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rocksdb/memtable/skiplistrep.cc

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// 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).
//
#include <random>
#include "db/memtable.h"
#include "memory/arena.h"
#include "memtable/inlineskiplist.h"
#include "rocksdb/memtablerep.h"
namespace ROCKSDB_NAMESPACE {
namespace {
class SkipListRep : public MemTableRep {
InlineSkipList<const MemTableRep::KeyComparator&> skip_list_;
const MemTableRep::KeyComparator& cmp_;
const SliceTransform* transform_;
const size_t lookahead_;
friend class LookaheadIterator;
public:
explicit SkipListRep(const MemTableRep::KeyComparator& compare,
Allocator* allocator, const SliceTransform* transform,
const size_t lookahead)
: MemTableRep(allocator),
skip_list_(compare, allocator),
cmp_(compare),
transform_(transform),
lookahead_(lookahead) {}
KeyHandle Allocate(const size_t len, char** buf) override {
*buf = skip_list_.AllocateKey(len);
return static_cast<KeyHandle>(*buf);
}
// Insert key into the list.
// REQUIRES: nothing that compares equal to key is currently in the list.
void Insert(KeyHandle handle) override {
skip_list_.Insert(static_cast<char*>(handle));
}
bool InsertKey(KeyHandle handle) override {
return skip_list_.Insert(static_cast<char*>(handle));
}
void InsertWithHint(KeyHandle handle, void** hint) override {
skip_list_.InsertWithHint(static_cast<char*>(handle), hint);
}
bool InsertKeyWithHint(KeyHandle handle, void** hint) override {
return skip_list_.InsertWithHint(static_cast<char*>(handle), hint);
}
void InsertWithHintConcurrently(KeyHandle handle, void** hint) override {
skip_list_.InsertWithHintConcurrently(static_cast<char*>(handle), hint);
}
bool InsertKeyWithHintConcurrently(KeyHandle handle, void** hint) override {
return skip_list_.InsertWithHintConcurrently(static_cast<char*>(handle),
hint);
}
void InsertConcurrently(KeyHandle handle) override {
skip_list_.InsertConcurrently(static_cast<char*>(handle));
}
bool InsertKeyConcurrently(KeyHandle handle) override {
return skip_list_.InsertConcurrently(static_cast<char*>(handle));
}
// Returns true iff an entry that compares equal to key is in the list.
bool Contains(const char* key) const override {
return skip_list_.Contains(key);
}
size_t ApproximateMemoryUsage() override {
// All memory is allocated through allocator; nothing to report here
return 0;
}
void Get(const LookupKey& k, void* callback_args,
bool (*callback_func)(void* arg, const char* entry)) override {
SkipListRep::Iterator iter(&skip_list_);
Slice dummy_slice;
for (iter.Seek(dummy_slice, k.memtable_key().data());
iter.Valid() && callback_func(callback_args, iter.key()); iter.Next()) {
}
}
uint64_t ApproximateNumEntries(const Slice& start_ikey,
const Slice& end_ikey) override {
std::string tmp;
uint64_t start_count =
skip_list_.EstimateCount(EncodeKey(&tmp, start_ikey));
uint64_t end_count = skip_list_.EstimateCount(EncodeKey(&tmp, end_ikey));
return (end_count >= start_count) ? (end_count - start_count) : 0;
}
void UniqueRandomSample(const uint64_t& num_entries,
const uint64_t& target_sample_size,
std::unordered_set<const char*>* entries) override {
entries->clear();
// Avoid divide-by-0.
assert(target_sample_size > 0);
assert(num_entries > 0);
// NOTE: the size of entries is not enforced to be exactly
// target_sample_size at the end of this function, it might be slightly
// greater or smaller.
SkipListRep::Iterator iter(&skip_list_);
// There are two methods to create the subset of samples (size m)
// from the table containing N elements:
// 1-Iterate linearly through the N memtable entries. For each entry i,
// add it to the sample set with a probability
// (target_sample_size - entries.size() ) / (N-i).
//
// 2-Pick m random elements without repetition.
// We pick Option 2 when m<sqrt(N) and
// Option 1 when m > sqrt(N).
if (target_sample_size >
static_cast<uint64_t>(std::sqrt(1.0 * num_entries))) {
Random* rnd = Random::GetTLSInstance();
iter.SeekToFirst();
uint64_t counter = 0, num_samples_left = target_sample_size;
for (; iter.Valid() && (num_samples_left > 0); iter.Next(), counter++) {
// Add entry to sample set with probability
// num_samples_left/(num_entries - counter).
if (rnd->Next() % (num_entries - counter) < num_samples_left) {
entries->insert(iter.key());
num_samples_left--;
}
}
} else {
// Option 2: pick m random elements with no duplicates.
// If Option 2 is picked, then target_sample_size<sqrt(N)
// Using a set spares the need to check for duplicates.
for (uint64_t i = 0; i < target_sample_size; i++) {
// We give it 5 attempts to find a non-duplicate
// With 5 attempts, the chances of returning `entries` set
// of size target_sample_size is:
// PROD_{i=1}^{target_sample_size-1} [1-(i/N)^5]
// which is monotonically increasing with N in the worse case
// of target_sample_size=sqrt(N), and is always >99.9% for N>4.
// At worst, for the final pick , when m=sqrt(N) there is
// a probability of p= 1/sqrt(N) chances to find a duplicate.
for (uint64_t j = 0; j < 5; j++) {
iter.RandomSeek();
// unordered_set::insert returns pair<iterator, bool>.
// The second element is true if an insert successfully happened.
// If element is already in the set, this bool will be false, and
// true otherwise.
if ((entries->insert(iter.key())).second) {
break;
}
}
}
}
}
~SkipListRep() override {}
// Iteration over the contents of a skip list
class Iterator : public MemTableRep::Iterator {
InlineSkipList<const MemTableRep::KeyComparator&>::Iterator iter_;
public:
// Initialize an iterator over the specified list.
// The returned iterator is not valid.
explicit Iterator(
const InlineSkipList<const MemTableRep::KeyComparator&>* list)
: iter_(list) {}
~Iterator() override {}
// Returns true iff the iterator is positioned at a valid node.
bool Valid() const override { return iter_.Valid(); }
// Returns the key at the current position.
// REQUIRES: Valid()
const char* key() const override { return iter_.key(); }
// Advances to the next position.
// REQUIRES: Valid()
void Next() override { iter_.Next(); }
// Advances to the previous position.
// REQUIRES: Valid()
void Prev() override { iter_.Prev(); }
// Advance to the first entry with a key >= target
void Seek(const Slice& user_key, const char* memtable_key) override {
if (memtable_key != nullptr) {
iter_.Seek(memtable_key);
} else {
iter_.Seek(EncodeKey(&tmp_, user_key));
}
}
// Retreat to the last entry with a key <= target
void SeekForPrev(const Slice& user_key, const char* memtable_key) override {
if (memtable_key != nullptr) {
iter_.SeekForPrev(memtable_key);
} else {
iter_.SeekForPrev(EncodeKey(&tmp_, user_key));
}
}
void RandomSeek() override { iter_.RandomSeek(); }
// Position at the first entry in list.
// Final state of iterator is Valid() iff list is not empty.
void SeekToFirst() override { iter_.SeekToFirst(); }
// Position at the last entry in list.
// Final state of iterator is Valid() iff list is not empty.
void SeekToLast() override { iter_.SeekToLast(); }
protected:
std::string tmp_; // For passing to EncodeKey
};
// Iterator over the contents of a skip list which also keeps track of the
// previously visited node. In Seek(), it examines a few nodes after it
// first, falling back to O(log n) search from the head of the list only if
// the target key hasn't been found.
class LookaheadIterator : public MemTableRep::Iterator {
public:
explicit LookaheadIterator(const SkipListRep& rep) :
rep_(rep), iter_(&rep_.skip_list_), prev_(iter_) {}
~LookaheadIterator() override {}
bool Valid() const override { return iter_.Valid(); }
const char* key() const override {
assert(Valid());
return iter_.key();
}
void Next() override {
assert(Valid());
bool advance_prev = true;
if (prev_.Valid()) {
auto k1 = rep_.UserKey(prev_.key());
auto k2 = rep_.UserKey(iter_.key());
if (k1.compare(k2) == 0) {
// same user key, don't move prev_
advance_prev = false;
} else if (rep_.transform_) {
// only advance prev_ if it has the same prefix as iter_
auto t1 = rep_.transform_->Transform(k1);
auto t2 = rep_.transform_->Transform(k2);
advance_prev = t1.compare(t2) == 0;
}
}
if (advance_prev) {
prev_ = iter_;
}
iter_.Next();
}
void Prev() override {
assert(Valid());
iter_.Prev();
prev_ = iter_;
}
void Seek(const Slice& internal_key, const char* memtable_key) override {
const char *encoded_key =
(memtable_key != nullptr) ?
memtable_key : EncodeKey(&tmp_, internal_key);
if (prev_.Valid() && rep_.cmp_(encoded_key, prev_.key()) >= 0) {
// prev_.key() is smaller or equal to our target key; do a quick
// linear search (at most lookahead_ steps) starting from prev_
iter_ = prev_;
size_t cur = 0;
while (cur++ <= rep_.lookahead_ && iter_.Valid()) {
if (rep_.cmp_(encoded_key, iter_.key()) <= 0) {
return;
}
Next();
}
}
iter_.Seek(encoded_key);
prev_ = iter_;
}
void SeekForPrev(const Slice& internal_key,
const char* memtable_key) override {
const char* encoded_key = (memtable_key != nullptr)
? memtable_key
: EncodeKey(&tmp_, internal_key);
iter_.SeekForPrev(encoded_key);
prev_ = iter_;
}
void SeekToFirst() override {
iter_.SeekToFirst();
prev_ = iter_;
}
void SeekToLast() override {
iter_.SeekToLast();
prev_ = iter_;
}
protected:
std::string tmp_; // For passing to EncodeKey
private:
const SkipListRep& rep_;
InlineSkipList<const MemTableRep::KeyComparator&>::Iterator iter_;
InlineSkipList<const MemTableRep::KeyComparator&>::Iterator prev_;
};
MemTableRep::Iterator* GetIterator(Arena* arena = nullptr) override {
if (lookahead_ > 0) {
void *mem =
arena ? arena->AllocateAligned(sizeof(SkipListRep::LookaheadIterator))
: operator new(sizeof(SkipListRep::LookaheadIterator));
return new (mem) SkipListRep::LookaheadIterator(*this);
} else {
void *mem =
arena ? arena->AllocateAligned(sizeof(SkipListRep::Iterator))
: operator new(sizeof(SkipListRep::Iterator));
return new (mem) SkipListRep::Iterator(&skip_list_);
}
}
};
}
MemTableRep* SkipListFactory::CreateMemTableRep(
const MemTableRep::KeyComparator& compare, Allocator* allocator,
const SliceTransform* transform, Logger* /*logger*/) {
return new SkipListRep(compare, allocator, transform, lookahead_);
}
} // namespace ROCKSDB_NAMESPACE