_t capacity, size_t estimated_value_size, |
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CacheMetadataChargePolicy metadata_charge_policy) { |
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size_t handle_charge = |
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CalcEstimatedHandleCharge(estimated_value_size, metadata_charge_policy); |
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assert(handle_charge > 0); |
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uint32_t num_entries = |
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static_cast<uint32_t>(capacity / (kLoadFactor * handle_charge)) + 1; |
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assert(num_entries <= uint32_t{1} << 31); |
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return FloorLog2((num_entries << 1) - 1); |
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} |
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void LRUCacheShard::SetCapacity(size_t capacity) { |
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autovector<LRUHandle> last_reference_list; |
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{ |
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DMutexLock l(mutex_); |
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if (capacity > capacity_) { |
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assert(false); // Not supported.
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} |
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capacity_ = capacity; |
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EvictFromLRU(0, &last_reference_list); |
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} |
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// Free the entries here outside of mutex for performance reasons.
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for (auto& h : last_reference_list) { |
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h.FreeData(); |
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} |
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} |
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void LRUCacheShard::SetStrictCapacityLimit(bool strict_capacity_limit) { |
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DMutexLock l(mutex_); |
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strict_capacity_limit_ = strict_capacity_limit; |
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} |
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Status LRUCacheShard::Insert(const Slice& key, uint32_t hash, void* value, |
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size_t charge, Cache::DeleterFn deleter, |
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LRUHandle** handle, Cache::Priority /*priority*/) { |
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if (key.size() != kCacheKeySize) { |
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return Status::NotSupported("FastLRUCache only supports key size " + |
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std::to_string(kCacheKeySize) + "B"); |
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} |
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LRUHandle tmp; |
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tmp.value = value; |
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tmp.deleter = deleter; |
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tmp.hash = hash; |
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tmp.CalcTotalCharge(charge, metadata_charge_policy_); |
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for (int i = 0; i < kCacheKeySize; i++) { |
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tmp.key_data[i] = key.data()[i]; |
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} |
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Status s = Status::OK(); |
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autovector<LRUHandle> last_reference_list; |
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{ |
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DMutexLock l(mutex_); |
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assert(table_.GetOccupancy() <= table_.GetOccupancyLimit()); |
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// Free the space following strict LRU policy until enough space
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// is freed or the lru list is empty.
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EvictFromLRU(tmp.total_charge, &last_reference_list); |
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if ((usage_ + tmp.total_charge > capacity_ && |
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(strict_capacity_limit_ || handle == nullptr)) || |
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table_.GetOccupancy() == table_.GetOccupancyLimit()) { |
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// There are two measures of capacity:
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// - Space (or charge) capacity: The maximum possible sum of the charges
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// of the elements.
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// - Table capacity: The number of slots in the hash table.
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// These are incomparable, in the sense that one doesn't imply the other.
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// Typically we will reach space capacity before table capacity---
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// if the user always inserts values with size equal to
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// estimated_value_size, then at most a kLoadFactor fraction of slots
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// will ever be occupied. But in some cases we may reach table capacity
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// before space capacity---if the user initially claims a very large
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// estimated_value_size but then inserts tiny values, more elements than
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// initially estimated will be inserted.
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// TODO(Guido) Some tests (at least two from cache_test, as well as the
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// stress tests) currently assume the table capacity is unbounded.
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if (handle == nullptr) { |
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// Don't insert the entry but still return ok, as if the entry inserted
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// into cache and get evicted immediately.
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last_reference_list.push_back(tmp); |
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} else { |
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if (table_.GetOccupancy() == table_.GetOccupancyLimit()) { |
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// TODO: Consider using a distinct status for this case, but usually
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// it will be handled the same way as reaching charge capacity limit
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s = Status::MemoryLimit( |
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"Insert failed because all slots in the hash table are full."); |
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} else { |
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s = Status::MemoryLimit( |
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"Insert failed because the total charge has exceeded the " |
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"capacity."); |
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} |
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} |
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} else { |
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// Insert into the cache. Note that the cache might get larger than its
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// capacity if not enough space was freed up.
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LRUHandle* old; |
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LRUHandle* h = table_.Insert(&tmp, &old); |
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assert(h != nullptr); // We're below occupancy, so this insertion should
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// never fail.
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usage_ += h->total_charge; |
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if (old != nullptr) { |
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s = Status::OkOverwritten(); |
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assert(old->IsVisible()); |
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table_.Exclude(old); |
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if (!old->HasRefs()) { |
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// old is on LRU because it's in cache and its reference count is 0.
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LRU_Remove(old); |
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table_.Remove(old); |
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assert(usage_ >= old->total_charge); |
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usage_ -= old->total_charge; |
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last_reference_list.push_back(*old); |
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} |
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} |
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if (handle == nullptr) { |
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LRU_Insert(h); |
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} else { |
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// If caller already holds a ref, no need to take one here.
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if (!h->HasRefs()) { |
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h->Ref(); |
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} |
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*handle = h; |
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} |
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} |
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} |
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// Free the entries here outside of mutex for performance reasons.
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for (auto& h : last_reference_list) { |
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h.FreeData(); |
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} |
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return s; |
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} |
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LRUHandle* LRUCacheShard::Lookup(const Slice& key, uint32_t hash) { |
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LRUHandle* h = nullptr; |
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{ |
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DMutexLock l(mutex_); |
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h = table_.Lookup(key, hash); |
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if (h != nullptr) { |
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assert(h->IsVisible()); |
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if (!h->HasRefs()) { |
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// The entry is in LRU since it's in hash and has no external
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// references.
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LRU_Remove(h); |
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} |
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h->Ref(); |
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} |
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} |
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return h; |
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} |
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bool LRUCacheShard::Ref(LRUHandle* h) { |
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DMutexLock l(mutex_); |
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// To create another reference - entry must be already externally referenced.
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assert(h->HasRefs()); |
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h->Ref(); |
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return true; |
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} |
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bool LRUCacheShard::Release(LRUHandle* h, bool erase_if_last_ref) { |
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if (h == nullptr) { |
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return false; |
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} |
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LRUHandle copy; |
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bool last_reference = false; |
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{ |
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DMutexLock l(mutex_); |
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last_reference = h->Unref(); |
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if (last_reference && h->IsVisible()) { |
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// The item is still in cache, and nobody else holds a reference to it.
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if (usage_ > capacity_ || erase_if_last_ref) { |
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// The LRU list must be empty since the cache is full.
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assert(lru_.next == &lru_ || erase_if_last_ref); |
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// Take this opportunity and remove the item.
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table_.Remove(h); |
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} else { |
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// Put the item back on the LRU list, and don't free it.
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LRU_Insert(h); |
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last_reference = false; |
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} |
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} |
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// If it was the last reference, then decrement the cache usage.
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if (last_reference) { |
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assert(usage_ >= h->total_charge); |
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usage_ -= h->total_charge; |
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copy = *h; |
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} |
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} |
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// Free the entry here outside of mutex for performance reasons.
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if (last_reference) { |
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copy.FreeData(); |
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} |
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return last_reference; |
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} |
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void LRUCacheShard::Erase(const Slice& key, uint32_t hash) { |
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LRUHandle copy; |
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bool last_reference = false; |
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{ |
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DMutexLock l(mutex_); |
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LRUHandle* h = table_.Lookup(key, hash); |
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if (h != nullptr) { |
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table_.Exclude(h); |
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if (!h->HasRefs()) { |
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// The entry is in LRU since it's in cache and has no external
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// references.
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LRU_Remove(h); |
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table_.Remove(h); |
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assert(usage_ >= h->total_charge); |
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usage_ -= h->total_charge; |
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last_reference = true; |
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copy = *h; |
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} |
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} |
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} |
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// Free the entry here outside of mutex for performance reasons.
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// last_reference will only be true if e != nullptr.
|
||||
if (last_reference) { |
||||
copy.FreeData(); |
||||
} |
||||
} |
||||
|
||||
size_t LRUCacheShard::GetUsage() const { |
||||
DMutexLock l(mutex_); |
||||
return usage_; |
||||
} |
||||
|
||||
size_t LRUCacheShard::GetPinnedUsage() const { |
||||
DMutexLock l(mutex_); |
||||
assert(usage_ >= lru_usage_); |
||||
return usage_ - lru_usage_; |
||||
} |
||||
|
||||
size_t LRUCacheShard::GetOccupancyCount() const { |
||||
DMutexLock l(mutex_); |
||||
return table_.GetOccupancy(); |
||||
} |
||||
|
||||
size_t LRUCacheShard::GetTableAddressCount() const { |
||||
DMutexLock l(mutex_); |
||||
return table_.GetTableSize(); |
||||
} |
||||
|
||||
LRUCache::LRUCache(size_t capacity, size_t estimated_value_size, |
||||
int num_shard_bits, bool strict_capacity_limit, |
||||
CacheMetadataChargePolicy metadata_charge_policy) |
||||
: ShardedCache(capacity, num_shard_bits, strict_capacity_limit, |
||||
nullptr /*allocator*/) { |
||||
assert(estimated_value_size > 0 || |
||||
metadata_charge_policy != kDontChargeCacheMetadata); |
||||
size_t per_shard = GetPerShardCapacity(); |
||||
InitShards([=](LRUCacheShard* cs) { |
||||
new (cs) LRUCacheShard(per_shard, estimated_value_size, |
||||
strict_capacity_limit, metadata_charge_policy); |
||||
}); |
||||
} |
||||
|
||||
void* LRUCache::Value(Handle* handle) { |
||||
return reinterpret_cast<const LRUHandle*>(handle)->value; |
||||
} |
||||
|
||||
size_t LRUCache::GetCharge(Handle* handle) const { |
||||
return reinterpret_cast<const LRUHandle*>(handle)->GetCharge( |
||||
GetShard(0).metadata_charge_policy_); |
||||
} |
||||
|
||||
Cache::DeleterFn LRUCache::GetDeleter(Handle* handle) const { |
||||
auto h = reinterpret_cast<const LRUHandle*>(handle); |
||||
return h->deleter; |
||||
} |
||||
|
||||
} // namespace fast_lru_cache
|
||||
|
||||
std::shared_ptr<Cache> NewFastLRUCache( |
||||
size_t capacity, size_t estimated_value_size, int num_shard_bits, |
||||
bool strict_capacity_limit, |
||||
CacheMetadataChargePolicy metadata_charge_policy) { |
||||
if (num_shard_bits >= 20) { |
||||
return nullptr; // The cache cannot be sharded into too many fine pieces.
|
||||
} |
||||
if (num_shard_bits < 0) { |
||||
num_shard_bits = GetDefaultCacheShardBits(capacity); |
||||
} |
||||
return std::make_shared<fast_lru_cache::LRUCache>( |
||||
capacity, estimated_value_size, num_shard_bits, strict_capacity_limit, |
||||
metadata_charge_policy); |
||||
} |
||||
|
||||
} // namespace ROCKSDB_NAMESPACE
|
||||
@ -1,476 +0,0 @@ |
||||
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved
|
||||
// This source code is licensed under both the GPLv2 (found in the
|
||||
// COPYING file in the root directory) and Apache 2.0 License
|
||||
// (found in the LICENSE.Apache file in the root directory).
|
||||
//
|
||||
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
|
||||
// Use of this source code is governed by a BSD-style license that can be
|
||||
// found in the LICENSE file. See the AUTHORS file for names of contributors.
|
||||
#pragma once |
||||
|
||||
#include <array> |
||||
#include <memory> |
||||
#include <string> |
||||
|
||||
#include "cache/cache_key.h" |
||||
#include "cache/sharded_cache.h" |
||||
#include "port/lang.h" |
||||
#include "port/malloc.h" |
||||
#include "port/port.h" |
||||
#include "rocksdb/secondary_cache.h" |
||||
#include "util/autovector.h" |
||||
#include "util/distributed_mutex.h" |
||||
|
||||
namespace ROCKSDB_NAMESPACE { |
||||
|
||||
namespace fast_lru_cache { |
||||
|
||||
// Forward declaration of friend class.
|
||||
class FastLRUCacheTest; |
||||
|
||||
// LRU cache implementation using an open-address hash table.
|
||||
//
|
||||
// Every slot in the hash table is an LRUHandle. Because handles can be
|
||||
// referenced externally, we can't discard them immediately once they are
|
||||
// deleted (via a delete or an LRU eviction) or replaced by a new version
|
||||
// (via an insert of the same key). The state of an element is defined by
|
||||
// the following two properties:
|
||||
// (R) Referenced: An element can be referenced externally (refs > 0), or not.
|
||||
// Importantly, an element can be evicted if and only if it's not
|
||||
// referenced. In particular, when an element becomes referenced, it's
|
||||
// temporarily taken out of the LRU list until all references to it
|
||||
// are dropped.
|
||||
// (V) Visible: An element can visible for lookups (IS_VISIBLE set), or not.
|
||||
// Initially, every element is visible. An element that is not visible is
|
||||
// called a ghost.
|
||||
// These properties induce 4 different states, with transitions defined as
|
||||
// follows:
|
||||
// - V --> not V: When a visible element is deleted or replaced by a new
|
||||
// version.
|
||||
// - Not V --> V: This cannot happen. A ghost remains in that state until it's
|
||||
// not referenced any more, at which point it's ready to be removed from the
|
||||
// hash table. (A ghost simply waits to transition to the afterlife---it will
|
||||
// never be visible again.)
|
||||
// - R --> not R: When all references to an element are dropped.
|
||||
// - Not R --> R: When an unreferenced element becomes referenced. This can only
|
||||
// happen if the element is V, since references to an element can only be
|
||||
// created when it's visible.
|
||||
//
|
||||
// Internally, the cache uses an open-addressed hash table to index the handles.
|
||||
// We use tombstone counters to keep track of displacements.
|
||||
// Because of the tombstones and the two possible visibility states of an
|
||||
// element, the table slots can be in 4 different states:
|
||||
// 1. Visible element (IS_ELEMENT set and IS_VISIBLE set): The slot contains a
|
||||
// key-value element.
|
||||
// 2. Ghost element (IS_ELEMENT set and IS_VISIBLE unset): The slot contains an
|
||||
// element that has been removed, but it's still referenced. It's invisible
|
||||
// to lookups.
|
||||
// 3. Tombstone (IS_ELEMENT unset and displacements > 0): The slot contains a
|
||||
// tombstone.
|
||||
// 4. Empty (IS_ELEMENT unset and displacements == 0): The slot is unused.
|
||||
// A slot that is an element can further have IS_VISIBLE set or not.
|
||||
// When a ghost is removed from the table, it can either transition to being a
|
||||
// tombstone or an empty slot, depending on the number of displacements of the
|
||||
// slot. In any case, the slot becomes available. When a handle is inserted
|
||||
// into that slot, it becomes a visible element again.
|
||||
|
||||
// The load factor p is a real number in (0, 1) such that at all
|
||||
// times at most a fraction p of all slots, without counting tombstones,
|
||||
// are occupied by elements. This means that the probability that a
|
||||
// random probe hits an empty slot is at most p, and thus at most 1/p probes
|
||||
// are required on average. For example, p = 70% implies that between 1 and 2
|
||||
// probes are needed on average (bear in mind that this reasoning doesn't
|
||||
// consider the effects of clustering over time).
|
||||
// Because the size of the hash table is always rounded up to the next
|
||||
// power of 2, p is really an upper bound on the actual load factor---the
|
||||
// actual load factor is anywhere between p/2 and p. This is a bit wasteful,
|
||||
// but bear in mind that slots only hold metadata, not actual values.
|
||||
// Since space cost is dominated by the values (the LSM blocks),
|
||||
// overprovisioning the table with metadata only increases the total cache space
|
||||
// usage by a tiny fraction.
|
||||
constexpr double kLoadFactor = 0.35; |
||||
|
||||
// The user can exceed kLoadFactor if the sizes of the inserted values don't
|
||||
// match estimated_value_size, or if strict_capacity_limit == false. To
|
||||
// avoid performance to plunge, we set a strict upper bound on the load factor.
|
||||
constexpr double kStrictLoadFactor = 0.7; |
||||
|
||||
// Arbitrary seeds.
|
||||
constexpr uint32_t kProbingSeed1 = 0xbc9f1d34; |
||||
constexpr uint32_t kProbingSeed2 = 0x7a2bb9d5; |
||||
|
||||
// An experimental (under development!) alternative to LRUCache
|
||||
|
||||
struct LRUHandle { |
||||
void* value; |
||||
Cache::DeleterFn deleter; |
||||
LRUHandle* next; |
||||
LRUHandle* prev; |
||||
size_t total_charge; // TODO(opt): Only allow uint32_t?
|
||||
// The hash of key(). Used for fast sharding and comparisons.
|
||||
uint32_t hash; |
||||
// The number of external refs to this entry.
|
||||
uint32_t refs; |
||||
|
||||
enum Flags : uint8_t { |
||||
// Whether the handle is visible to Lookups.
|
||||
IS_VISIBLE = (1 << 0), |
||||
// Whether the slot is in use by an element.
|
||||
IS_ELEMENT = (1 << 1), |
||||
}; |
||||
uint8_t flags; |
||||
|
||||
// The number of elements that hash to this slot or a lower one,
|
||||
// but wind up in a higher slot.
|
||||
uint32_t displacements; |
||||
|
||||
std::array<char, kCacheKeySize> key_data; |
||||
|
||||
LRUHandle() { |
||||
value = nullptr; |
||||
deleter = nullptr; |
||||
next = nullptr; |
||||
prev = nullptr; |
||||
total_charge = 0; |
||||
hash = 0; |
||||
refs = 0; |
||||
flags = 0; |
||||
displacements = 0; |
||||
key_data.fill(0); |
||||
} |
||||
|
||||
Slice key() const { return Slice(key_data.data(), kCacheKeySize); } |
||||
|
||||
// For HandleImpl concept
|
||||
uint32_t GetHash() const { return hash; } |
||||
|
||||
// Increase the reference count by 1.
|
||||
void Ref() { refs++; } |
||||
|
||||
// Just reduce the reference count by 1. Return true if it was last reference.
|
||||
bool Unref() { |
||||
assert(refs > 0); |
||||
refs--; |
||||
return refs == 0; |
||||
} |
||||
|
||||
// Return true if there are external refs, false otherwise.
|
||||
bool HasRefs() const { return refs > 0; } |
||||
|
||||
bool IsVisible() const { return flags & IS_VISIBLE; } |
||||
|
||||
void SetIsVisible(bool is_visible) { |
||||
if (is_visible) { |
||||
flags |= IS_VISIBLE; |
||||
} else { |
||||
flags &= ~IS_VISIBLE; |
||||
} |
||||
} |
||||
|
||||
bool IsElement() const { return flags & IS_ELEMENT; } |
||||
|
||||
void SetIsElement(bool is_element) { |
||||
if (is_element) { |
||||
flags |= IS_ELEMENT; |
||||
} else { |
||||
flags &= ~IS_ELEMENT; |
||||
} |
||||
} |
||||
|
||||
void FreeData() { |
||||
assert(refs == 0); |
||||
if (deleter) { |
||||
(*deleter)(key(), value); |
||||
} |
||||
} |
||||
|
||||
// Calculate the memory usage by metadata.
|
||||
inline size_t CalcMetaCharge( |
||||
CacheMetadataChargePolicy metadata_charge_policy) const { |
||||
if (metadata_charge_policy != kFullChargeCacheMetadata) { |
||||
return 0; |
||||
} else { |
||||
// #ifdef ROCKSDB_MALLOC_USABLE_SIZE
|
||||
// return malloc_usable_size(
|
||||
// const_cast<void*>(static_cast<const void*>(this)));
|
||||
// #else
|
||||
// TODO(Guido) malloc_usable_size only works when we call it on
|
||||
// a pointer allocated with malloc. Because our handles are all
|
||||
// allocated in a single shot as an array, the user can't call
|
||||
// CalcMetaCharge (or CalcTotalCharge or GetCharge) on a handle
|
||||
// pointer returned by the cache. Moreover, malloc_usable_size
|
||||
// expects a heap-allocated handle, but sometimes in our code we
|
||||
// wish to pass a stack-allocated handle (this is only a performance
|
||||
// concern).
|
||||
// What is the right way to compute metadata charges with pre-allocated
|
||||
// handles?
|
||||
return sizeof(LRUHandle); |
||||
// #endif
|
||||
} |
||||
} |
||||
|
||||
inline void CalcTotalCharge( |
||||
size_t charge, CacheMetadataChargePolicy metadata_charge_policy) { |
||||
total_charge = charge + CalcMetaCharge(metadata_charge_policy); |
||||
} |
||||
|
||||
inline size_t GetCharge( |
||||
CacheMetadataChargePolicy metadata_charge_policy) const { |
||||
size_t meta_charge = CalcMetaCharge(metadata_charge_policy); |
||||
assert(total_charge >= meta_charge); |
||||
return total_charge - meta_charge; |
||||
} |
||||
|
||||
inline bool IsEmpty() { |
||||
return !this->IsElement() && this->displacements == 0; |
||||
} |
||||
|
||||
inline bool IsTombstone() { |
||||
return !this->IsElement() && this->displacements > 0; |
||||
} |
||||
|
||||
inline bool Matches(const Slice& some_key, uint32_t some_hash) { |
||||
return this->IsElement() && this->hash == some_hash && |
||||
this->key() == some_key; |
||||
} |
||||
}; |
||||
|
||||
class LRUHandleTable { |
||||
public: |
||||
explicit LRUHandleTable(int hash_bits); |
||||
~LRUHandleTable(); |
||||
|
||||
// Returns a pointer to a visible element matching the key/hash, or
|
||||
// nullptr if not present.
|
||||
LRUHandle* Lookup(const Slice& key, uint32_t hash); |
||||
|
||||
// Inserts a copy of h into the hash table.
|
||||
// Returns a pointer to the inserted handle, or nullptr if no slot
|
||||
// available was found. If an existing visible element matching the
|
||||
// key/hash is already present in the hash table, the argument old
|
||||
// is set to pointe to it; otherwise, it's set to nullptr.
|
||||
LRUHandle* Insert(LRUHandle* h, LRUHandle** old); |
||||
|
||||
// Removes h from the hash table. The handle must already be off
|
||||
// the LRU list.
|
||||
void Remove(LRUHandle* h); |
||||
|
||||
// Turns a visible element h into a ghost (i.e., not visible).
|
||||
void Exclude(LRUHandle* h); |
||||
|
||||
// Assigns a copy of h to the given slot.
|
||||
void Assign(int slot, LRUHandle* h); |
||||
|
||||
template <typename T> |
||||
void ApplyToEntriesRange(T func, size_t index_begin, size_t index_end) { |
||||
for (size_t i = index_begin; i < index_end; i++) { |
||||
LRUHandle* h = &array_[i]; |
||||
if (h->IsVisible()) { |
||||
func(h); |
||||
} |
||||
} |
||||
} |
||||
|
||||
uint32_t GetTableSize() const { return uint32_t{1} << length_bits_; } |
||||
|
||||
int GetLengthBits() const { return length_bits_; } |
||||
|
||||
uint32_t GetOccupancyLimit() const { return occupancy_limit_; } |
||||
|
||||
uint32_t GetOccupancy() const { return occupancy_; } |
||||
|
||||
// Returns x mod 2^{length_bits_}.
|
||||
uint32_t ModTableSize(uint32_t x) { return x & length_bits_mask_; } |
||||
|
||||
private: |
||||
int FindVisibleElement(const Slice& key, uint32_t hash, int& probe, |
||||
int displacement); |
||||
|
||||
int FindAvailableSlot(const Slice& key, int& probe, int displacement); |
||||
|
||||
int FindVisibleElementOrAvailableSlot(const Slice& key, uint32_t hash, |
||||
int& probe, int displacement); |
||||
|
||||
// Returns the index of the first slot probed (hashing with
|
||||
// the given key) with a handle e such that cond(e) is true.
|
||||
// Otherwise, if no match is found, returns -1.
|
||||
// For every handle e probed except the final slot, updates
|
||||
// e->displacements += displacement.
|
||||
// The argument probe is modified such that consecutive calls
|
||||
// to FindSlot continue probing right after where the previous
|
||||
// call left.
|
||||
int FindSlot(const Slice& key, std::function<bool(LRUHandle*)> cond, |
||||
int& probe, int displacement); |
||||
|
||||
// Number of hash bits used for table index.
|
||||
// The size of the table is 1 << length_bits_.
|
||||
int length_bits_; |
||||
|
||||
const uint32_t length_bits_mask_; |
||||
|
||||
// Number of elements in the table.
|
||||
uint32_t occupancy_; |
||||
|
||||
// Maximum number of elements the user can store in the table.
|
||||
uint32_t occupancy_limit_; |
||||
|
||||
std::unique_ptr<LRUHandle[]> array_; |
||||
}; |
||||
|
||||
// A single shard of sharded cache.
|
||||
class ALIGN_AS(CACHE_LINE_SIZE) LRUCacheShard final : public CacheShardBase { |
||||
public: |
||||
LRUCacheShard(size_t capacity, size_t estimated_value_size, |
||||
bool strict_capacity_limit, |
||||
CacheMetadataChargePolicy metadata_charge_policy); |
||||
|
||||
// For CacheShard concept
|
||||
using HandleImpl = LRUHandle; |
||||
|
||||
// Keep 32-bit hashing for now (FIXME: upgrade to 64-bit)
|
||||
using HashVal = uint32_t; |
||||
using HashCref = uint32_t; |
||||
static inline HashVal ComputeHash(const Slice& key) { |
||||
return Lower32of64(GetSliceNPHash64(key)); |
||||
} |
||||
static inline uint32_t HashPieceForSharding(HashCref hash) { return hash; } |
||||
|
||||
// Separate from constructor so caller can easily make an array of LRUCache
|
||||
// if current usage is more than new capacity, the function will attempt to
|
||||
// free the needed space.
|
||||
void SetCapacity(size_t capacity); |
||||
|
||||
// Set the flag to reject insertion if cache if full.
|
||||
void SetStrictCapacityLimit(bool strict_capacity_limit); |
||||
|
||||
// Like Cache methods, but with an extra "hash" parameter.
|
||||
// Insert an item into the hash table and, if handle is null, insert into
|
||||
// the LRU list. Older items are evicted as necessary. If the cache is full
|
||||
// and free_handle_on_fail is true, the item is deleted and handle is set to
|
||||
// nullptr.
|
||||
Status Insert(const Slice& key, uint32_t hash, void* value, size_t charge, |
||||
Cache::DeleterFn deleter, LRUHandle** handle, |
||||
Cache::Priority priority); |
||||
|
||||
Status Insert(const Slice& key, uint32_t hash, void* value, |
||||
const Cache::CacheItemHelper* helper, size_t charge, |
||||
LRUHandle** handle, Cache::Priority priority) { |
||||
return Insert(key, hash, value, charge, helper->del_cb, handle, priority); |
||||
} |
||||
|
||||
LRUHandle* Lookup(const Slice& key, uint32_t hash, |
||||
const Cache::CacheItemHelper* /*helper*/, |
||||
const Cache::CreateCallback& /*create_cb*/, |
||||
Cache::Priority /*priority*/, bool /*wait*/, |
||||
Statistics* /*stats*/) { |
||||
return Lookup(key, hash); |
||||
} |
||||
LRUHandle* Lookup(const Slice& key, uint32_t hash); |
||||
|
||||
bool Release(LRUHandle* handle, bool /*useful*/, bool erase_if_last_ref) { |
||||
return Release(handle, erase_if_last_ref); |
||||
} |
||||
bool IsReady(LRUHandle* /*handle*/) { return true; } |
||||
void Wait(LRUHandle* /*handle*/) {} |
||||
|
||||
bool Ref(LRUHandle* handle); |
||||
bool Release(LRUHandle* handle, bool erase_if_last_ref = false); |
||||
void Erase(const Slice& key, uint32_t hash); |
||||
|
||||
size_t GetUsage() const; |
||||
size_t GetPinnedUsage() const; |
||||
size_t GetOccupancyCount() const; |
||||
size_t GetTableAddressCount() const; |
||||
|
||||
void ApplyToSomeEntries( |
||||
const std::function<void(const Slice& key, void* value, size_t charge, |
||||
DeleterFn deleter)>& callback, |
||||
size_t average_entries_per_lock, size_t* state); |
||||
|
||||
void EraseUnRefEntries(); |
||||
|
||||
private: |
||||
friend class LRUCache; |
||||
friend class FastLRUCacheTest; |
||||
|
||||
void LRU_Remove(LRUHandle* e); |
||||
void LRU_Insert(LRUHandle* e); |
||||
|
||||
// Free some space following strict LRU policy until enough space
|
||||
// to hold (usage_ + charge) is freed or the LRU list is empty
|
||||
// This function is not thread safe - it needs to be executed while
|
||||
// holding the mutex_.
|
||||
void EvictFromLRU(size_t charge, autovector<LRUHandle>* deleted); |
||||
|
||||
// Returns the charge of a single handle.
|
||||
static size_t CalcEstimatedHandleCharge( |
||||
size_t estimated_value_size, |
||||
CacheMetadataChargePolicy metadata_charge_policy); |
||||
|
||||
// Returns the number of bits used to hash an element in the hash
|
||||
// table.
|
||||
static int CalcHashBits(size_t capacity, size_t estimated_value_size, |
||||
CacheMetadataChargePolicy metadata_charge_policy); |
||||
|
||||
// Initialized before use.
|
||||
size_t capacity_; |
||||
|
||||
// Whether to reject insertion if cache reaches its full capacity.
|
||||
bool strict_capacity_limit_; |
||||
|
||||
// Dummy head of LRU list.
|
||||
// lru.prev is newest entry, lru.next is oldest entry.
|
||||
// LRU contains items which can be evicted, ie reference only by cache
|
||||
LRUHandle lru_; |
||||
|
||||
// Pointer to head of low-pri pool in LRU list.
|
||||
LRUHandle* lru_low_pri_; |
||||
|
||||
// ------------^^^^^^^^^^^^^-----------
|
||||
// Not frequently modified data members
|
||||
// ------------------------------------
|
||||
//
|
||||
// We separate data members that are updated frequently from the ones that
|
||||
// are not frequently updated so that they don't share the same cache line
|
||||
// which will lead into false cache sharing
|
||||
//
|
||||
// ------------------------------------
|
||||
// Frequently modified data members
|
||||
// ------------vvvvvvvvvvvvv-----------
|
||||
LRUHandleTable table_; |
||||
|
||||
// Memory size for entries residing in the cache.
|
||||
size_t usage_; |
||||
|
||||
// Memory size for entries residing only in the LRU list.
|
||||
size_t lru_usage_; |
||||
|
||||
// mutex_ protects the following state.
|
||||
// We don't count mutex_ as the cache's internal state so semantically we
|
||||
// don't mind mutex_ invoking the non-const actions.
|
||||
mutable DMutex mutex_; |
||||
}; |
||||
|
||||
class LRUCache |
||||
#ifdef NDEBUG |
||||
final |
||||
#endif |
||||
: public ShardedCache<LRUCacheShard> { |
||||
public: |
||||
LRUCache(size_t capacity, size_t estimated_value_size, int num_shard_bits, |
||||
bool strict_capacity_limit, |
||||
CacheMetadataChargePolicy metadata_charge_policy = |
||||
kDontChargeCacheMetadata); |
||||
const char* Name() const override { return "LRUCache"; } |
||||
void* Value(Handle* handle) override; |
||||
size_t GetCharge(Handle* handle) const override; |
||||
DeleterFn GetDeleter(Handle* handle) const override; |
||||
}; |
||||
} // namespace fast_lru_cache
|
||||
|
||||
std::shared_ptr<Cache> NewFastLRUCache( |
||||
size_t capacity, size_t estimated_value_size, int num_shard_bits, |
||||
bool strict_capacity_limit, |
||||
CacheMetadataChargePolicy metadata_charge_policy); |
||||
|
||||
} // namespace ROCKSDB_NAMESPACE
|
||||
Loading…
Reference in new issue