Summary: Clock-based cache implemenetation aim to have better concurreny than default LRU cache. See inline comments for implementation details. Test Plan: Update cache_test to run on both LRUCache and ClockCache. Adding some new tests to catch some of the bugs that I fixed while implementing the cache. Reviewers: kradhakrishnan, sdong Reviewed By: sdong Subscribers: andrewkr, dhruba, leveldb Differential Revision: https://reviews.facebook.net/D61647main
Yi Wu
8 years ago
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ff17a2abf3
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under the BSD-style license found in the
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// LICENSE file in the root directory of this source tree. An additional grant
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// of patent rights can be found in the PATENTS file in the same 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 "util/clock_cache.h" |
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#ifndef SUPPORT_CLOCK_CACHE |
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namespace rocksdb { |
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std::shared_ptr<Cache> NewClockCache(size_t capacity, int num_shard_bits, |
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bool strict_capacity_limit) { |
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// Clock cache not supported.
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return nullptr; |
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} |
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} // namespace rocksdb
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#else |
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#include <assert.h> |
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#include <atomic> |
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#include <deque> |
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#include <limits> |
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#include "tbb/concurrent_hash_map.h" |
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#include "port/port.h" |
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#include "util/autovector.h" |
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#include "util/mutexlock.h" |
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#include "util/sharded_cache.h" |
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namespace rocksdb { |
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namespace { |
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// An implementation of the Cache interface based on CLOCK algorithm, with
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// better concurrent performance than LRUCache. The idea of CLOCK algorithm
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// is to maintain all cache entries in a circular list, and an iterator
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// (the "head") pointing to the last examined entry. Eviction starts from the
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// current head. Each entry is given a second chance before eviction, if it
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// has been access since last examine. In contrast to LRU, no modification
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// to the internal data-structure (except for flipping the usage bit) needs
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// to be done upon lookup. This gives us oppertunity to implement a cache
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// with better concurrency.
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//
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// Each cache entry is represented by a cache handle, and all the handles
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// are arranged in a circular list, as describe above. Upon erase of an entry,
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// we never remove the handle. Instead, the handle is put into a recycle bin
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// to be re-use. This is to avoid memory dealocation, which is hard to deal
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// with in concurrent environment.
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//
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// The cache also maintains a concurrent hash map for lookup. Any concurrent
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// hash map implementation should do the work. We currently use
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// tbb::concurrent_hash_map because it supports concurrent erase.
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//
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// Each cache handle has the following flags and counters, which are squeeze
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// in an atomic interger, to make sure the handle always be in a consistent
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// state:
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//
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// * In-cache bit: whether the entry is reference by the cache itself. If
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// an entry is in cache, its key would also be available in the hash map.
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// * Usage bit: whether the entry has been access by user since last
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// examine for eviction. Can be reset by eviction.
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// * Reference count: reference count by user.
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//
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// An entry can be reference only when it's in cache. An entry can be evicted
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// only when it is in cache, has no usage since last examine, and reference
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// count is zero.
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//
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// The follow figure shows a possible layout of the cache. Boxes represents
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// cache handles and numbers in each box being in-cache bit, usage bit and
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// reference count respectively.
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//
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// hash map:
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// +-------+--------+
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// | key | handle |
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// +-------+--------+
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// | "foo" | 5 |-------------------------------------+
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// +-------+--------+ |
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// | "bar" | 2 |--+ |
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// +-------+--------+ | |
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// | |
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// head | |
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// | | |
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// circular list: | | |
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// +-------+ +-------+ +-------+ +-------+ +-------+ +-------
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// |(0,0,0)|---|(1,1,0)|---|(0,0,0)|---|(0,1,3)|---|(1,0,0)|---| ...
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// +-------+ +-------+ +-------+ +-------+ +-------+ +-------
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// | |
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// +-------+ +-----------+
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// | |
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// +---+---+
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// recycle bin: | 1 | 3 |
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// +---+---+
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//
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// Suppose we try to insert "baz" into the cache at this point and the cache is
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// full. The cache will first look for entries to evict, starting from where
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// head points to (the second entry). It resets usage bit of the second entry,
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// skips the third and fourth entry since they are not in cache, and finally
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// evict the fifth entry ("foo"). It looks at recycle bin for available handle,
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// grabs handle 3, and insert the key into the handle. The following figure
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// shows the resulting layout.
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//
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// hash map:
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// +-------+--------+
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// | key | handle |
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// +-------+--------+
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// | "baz" | 3 |-------------+
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// +-------+--------+ |
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// | "bar" | 2 |--+ |
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// +-------+--------+ | |
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// | |
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// | | head
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// | | |
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// circular list: | | |
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// +-------+ +-------+ +-------+ +-------+ +-------+ +-------
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// |(0,0,0)|---|(1,0,0)|---|(1,0,0)|---|(0,1,3)|---|(0,0,0)|---| ...
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// +-------+ +-------+ +-------+ +-------+ +-------+ +-------
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// | |
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// +-------+ +-----------------------------------+
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// | |
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// +---+---+
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// recycle bin: | 1 | 5 |
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// +---+---+
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//
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// A global mutex guards the circular list, the head, and the recycle bin.
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// We additionally require that modifying the hash map needs to hold the mutex.
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// As such, Modifying the cache (such as Insert() and Erase()) require to
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// hold the mutex. Lookup() only access the hash map and the flags associated
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// with each handle, and don't require explicit locking. Release() has to
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// acquire the mutex only when it releases the last reference to the entry and
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// the entry has been erased from cache explicitly. A future improvement could
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// be to remove the mutex completely.
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//
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// Benchmark:
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// We run readrandom db_bench on a test DB of size 13GB, with size of each
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// level:
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//
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// Level Files Size(MB)
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// -------------------------
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// L0 1 0.01
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// L1 18 17.32
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// L2 230 182.94
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// L3 1186 1833.63
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// L4 4602 8140.30
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//
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// We test with both 32 and 16 read threads, with 2GB cache size (the whole DB
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// doesn't fits in) and 64GB cache size (the whole DB can fit in cache), and
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// whether to put index and filter blocks in block cache. The benchmark runs
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// with
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// with RocksDB 4.10. We got the following result:
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//
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// Threads Cache Cache ClockCache LRUCache
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// Size Index/Filter Throughput(MB/s) Hit Throughput(MB/s) Hit
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// 32 2GB yes 466.7 85.9% 433.7 86.5%
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// 32 2GB no 529.9 72.7% 532.7 73.9%
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// 32 64GB yes 649.9 99.9% 507.9 99.9%
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// 32 64GB no 740.4 99.9% 662.8 99.9%
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// 16 2GB yes 278.4 85.9% 283.4 86.5%
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// 16 2GB no 318.6 72.7% 335.8 73.9%
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// 16 64GB yes 391.9 99.9% 353.3 99.9%
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// 16 64GB no 433.8 99.8% 419.4 99.8%
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// Cache entry meta data.
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struct CacheHandle { |
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Slice key; |
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uint32_t hash; |
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void* value; |
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size_t charge; |
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void (*deleter)(const Slice&, void* value); |
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// Flags and counters associated with the cache handle:
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// lowest bit: n-cache bit
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// second lowest bit: usage bit
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// the rest bits: reference count
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// The handle is unused when flags equals to 0. The thread decreases the count
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// to 0 is responsible to put the handle back to recycle_ and cleanup memory.
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std::atomic<uint32_t> flags; |
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CacheHandle() = default; |
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CacheHandle(const CacheHandle& a) { *this = a; } |
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CacheHandle& operator=(const CacheHandle& a) { |
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// Only copy members needed for deletion.
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key = a.key; |
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value = a.value; |
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deleter = a.deleter; |
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return *this; |
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} |
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}; |
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// Key of hash map. We store hash value with the key for convenience.
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struct CacheKey { |
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Slice key; |
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uint32_t hash_value; |
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CacheKey() = default; |
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CacheKey(const Slice& k, uint32_t h) { |
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key = k; |
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hash_value = h; |
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} |
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static bool equal(const CacheKey& a, const CacheKey& b) { |
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return a.hash_value == b.hash_value && a.key == b.key; |
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} |
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static size_t hash(const CacheKey& a) { |
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return static_cast<size_t>(a.hash_value); |
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} |
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}; |
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struct CleanupContext { |
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// List of values to be deleted, along with the key and deleter.
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autovector<CacheHandle> to_delete_value; |
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// List of keys to be deleted.
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autovector<const char*> to_delete_key; |
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}; |
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// A cache shard which maintains its own CLOCK cache.
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class ClockCacheShard : public CacheShard { |
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public: |
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// Hash map type.
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typedef tbb::concurrent_hash_map<CacheKey, CacheHandle*, CacheKey> HashTable; |
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ClockCacheShard(); |
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~ClockCacheShard() = default; |
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// Interfaces
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virtual void SetCapacity(size_t capacity) override; |
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virtual void SetStrictCapacityLimit(bool strict_capacity_limit) override; |
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virtual Status Insert(const Slice& key, uint32_t hash, void* value, |
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size_t charge, |
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void (*deleter)(const Slice& key, void* value), |
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Cache::Handle** handle) override; |
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virtual Cache::Handle* Lookup(const Slice& key, uint32_t hash) override; |
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virtual void Release(Cache::Handle* handle) override; |
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virtual void Erase(const Slice& key, uint32_t hash) override; |
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virtual size_t GetUsage() const override; |
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virtual size_t GetPinnedUsage() const override; |
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virtual void EraseUnRefEntries() override; |
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virtual void ApplyToAllCacheEntries(void (*callback)(void*, size_t), |
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bool thread_safe) override; |
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private: |
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static const uint32_t kInCacheBit = 1; |
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static const uint32_t kUsageBit = 2; |
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static const uint32_t kRefsOffset = 2; |
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static const uint32_t kOneRef = 1 << kRefsOffset; |
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// Helper functions to extract cache handle flags and counters.
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static bool InCache(uint32_t flags) { return flags & kInCacheBit; } |
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static bool HasUsage(uint32_t flags) { return flags & kUsageBit; } |
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static uint32_t CountRefs(uint32_t flags) { return flags >> kRefsOffset; } |
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// If the entry in in cache, increase reference count and return true.
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// Return false otherwise.
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//
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// Not necessary to hold mutex_ before being called.
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bool Ref(CacheHandle* handle); |
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// Decrease reference count of the entry. If this decreases the count to 0,
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// recycle the entry. If set_usage is true, also set the usage bit.
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//
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// Not necessary to hold mutex_ before being called.
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void Unref(CacheHandle* handle, bool set_usage, CleanupContext* context); |
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// Unset in-cache bit of the entry. Recycle the handle if necessary.
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//
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// Has to hold mutex_ before being called.
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void UnsetInCache(CacheHandle* handle, CleanupContext* context); |
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// Put the handle back to recycle_ list, and put the value associated with
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// it into to-be-deleted list. It doesn't cleanup the key as it might be
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// reused by another handle.
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//
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// Has to hold mutex_ before being called.
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void RecycleHandle(CacheHandle* handle, CleanupContext* context); |
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// Remove the key from hash map. Put the key associated with the entry into
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// to be deleted list.
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//
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// Has to hold mutex_ before being called.
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void EraseKey(CacheHandle* handle, CleanupContext* context); |
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// Delete keys and values in to-be-deleted list. Call the method without
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// holding mutex, as destructors can be expensive.
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void Cleanup(const CleanupContext& context); |
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// Examine the handle for eviction. If the handle is in cache, usage bit is
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// not set, and referece count is 0, evict it from cache. Otherwise unset
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// the usage bit.
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//
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// Has to hold mutex_ before being called.
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bool TryEvict(CacheHandle* value, CleanupContext* context); |
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// Scan through the circular list, evict entries until we get enough capacity
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// for new cache entry of specific size. Return true if success, false
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// otherwise.
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//
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// Has to hold mutex_ before being called.
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bool EvictFromCache(size_t charge, CleanupContext* context); |
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CacheHandle* Insert(const Slice& key, uint32_t hash, void* value, |
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size_t change, |
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void (*deleter)(const Slice& key, void* value), |
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bool hold_reference, CleanupContext* context); |
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// Guards list_, head_, and recycle_. In addition, updating table_ also has
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// to hold the mutex, to avoid the cache being in inconsistent state.
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mutable port::Mutex mutex_; |
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// The circular list of cache handles. Initially the list is empty. Once a
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// handle is needed by insertion, and no more handles are available in
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// recycle bin, one more handle is appended to the end.
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//
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// We use std::deque for the circular list because we want to make sure
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// pointers to handles are valid through out the life-cycle of the cache
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// (in contrast to std::vector), and be able to grow the list (in contrast
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// to statically allocated arrays).
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std::deque<CacheHandle> list_; |
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// Pointer to the next handle in the circular list to be examine for
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// eviction.
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size_t head_; |
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// Recycle bin of cache handles.
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autovector<CacheHandle*> recycle_; |
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// Maximum cache size.
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std::atomic<size_t> capacity_; |
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// Current total size of the cache.
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std::atomic<size_t> usage_; |
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// Total un-released cache size.
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std::atomic<size_t> pinned_usage_; |
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// Whether allow insert into cache if cache is full.
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std::atomic<bool> strict_capacity_limit_; |
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// Hash table (tbb::concurrent_hash_map) for lookup.
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HashTable table_; |
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}; |
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ClockCacheShard::ClockCacheShard() |
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: head_(0), usage_(0), pinned_usage_(0), strict_capacity_limit_(false) {} |
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size_t ClockCacheShard::GetUsage() const { |
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return usage_.load(std::memory_order_relaxed); |
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} |
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size_t ClockCacheShard::GetPinnedUsage() const { |
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return pinned_usage_.load(std::memory_order_relaxed); |
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} |
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void ClockCacheShard::ApplyToAllCacheEntries(void (*callback)(void*, size_t), |
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bool thread_safe) { |
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if (thread_safe) { |
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mutex_.Lock(); |
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} |
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for (auto& handle : list_) { |
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// Use relaxed semantics instead of acquire semantics since we are either
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// holding mutex, or don't have thread safe requirement.
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uint32_t flags = handle.flags.load(std::memory_order_relaxed); |
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if (InCache(flags)) { |
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callback(handle.value, handle.charge); |
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} |
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} |
||||||
|
if (thread_safe) { |
||||||
|
mutex_.Unlock(); |
||||||
|
} |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::RecycleHandle(CacheHandle* handle, |
||||||
|
CleanupContext* context) { |
||||||
|
mutex_.AssertHeld(); |
||||||
|
assert(!InCache(handle->flags) && CountRefs(handle->flags) == 0); |
||||||
|
// Only cleanup the value. The key may be reused by another handle.
|
||||||
|
context->to_delete_value.emplace_back(*handle); |
||||||
|
recycle_.push_back(handle); |
||||||
|
usage_.fetch_sub(handle->charge, std::memory_order_relaxed); |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::EraseKey(CacheHandle* handle, CleanupContext* context) { |
||||||
|
mutex_.AssertHeld(); |
||||||
|
assert(!InCache(handle->flags)); |
||||||
|
bool erased __attribute__((__unused__)) = |
||||||
|
table_.erase(CacheKey(handle->key, handle->hash)); |
||||||
|
assert(erased); |
||||||
|
context->to_delete_key.push_back(handle->key.data()); |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::Cleanup(const CleanupContext& context) { |
||||||
|
for (const CacheHandle& handle : context.to_delete_value) { |
||||||
|
if (handle.deleter) { |
||||||
|
(*handle.deleter)(handle.key, handle.value); |
||||||
|
} |
||||||
|
} |
||||||
|
for (const char* key : context.to_delete_key) { |
||||||
|
delete[] key; |
||||||
|
} |
||||||
|
} |
||||||
|
|
||||||
|
bool ClockCacheShard::Ref(CacheHandle* handle) { |
||||||
|
// CAS loop to increase reference count.
|
||||||
|
uint32_t flags = handle->flags.load(std::memory_order_relaxed); |
||||||
|
while (InCache(flags)) { |
||||||
|
// Use acquire semantics on success, as further operations on the cache
|
||||||
|
// entry has to be order after reference count is increased.
|
||||||
|
if (handle->flags.compare_exchange_weak(flags, flags + kOneRef, |
||||||
|
std::memory_order_acquire, |
||||||
|
std::memory_order_relaxed)) { |
||||||
|
if (CountRefs(flags) == 0) { |
||||||
|
// No reference count before the operation.
|
||||||
|
pinned_usage_.fetch_add(handle->charge, std::memory_order_relaxed); |
||||||
|
} |
||||||
|
return true; |
||||||
|
} |
||||||
|
} |
||||||
|
return false; |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::Unref(CacheHandle* handle, bool set_usage, |
||||||
|
CleanupContext* context) { |
||||||
|
if (set_usage) { |
||||||
|
handle->flags.fetch_or(kUsageBit, std::memory_order_relaxed); |
||||||
|
} |
||||||
|
// Use acquire-release semantics as previous operations on the cache entry
|
||||||
|
// has to be order before reference count is decreased, and potential cleanup
|
||||||
|
// of the entry has to be order after.
|
||||||
|
uint32_t flags = handle->flags.fetch_sub(kOneRef, std::memory_order_acq_rel); |
||||||
|
assert(CountRefs(flags) > 0); |
||||||
|
if (CountRefs(flags) == 1) { |
||||||
|
// this is the last reference.
|
||||||
|
pinned_usage_.fetch_sub(handle->charge, std::memory_order_relaxed); |
||||||
|
// Cleanup if it is the last reference.
|
||||||
|
if (!InCache(flags)) { |
||||||
|
MutexLock l(&mutex_); |
||||||
|
RecycleHandle(handle, context); |
||||||
|
} |
||||||
|
} |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::UnsetInCache(CacheHandle* handle, |
||||||
|
CleanupContext* context) { |
||||||
|
mutex_.AssertHeld(); |
||||||
|
// Use acquire-release semantics as previous operations on the cache entry
|
||||||
|
// has to be order before reference count is decreased, and potential cleanup
|
||||||
|
// of the entry has to be order after.
|
||||||
|
uint32_t flags = |
||||||
|
handle->flags.fetch_and(~kInCacheBit, std::memory_order_acq_rel); |
||||||
|
// Cleanup if it is the last reference.
|
||||||
|
if (InCache(flags) && CountRefs(flags) == 0) { |
||||||
|
RecycleHandle(handle, context); |
||||||
|
} |
||||||
|
} |
||||||
|
|
||||||
|
bool ClockCacheShard::TryEvict(CacheHandle* handle, CleanupContext* context) { |
||||||
|
mutex_.AssertHeld(); |
||||||
|
uint32_t flags = kInCacheBit; |
||||||
|
if (handle->flags.compare_exchange_strong(flags, 0, std::memory_order_acquire, |
||||||
|
std::memory_order_relaxed)) { |
||||||
|
RecycleHandle(handle, context); |
||||||
|
EraseKey(handle, context); |
||||||
|
return true; |
||||||
|
} |
||||||
|
handle->flags.fetch_and(~kUsageBit, std::memory_order_relaxed); |
||||||
|
return false; |
||||||
|
} |
||||||
|
|
||||||
|
bool ClockCacheShard::EvictFromCache(size_t charge, CleanupContext* context) { |
||||||
|
size_t usage = usage_.load(std::memory_order_relaxed); |
||||||
|
size_t capacity = capacity_.load(std::memory_order_relaxed); |
||||||
|
if (usage == 0) { |
||||||
|
return charge <= capacity; |
||||||
|
} |
||||||
|
size_t new_head = head_; |
||||||
|
bool second_iteration = false; |
||||||
|
while (usage + charge > capacity) { |
||||||
|
assert(new_head < list_.size()); |
||||||
|
if (TryEvict(&list_[new_head], context)) { |
||||||
|
usage = usage_.load(std::memory_order_relaxed); |
||||||
|
} |
||||||
|
new_head = (new_head + 1 >= list_.size()) ? 0 : new_head + 1; |
||||||
|
if (new_head == head_) { |
||||||
|
if (second_iteration) { |
||||||
|
return false; |
||||||
|
} else { |
||||||
|
second_iteration = true; |
||||||
|
} |
||||||
|
} |
||||||
|
} |
||||||
|
head_ = new_head; |
||||||
|
return true; |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::SetCapacity(size_t capacity) { |
||||||
|
CleanupContext context; |
||||||
|
{ |
||||||
|
MutexLock l(&mutex_); |
||||||
|
capacity_.store(capacity, std::memory_order_relaxed); |
||||||
|
EvictFromCache(0, &context); |
||||||
|
} |
||||||
|
Cleanup(context); |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::SetStrictCapacityLimit(bool strict_capacity_limit) { |
||||||
|
strict_capacity_limit_.store(strict_capacity_limit, |
||||||
|
std::memory_order_relaxed); |
||||||
|
} |
||||||
|
|
||||||
|
CacheHandle* ClockCacheShard::Insert( |
||||||
|
const Slice& key, uint32_t hash, void* value, size_t charge, |
||||||
|
void (*deleter)(const Slice& key, void* value), bool hold_reference, |
||||||
|
CleanupContext* context) { |
||||||
|
MutexLock l(&mutex_); |
||||||
|
bool success = EvictFromCache(charge, context); |
||||||
|
bool strict = strict_capacity_limit_.load(std::memory_order_relaxed); |
||||||
|
if (!success && strict) { |
||||||
|
return nullptr; |
||||||
|
} |
||||||
|
// Grab available handle from recycle bin. If recycle bin is empty, create
|
||||||
|
// and append new handle to end of circular list.
|
||||||
|
CacheHandle* handle = nullptr; |
||||||
|
if (!recycle_.empty()) { |
||||||
|
handle = recycle_.back(); |
||||||
|
recycle_.pop_back(); |
||||||
|
} else { |
||||||
|
list_.emplace_back(); |
||||||
|
handle = &list_.back(); |
||||||
|
} |
||||||
|
// Fill handle.
|
||||||
|
handle->key = key; |
||||||
|
handle->hash = hash; |
||||||
|
handle->value = value; |
||||||
|
handle->charge = charge; |
||||||
|
handle->deleter = deleter; |
||||||
|
uint32_t flags = hold_reference ? kInCacheBit + kOneRef : kInCacheBit; |
||||||
|
handle->flags.store(flags, std::memory_order_relaxed); |
||||||
|
HashTable::accessor accessor; |
||||||
|
if (table_.find(accessor, CacheKey(key, hash))) { |
||||||
|
// Key exists. Replace with new handle, but keep the existing key since
|
||||||
|
// the key in hash table is back by the existing one. The new key will be
|
||||||
|
// deleted by Cleanup().
|
||||||
|
CacheHandle* existing_handle = accessor->second; |
||||||
|
context->to_delete_key.push_back(handle->key.data()); |
||||||
|
handle->key = existing_handle->key; |
||||||
|
accessor->second = handle; |
||||||
|
accessor.release(); |
||||||
|
UnsetInCache(existing_handle, context); |
||||||
|
} else { |
||||||
|
table_.insert(HashTable::value_type(CacheKey(key, hash), handle)); |
||||||
|
} |
||||||
|
if (hold_reference) { |
||||||
|
pinned_usage_.fetch_add(charge, std::memory_order_relaxed); |
||||||
|
} |
||||||
|
usage_.fetch_add(charge, std::memory_order_relaxed); |
||||||
|
return handle; |
||||||
|
} |
||||||
|
|
||||||
|
Status ClockCacheShard::Insert(const Slice& key, uint32_t hash, void* value, |
||||||
|
size_t charge, |
||||||
|
void (*deleter)(const Slice& key, void* value), |
||||||
|
Cache::Handle** h) { |
||||||
|
CleanupContext context; |
||||||
|
HashTable::accessor accessor; |
||||||
|
char* key_data = new char[key.size()]; |
||||||
|
memcpy(key_data, key.data(), key.size()); |
||||||
|
Slice key_copy(key_data, key.size()); |
||||||
|
CacheHandle* handle = |
||||||
|
Insert(key_copy, hash, value, charge, deleter, h != nullptr, &context); |
||||||
|
Status s; |
||||||
|
if (h != nullptr) { |
||||||
|
*h = reinterpret_cast<Cache::Handle*>(handle); |
||||||
|
} |
||||||
|
if (handle == nullptr) { |
||||||
|
s = Status::Incomplete("Insert failed due to LRU cache being full."); |
||||||
|
} |
||||||
|
Cleanup(context); |
||||||
|
return s; |
||||||
|
} |
||||||
|
|
||||||
|
Cache::Handle* ClockCacheShard::Lookup(const Slice& key, uint32_t hash) { |
||||||
|
HashTable::const_accessor accessor; |
||||||
|
if (!table_.find(accessor, CacheKey(key, hash))) { |
||||||
|
return nullptr; |
||||||
|
} |
||||||
|
CacheHandle* handle = accessor->second; |
||||||
|
accessor.release(); |
||||||
|
// Ref() could fail if another thread sneak in and evict/erase the cache
|
||||||
|
// entry before we are able to hold reference.
|
||||||
|
if (!Ref(handle)) { |
||||||
|
return nullptr; |
||||||
|
} |
||||||
|
// Double check the key since the handle may now representing another key
|
||||||
|
// if other threads sneak in, evict/erase the entry and re-used the handle
|
||||||
|
// for another cache entry.
|
||||||
|
if (hash != handle->hash || key != handle->key) { |
||||||
|
CleanupContext context; |
||||||
|
Unref(handle, false, &context); |
||||||
|
// It is possible Unref() delete the entry, so we need to cleanup.
|
||||||
|
Cleanup(context); |
||||||
|
return nullptr; |
||||||
|
} |
||||||
|
return reinterpret_cast<Cache::Handle*>(handle); |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::Release(Cache::Handle* h) { |
||||||
|
CleanupContext context; |
||||||
|
CacheHandle* handle = reinterpret_cast<CacheHandle*>(h); |
||||||
|
Unref(handle, true, &context); |
||||||
|
Cleanup(context); |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::Erase(const Slice& key, uint32_t hash) { |
||||||
|
CleanupContext context; |
||||||
|
{ |
||||||
|
MutexLock l(&mutex_); |
||||||
|
HashTable::accessor accessor; |
||||||
|
if (table_.find(accessor, CacheKey(key, hash))) { |
||||||
|
CacheHandle* handle = accessor->second; |
||||||
|
table_.erase(accessor); |
||||||
|
UnsetInCache(handle, &context); |
||||||
|
} |
||||||
|
} |
||||||
|
Cleanup(context); |
||||||
|
} |
||||||
|
|
||||||
|
void ClockCacheShard::EraseUnRefEntries() { |
||||||
|
CleanupContext context; |
||||||
|
{ |
||||||
|
MutexLock l(&mutex_); |
||||||
|
table_.clear(); |
||||||
|
for (auto& handle : list_) { |
||||||
|
UnsetInCache(&handle, &context); |
||||||
|
} |
||||||
|
} |
||||||
|
Cleanup(context); |
||||||
|
} |
||||||
|
|
||||||
|
class ClockCache : public ShardedCache { |
||||||
|
public: |
||||||
|
ClockCache(size_t capacity, int num_shard_bits, bool strict_capacity_limit) |
||||||
|
: ShardedCache(capacity, num_shard_bits, strict_capacity_limit) { |
||||||
|
int num_shards = 1 << num_shard_bits; |
||||||
|
shards_ = new ClockCacheShard[num_shards]; |
||||||
|
SetCapacity(capacity); |
||||||
|
SetStrictCapacityLimit(strict_capacity_limit); |
||||||
|
} |
||||||
|
|
||||||
|
virtual ~ClockCache() { delete[] shards_; } |
||||||
|
|
||||||
|
virtual const char* Name() const override { return "ClockCache"; } |
||||||
|
|
||||||
|
virtual CacheShard* GetShard(int shard) override { |
||||||
|
return reinterpret_cast<CacheShard*>(&shards_[shard]); |
||||||
|
} |
||||||
|
|
||||||
|
virtual const CacheShard* GetShard(int shard) const override { |
||||||
|
return reinterpret_cast<CacheShard*>(&shards_[shard]); |
||||||
|
} |
||||||
|
|
||||||
|
virtual void* Value(Handle* handle) override { |
||||||
|
return reinterpret_cast<const CacheHandle*>(handle)->value; |
||||||
|
} |
||||||
|
|
||||||
|
virtual size_t GetCharge(Handle* handle) const override { |
||||||
|
return reinterpret_cast<const CacheHandle*>(handle)->charge; |
||||||
|
} |
||||||
|
|
||||||
|
virtual uint32_t GetHash(Handle* handle) const override { |
||||||
|
return reinterpret_cast<const CacheHandle*>(handle)->hash; |
||||||
|
} |
||||||
|
|
||||||
|
virtual void DisownData() override { shards_ = nullptr; } |
||||||
|
|
||||||
|
private: |
||||||
|
ClockCacheShard* shards_; |
||||||
|
}; |
||||||
|
|
||||||
|
} // end anonymous namespace
|
||||||
|
|
||||||
|
std::shared_ptr<Cache> NewClockCache(size_t capacity, int num_shard_bits, |
||||||
|
bool strict_capacity_limit) { |
||||||
|
return std::make_shared<ClockCache>(capacity, num_shard_bits, |
||||||
|
strict_capacity_limit); |
||||||
|
} |
||||||
|
|
||||||
|
} // namespace rocksdb
|
||||||
|
|
||||||
|
#endif // SUPPORT_CLOCK_CACHE
|
||||||
@ -0,0 +1,16 @@ |
|||||||
|
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
|
||||||
|
// This source code is licensed under the BSD-style license found in the
|
||||||
|
// LICENSE file in the root directory of this source tree. An additional grant
|
||||||
|
// of patent rights can be found in the PATENTS file in the same 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 "rocksdb/cache.h" |
||||||
|
|
||||||
|
#if defined(TBB) && !defined(ROCKSDB_LITE) |
||||||
|
#define SUPPORT_CLOCK_CACHE |
||||||
|
#endif |
||||||
Loading…
Reference in new issue