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Introduce ClockCache

Browse Source 
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/D61647
This commit is contained in:
Yi Wu 2016-08-19 12:28:19 -07:00
parent ff17a2abf3
commit 4cc37f59e5
11 changed files with 874 additions and 67 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
CMakeLists.txt
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@ -193,6 +193,7 @@ set(SOURCES
util/arena.cc
util/bloom.cc
util/build_version.cc
util/clock_cache.cc
util/coding.cc
util/compaction_job_stats_impl.cc
util/comparator.cc

2
HISTORY.md
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@ -1,5 +1,7 @@
# Rocksdb Change Log
## Unreleased
### New Features
* Introduce NewClockCache, which is based on CLOCK algorithm with better concurrent performance in some cases. It can be used to replace the default LRU-based block cache and table cache. To use it, RocksDB need to be linked with TBB lib.
## 4.11.0 (8/1/2016)
### Public API Change

15
include/rocksdb/cache.h
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@ -2,6 +2,7 @@
// 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.
@ -19,8 +20,7 @@
// they want something more sophisticated (like scan-resistance, a
// custom eviction policy, variable cache sizing, etc.)
#ifndef STORAGE_ROCKSDB_INCLUDE_CACHE_H_
#define STORAGE_ROCKSDB_INCLUDE_CACHE_H_
#pragma once
#include <stdint.h>
#include <memory>
@ -38,6 +38,15 @@ extern std::shared_ptr<Cache> NewLRUCache(size_t capacity,
int num_shard_bits = 6,
bool strict_capacity_limit = false);
// Similar to NewLRUCache, but create a cache based on CLOCK algorithm with
// better concurrent performance in some cases. See util/clock_cache.cc for
// more detail.
//
// Return nullptr if it is not supported.
extern std::shared_ptr<Cache> NewClockCache(size_t capacity,
int num_shard_bits = 6,
bool strict_capacity_limit = false);
class Cache {
public:
Cache() {}
@ -153,5 +162,3 @@ class Cache {
};
} // namespace rocksdb
#endif // STORAGE_ROCKSDB_UTIL_CACHE_H_

1
src.mk
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@ -88,6 +88,7 @@ LIB_SOURCES = \
util/arena.cc \
util/bloom.cc \
util/build_version.cc \
util/clock_cache.cc \
util/coding.cc \
util/comparator.cc \
util/compaction_job_stats_impl.cc \

48
tools/db_bench_tool.cc
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@ -349,13 +349,20 @@ DEFINE_int32(universal_compression_size_percent, -1,
DEFINE_bool(universal_allow_trivial_move, false,
"Allow trivial move in universal compaction.");
DEFINE_int64(cache_size, -1,
"Number of bytes to use as a cache of uncompressed"
" data. Negative means use default settings.");
DEFINE_int64(cache_size, 8 << 20, // 8MB
"Number of bytes to use as a cache of uncompressed data");
DEFINE_int32(cache_numshardbits, 6,
"Number of shards for the block cache"
" is 2 ** cache_numshardbits. Negative means use default settings."
" This is applied only if FLAGS_cache_size is non-negative.");
DEFINE_bool(use_clock_cache, false,
"Replace default LRU block cache with clock cache.");
DEFINE_int64(simcache_size, -1,
"Number of bytes to use as a simcache of "
"uncompressed data. Negative means use default settings.");
"uncompressed data. Nagative value disables simcache.");
DEFINE_bool(cache_index_and_filter_blocks, false,
"Cache index/filter blocks in block cache.");
@ -433,9 +440,6 @@ static bool ValidateCacheNumshardbits(const char* flagname, int32_t value) {
}
return true;
}
DEFINE_int32(cache_numshardbits, -1, "Number of shards for the block cache"
" is 2 ** cache_numshardbits. Negative means use default settings."
" This is applied only if FLAGS_cache_size is non-negative.");
DEFINE_bool(verify_checksum, false, "Verify checksum for every block read"
" from storage");
@ -1877,20 +1881,26 @@ class Benchmark {
std::shared_ptr<TimestampEmulator> timestamp_emulator_;
};
std::shared_ptr<Cache> NewCache(int64_t capacity) {
if (capacity <= 0) {
return nullptr;
}
if (FLAGS_use_clock_cache) {
auto cache = NewClockCache((size_t)capacity, FLAGS_cache_numshardbits);
if (!cache) {
fprintf(stderr, "Clock cache not supported.");
exit(1);
}
return cache;
} else {
return NewLRUCache((size_t)capacity, FLAGS_cache_numshardbits);
}
}
public:
Benchmark()
: cache_(
FLAGS_cache_size >= 0
? (FLAGS_cache_numshardbits >= 1
? NewLRUCache(FLAGS_cache_size, FLAGS_cache_numshardbits)
: NewLRUCache(FLAGS_cache_size))
: nullptr),
compressed_cache_(FLAGS_compressed_cache_size >= 0
? (FLAGS_cache_numshardbits >= 1
? NewLRUCache(FLAGS_compressed_cache_size,
FLAGS_cache_numshardbits)
: NewLRUCache(FLAGS_compressed_cache_size))
: nullptr),
: cache_(NewCache(FLAGS_cache_size)),
compressed_cache_(NewCache(FLAGS_compressed_cache_size)),
filter_policy_(FLAGS_bloom_bits >= 0
? NewBloomFilterPolicy(FLAGS_bloom_bits,
FLAGS_use_block_based_filter)

2
tools/db_crashtest.py
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@ -19,6 +19,7 @@ import argparse
default_params = {
"block_size": 16384,
"cache_size": 1048576,
"use_clock_cache": lambda: random.choice(["true", "false"]),
"delpercent": 5,
"destroy_db_initially": 0,
"disable_data_sync": 0,
@ -84,6 +85,7 @@ whitebox_default_params = {
simple_default_params = {
"block_size": 16384,
"cache_size": 1048576,
"use_clock_cache": lambda: random.choice(["true", "false"]),
"column_families": 1,
"delpercent": 5,
"destroy_db_initially": 0,

33
tools/db_stress.cc
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@ -228,6 +228,9 @@ DEFINE_int32(set_in_place_one_in, 0,
DEFINE_int64(cache_size, 2LL * KB * KB * KB,
"Number of bytes to use as a cache of uncompressed data.");
DEFINE_bool(use_clock_cache, false,
"Replace default LRU block cache with clock cache.");
DEFINE_uint64(subcompactions, 1,
"Maximum number of subcompactions to divide L0-L1 compactions "
"into.");
@ -993,15 +996,13 @@ class DbStressListener : public EventListener {
class StressTest {
public:
StressTest()
: cache_(NewLRUCache(FLAGS_cache_size)),
compressed_cache_(FLAGS_compressed_cache_size >= 0
? NewLRUCache(FLAGS_compressed_cache_size)
: nullptr),
: cache_(NewCache(FLAGS_cache_size)),
compressed_cache_(NewLRUCache(FLAGS_compressed_cache_size)),
filter_policy_(FLAGS_bloom_bits >= 0
? FLAGS_use_block_based_filter
? NewBloomFilterPolicy(FLAGS_bloom_bits, true)
: NewBloomFilterPolicy(FLAGS_bloom_bits, false)
: nullptr),
? FLAGS_use_block_based_filter
? NewBloomFilterPolicy(FLAGS_bloom_bits, true)
: NewBloomFilterPolicy(FLAGS_bloom_bits, false)
: nullptr),
db_(nullptr),
new_column_family_name_(1),
num_times_reopened_(0) {
@ -1025,6 +1026,22 @@ class StressTest {
delete db_;
}
std::shared_ptr<Cache> NewCache(size_t capacity) {
if (capacity <= 0) {
return nullptr;
}
if (FLAGS_use_clock_cache) {
auto cache = NewClockCache((size_t)capacity);
if (!cache) {
fprintf(stderr, "Clock cache not supported.");
exit(1);
}
return cache;
} else {
return NewLRUCache((size_t)capacity);
}
}
bool BuildOptionsTable() {
if (FLAGS_set_options_one_in <= 0) {
return true;

16
util/cache_bench.cc
View File

@ -46,6 +46,8 @@ DEFINE_int32(lookup_percent, 50,
DEFINE_int32(erase_percent, 10,
"Ratio of erase to total workload (expressed as a percentage)");
DEFINE_bool(use_clock_cache, false, "");
namespace rocksdb {
class CacheBench;
@ -129,9 +131,17 @@ struct ThreadState {
class CacheBench {
public:
CacheBench() :
cache_(NewLRUCache(FLAGS_cache_size, FLAGS_num_shard_bits)),
num_threads_(FLAGS_threads) {}
CacheBench() : num_threads_(FLAGS_threads) {
if (FLAGS_use_clock_cache) {
cache_ = NewClockCache(FLAGS_cache_size, FLAGS_num_shard_bits);
if (!cache_) {
fprintf(stderr, "Clock cache not supported.\n");
exit(1);
}
} else {
cache_ = NewLRUCache(FLAGS_cache_size, FLAGS_num_shard_bits);
}
}
~CacheBench() {}

107
util/cache_test.cc
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@ -10,9 +10,11 @@
#include "rocksdb/cache.h"
#include <forward_list>
#include <vector>
#include <string>
#include <functional>
#include <iostream>
#include <string>
#include <vector>
#include "util/clock_cache.h"
#include "util/coding.h"
#include "util/string_util.h"
#include "util/testharness.h"
@ -34,7 +36,16 @@ static int DecodeValue(void* v) {
return static_cast<int>(reinterpret_cast<uintptr_t>(v));
}
class CacheTest : public testing::Test {
typedef std::function<std::shared_ptr<Cache>(size_t, int, bool)> NewCache;
void dumbDeleter(const Slice& key, void* value) {}
void eraseDeleter(const Slice& key, void* value) {
Cache* cache = reinterpret_cast<Cache*>(value);
cache->Erase("foo");
}
class CacheTest : public testing::TestWithParam<NewCache> {
public:
static CacheTest* current_;
@ -54,15 +65,17 @@ class CacheTest : public testing::Test {
shared_ptr<Cache> cache_;
shared_ptr<Cache> cache2_;
CacheTest() :
cache_(NewLRUCache(kCacheSize, kNumShardBits)),
cache2_(NewLRUCache(kCacheSize2, kNumShardBits2)) {
CacheTest()
: cache_(GetNewCache()(kCacheSize, kNumShardBits, false)),
cache2_(GetNewCache()(kCacheSize2, kNumShardBits2, false)) {
current_ = this;
}
~CacheTest() {
}
NewCache GetNewCache() { return GetParam(); }
int Lookup(shared_ptr<Cache> cache, int key) {
Cache::Handle* handle = cache->Lookup(EncodeKey(key));
const int r = (handle == nullptr) ? -1 : DecodeValue(cache->Value(handle));
@ -108,14 +121,10 @@ class CacheTest : public testing::Test {
};
CacheTest* CacheTest::current_;
namespace {
void dumbDeleter(const Slice& key, void* value) { }
} // namespace
TEST_F(CacheTest, UsageTest) {
TEST_P(CacheTest, UsageTest) {
// cache is shared_ptr and will be automatically cleaned up.
const uint64_t kCapacity = 100000;
auto cache = NewLRUCache(kCapacity, 8);
auto cache = GetNewCache()(kCapacity, 8, false);
size_t usage = 0;
char value[10] = "abcdef";
@ -140,10 +149,10 @@ TEST_F(CacheTest, UsageTest) {
ASSERT_LT(kCapacity * 0.95, cache->GetUsage());
}
TEST_F(CacheTest, PinnedUsageTest) {
TEST_P(CacheTest, PinnedUsageTest) {
// cache is shared_ptr and will be automatically cleaned up.
const uint64_t kCapacity = 100000;
auto cache = NewLRUCache(kCapacity, 8);
auto cache = GetNewCache()(kCapacity, 8, false);
size_t pinned_usage = 0;
char value[10] = "abcdef";
@ -192,7 +201,7 @@ TEST_F(CacheTest, PinnedUsageTest) {
}
}
TEST_F(CacheTest, HitAndMiss) {
TEST_P(CacheTest, HitAndMiss) {
ASSERT_EQ(-1, Lookup(100));
Insert(100, 101);
@ -215,7 +224,13 @@ TEST_F(CacheTest, HitAndMiss) {
ASSERT_EQ(101, deleted_values_[0]);
}
TEST_F(CacheTest, Erase) {
TEST_P(CacheTest, InsertSameKey) {
Insert(1, 1);
Insert(1, 2);
ASSERT_EQ(2, Lookup(1));
}
TEST_P(CacheTest, Erase) {
Erase(200);
ASSERT_EQ(0U, deleted_keys_.size());
@ -234,7 +249,7 @@ TEST_F(CacheTest, Erase) {
ASSERT_EQ(1U, deleted_keys_.size());
}
TEST_F(CacheTest, EntriesArePinned) {
TEST_P(CacheTest, EntriesArePinned) {
Insert(100, 101);
Cache::Handle* h1 = cache_->Lookup(EncodeKey(100));
ASSERT_EQ(101, DecodeValue(cache_->Value(h1)));
@ -264,21 +279,20 @@ TEST_F(CacheTest, EntriesArePinned) {
ASSERT_EQ(0U, cache_->GetUsage());
}
TEST_F(CacheTest, EvictionPolicy) {
TEST_P(CacheTest, EvictionPolicy) {
Insert(100, 101);
Insert(200, 201);
// Frequently used entry must be kept around
for (int i = 0; i < kCacheSize + 100; i++) {
Insert(1000+i, 2000+i);
ASSERT_EQ(2000+i, Lookup(1000+i));
ASSERT_EQ(101, Lookup(100));
}
ASSERT_EQ(101, Lookup(100));
ASSERT_EQ(-1, Lookup(200));
}
TEST_F(CacheTest, EvictionPolicyRef) {
TEST_P(CacheTest, EvictionPolicyRef) {
Insert(100, 101);
Insert(101, 102);
Insert(102, 103);
@ -326,7 +340,24 @@ TEST_F(CacheTest, EvictionPolicyRef) {
cache_->Release(h204);
}
TEST_F(CacheTest, ErasedHandleState) {
TEST_P(CacheTest, EvictEmptyCache) {
// Insert item large than capacity to trigger eviction on empty cache.
auto cache = GetNewCache()(1, 0, false);
ASSERT_OK(cache->Insert("foo", nullptr, 10, dumbDeleter));
}
TEST_P(CacheTest, EraseFromDeleter) {
// Have deleter which will erase item from cache, which will re-enter
// the cache at that point.
std::shared_ptr<Cache> cache = GetNewCache()(10, 0, false);
ASSERT_OK(cache->Insert("foo", nullptr, 1, dumbDeleter));
ASSERT_OK(cache->Insert("bar", cache.get(), 1, eraseDeleter));
cache->Erase("bar");
ASSERT_EQ(nullptr, cache->Lookup("foo"));
ASSERT_EQ(nullptr, cache->Lookup("bar"));
}
TEST_P(CacheTest, ErasedHandleState) {
// insert a key and get two handles
Insert(100, 1000);
Cache::Handle* h1 = cache_->Lookup(EncodeKey(100));
@ -348,7 +379,7 @@ TEST_F(CacheTest, ErasedHandleState) {
cache_->Release(h2);
}
TEST_F(CacheTest, HeavyEntries) {
TEST_P(CacheTest, HeavyEntries) {
// Add a bunch of light and heavy entries and then count the combined
// size of items still in the cache, which must be approximately the
// same as the total capacity.
@ -375,7 +406,7 @@ TEST_F(CacheTest, HeavyEntries) {
ASSERT_LE(cached_weight, kCacheSize + kCacheSize/10);
}
TEST_F(CacheTest, NewId) {
TEST_P(CacheTest, NewId) {
uint64_t a = cache_->NewId();
uint64_t b = cache_->NewId();
ASSERT_NE(a, b);
@ -383,12 +414,10 @@ TEST_F(CacheTest, NewId) {
class Value {
private:
size_t v_;
public:
explicit Value(size_t v) : v_(v) { }
~Value() { std::cout << v_ << " is destructed\n"; }
size_t v_;
};
namespace {
@ -397,12 +426,12 @@ void deleter(const Slice& key, void* value) {
}
} // namespace
TEST_F(CacheTest, SetCapacity) {
TEST_P(CacheTest, SetCapacity) {
// test1: increase capacity
// lets create a cache with capacity 5,
// then, insert 5 elements, then increase capacity
// to 10, returned capacity should be 10, usage=5
std::shared_ptr<Cache> cache = NewLRUCache(5, 0);
std::shared_ptr<Cache> cache = GetNewCache()(5, 0, false);
std::vector<Cache::Handle*> handles(10);
// Insert 5 entries, but not releasing.
for (size_t i = 0; i < 5; i++) {
@ -442,7 +471,7 @@ TEST_F(CacheTest, SetCapacity) {
}
}
TEST_F(CacheTest, SetStrictCapacityLimit) {
TEST_P(CacheTest, SetStrictCapacityLimit) {
// test1: set the flag to false. Insert more keys than capacity. See if they
// all go through.
std::shared_ptr<Cache> cache = NewLRUCache(5, 0, false);
@ -489,11 +518,11 @@ TEST_F(CacheTest, SetStrictCapacityLimit) {
}
}
TEST_F(CacheTest, OverCapacity) {
TEST_P(CacheTest, OverCapacity) {
size_t n = 10;
// a LRUCache with n entries and one shard only
std::shared_ptr<Cache> cache = NewLRUCache(n, 0);
std::shared_ptr<Cache> cache = GetNewCache()(n, 0, false);
std::vector<Cache::Handle*> handles(n+1);
@ -508,7 +537,6 @@ TEST_F(CacheTest, OverCapacity) {
for (size_t i = 0; i < n + 1; i++) {
std::string key = ToString(i+1);
auto h = cache->Lookup(key);
std::cout << key << (h?" found\n":" not found\n");
ASSERT_TRUE(h != nullptr);
if (h) cache->Release(h);
}
@ -518,6 +546,8 @@ TEST_F(CacheTest, OverCapacity) {
for (size_t i = 0; i < n + 1; i++) {
cache->Release(handles[i]);
}
// Make sure eviction is triggered.
cache->SetCapacity(n);
// cache is under capacity now since elements were released
ASSERT_EQ(n, cache->GetUsage());
@ -544,7 +574,7 @@ void callback(void* entry, size_t charge) {
}
};
TEST_F(CacheTest, ApplyToAllCacheEntiresTest) {
TEST_P(CacheTest, ApplyToAllCacheEntiresTest) {
std::vector<std::pair<int, int>> inserted;
callback_state.clear();
@ -559,6 +589,17 @@ TEST_F(CacheTest, ApplyToAllCacheEntiresTest) {
ASSERT_TRUE(inserted == callback_state);
}
shared_ptr<Cache> (*newLRUCache)(size_t, int, bool) = NewLRUCache;
#ifdef SUPPORT_CLOCK_CACHE
shared_ptr<Cache> (*newClockCache)(size_t, int, bool) = NewClockCache;
INSTANTIATE_TEST_CASE_P(CacheTestInstance, CacheTest,
testing::Values(NewCache(newLRUCache),
NewCache(newClockCache)));
#else
INSTANTIATE_TEST_CASE_P(CacheTestInstance, CacheTest,
testing::Values(NewCache(newLRUCache)));
#endif // SUPPORT_CLOCK_CACHE
} // namespace rocksdb
int main(int argc, char** argv) {

700
util/clock_cache.cc Normal file
View File

@ -0,0 +1,700 @@
// 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.
#include "util/clock_cache.h"
#ifndef SUPPORT_CLOCK_CACHE
namespace rocksdb {
std::shared_ptr<Cache> NewClockCache(size_t capacity, int num_shard_bits,
bool strict_capacity_limit) {
// Clock cache not supported.
return nullptr;
}
} // namespace rocksdb
#else
#include <assert.h>
#include <atomic>
#include <deque>
#include <limits>
#include "tbb/concurrent_hash_map.h"
#include "port/port.h"
#include "util/autovector.h"
#include "util/mutexlock.h"
#include "util/sharded_cache.h"
namespace rocksdb {
namespace {
// An implementation of the Cache interface based on CLOCK algorithm, with
// better concurrent performance than LRUCache. The idea of CLOCK algorithm
// is to maintain all cache entries in a circular list, and an iterator
// (the "head") pointing to the last examined entry. Eviction starts from the
// current head. Each entry is given a second chance before eviction, if it
// has been access since last examine. In contrast to LRU, no modification
// to the internal data-structure (except for flipping the usage bit) needs
// to be done upon lookup. This gives us oppertunity to implement a cache
// with better concurrency.
//
// Each cache entry is represented by a cache handle, and all the handles
// are arranged in a circular list, as describe above. Upon erase of an entry,
// we never remove the handle. Instead, the handle is put into a recycle bin
// to be re-use. This is to avoid memory dealocation, which is hard to deal
// with in concurrent environment.
//
// The cache also maintains a concurrent hash map for lookup. Any concurrent
// hash map implementation should do the work. We currently use
// tbb::concurrent_hash_map because it supports concurrent erase.
//
// Each cache handle has the following flags and counters, which are squeeze
// in an atomic interger, to make sure the handle always be in a consistent
// state:
//
// * In-cache bit: whether the entry is reference by the cache itself. If
// an entry is in cache, its key would also be available in the hash map.
// * Usage bit: whether the entry has been access by user since last
// examine for eviction. Can be reset by eviction.
// * Reference count: reference count by user.
//
// An entry can be reference only when it's in cache. An entry can be evicted
// only when it is in cache, has no usage since last examine, and reference
// count is zero.
//
// The follow figure shows a possible layout of the cache. Boxes represents
// cache handles and numbers in each box being in-cache bit, usage bit and
// reference count respectively.
//
// hash map:
// +-------+--------+
// | key | handle |
// +-------+--------+
// | "foo" | 5 |-------------------------------------+
// +-------+--------+ |
// | "bar" | 2 |--+ |
// +-------+--------+ | |
// | |
// head | |
// | | |
// circular list: | | |
// +-------+ +-------+ +-------+ +-------+ +-------+ +-------
// |(0,0,0)|---|(1,1,0)|---|(0,0,0)|---|(0,1,3)|---|(1,0,0)|---| ...
// +-------+ +-------+ +-------+ +-------+ +-------+ +-------
// | |
// +-------+ +-----------+
// | |
// +---+---+
// recycle bin: | 1 | 3 |
// +---+---+
//
// Suppose we try to insert "baz" into the cache at this point and the cache is
// full. The cache will first look for entries to evict, starting from where
// head points to (the second entry). It resets usage bit of the second entry,
// skips the third and fourth entry since they are not in cache, and finally
// evict the fifth entry ("foo"). It looks at recycle bin for available handle,
// grabs handle 3, and insert the key into the handle. The following figure
// shows the resulting layout.
//
// hash map:
// +-------+--------+
// | key | handle |
// +-------+--------+
// | "baz" | 3 |-------------+
// +-------+--------+ |
// | "bar" | 2 |--+ |
// +-------+--------+ | |
// | |
// | | head
// | | |
// circular list: | | |
// +-------+ +-------+ +-------+ +-------+ +-------+ +-------
// |(0,0,0)|---|(1,0,0)|---|(1,0,0)|---|(0,1,3)|---|(0,0,0)|---| ...
// +-------+ +-------+ +-------+ +-------+ +-------+ +-------
// | |
// +-------+ +-----------------------------------+
// | |
// +---+---+
// recycle bin: | 1 | 5 |
// +---+---+
//
// A global mutex guards the circular list, the head, and the recycle bin.
// We additionally require that modifying the hash map needs to hold the mutex.
// As such, Modifying the cache (such as Insert() and Erase()) require to
// hold the mutex. Lookup() only access the hash map and the flags associated
// with each handle, and don't require explicit locking. Release() has to
// acquire the mutex only when it releases the last reference to the entry and
// the entry has been erased from cache explicitly. A future improvement could
// be to remove the mutex completely.
//
// Benchmark:
// We run readrandom db_bench on a test DB of size 13GB, with size of each
// level:
//
// Level Files Size(MB)
// -------------------------
// L0 1 0.01
// L1 18 17.32
// L2 230 182.94
// L3 1186 1833.63
// L4 4602 8140.30
//
// We test with both 32 and 16 read threads, with 2GB cache size (the whole DB
// doesn't fits in) and 64GB cache size (the whole DB can fit in cache), and
// whether to put index and filter blocks in block cache. The benchmark runs
// with
// with RocksDB 4.10. We got the following result:
//
// Threads Cache Cache ClockCache LRUCache
// Size Index/Filter Throughput(MB/s) Hit Throughput(MB/s) Hit
// 32 2GB yes 466.7 85.9% 433.7 86.5%
// 32 2GB no 529.9 72.7% 532.7 73.9%
// 32 64GB yes 649.9 99.9% 507.9 99.9%
// 32 64GB no 740.4 99.9% 662.8 99.9%
// 16 2GB yes 278.4 85.9% 283.4 86.5%
// 16 2GB no 318.6 72.7% 335.8 73.9%
// 16 64GB yes 391.9 99.9% 353.3 99.9%
// 16 64GB no 433.8 99.8% 419.4 99.8%
// Cache entry meta data.
struct CacheHandle {
Slice key;
uint32_t hash;
void* value;
size_t charge;
void (*deleter)(const Slice&, void* value);
// Flags and counters associated with the cache handle:
// lowest bit: n-cache bit
// second lowest bit: usage bit
// the rest bits: reference count
// The handle is unused when flags equals to 0. The thread decreases the count
// to 0 is responsible to put the handle back to recycle_ and cleanup memory.
std::atomic<uint32_t> flags;
CacheHandle() = default;
CacheHandle(const CacheHandle& a) { *this = a; }
CacheHandle& operator=(const CacheHandle& a) {
// Only copy members needed for deletion.
key = a.key;
value = a.value;
deleter = a.deleter;
return *this;
}
};
// Key of hash map. We store hash value with the key for convenience.
struct CacheKey {
Slice key;
uint32_t hash_value;
CacheKey() = default;
CacheKey(const Slice& k, uint32_t h) {
key = k;
hash_value = h;
}
static bool equal(const CacheKey& a, const CacheKey& b) {
return a.hash_value == b.hash_value && a.key == b.key;
}
static size_t hash(const CacheKey& a) {
return static_cast<size_t>(a.hash_value);
}
};
struct CleanupContext {
// List of values to be deleted, along with the key and deleter.
autovector<CacheHandle> to_delete_value;
// List of keys to be deleted.
autovector<const char*> to_delete_key;
};
// A cache shard which maintains its own CLOCK cache.
class ClockCacheShard : public CacheShard {
public:
// Hash map type.
typedef tbb::concurrent_hash_map<CacheKey, CacheHandle*, CacheKey> HashTable;
ClockCacheShard();
~ClockCacheShard() = default;
// Interfaces
virtual void SetCapacity(size_t capacity) override;
virtual void SetStrictCapacityLimit(bool strict_capacity_limit) override;
virtual Status Insert(const Slice& key, uint32_t hash, void* value,
size_t charge,
void (*deleter)(const Slice& key, void* value),
Cache::Handle** handle) override;
virtual Cache::Handle* Lookup(const Slice& key, uint32_t hash) override;
virtual void Release(Cache::Handle* handle) override;
virtual void Erase(const Slice& key, uint32_t hash) override;
virtual size_t GetUsage() const override;
virtual size_t GetPinnedUsage() const override;
virtual void EraseUnRefEntries() override;
virtual void ApplyToAllCacheEntries(void (*callback)(void*, size_t),
bool thread_safe) override;
private:
static const uint32_t kInCacheBit = 1;
static const uint32_t kUsageBit = 2;
static const uint32_t kRefsOffset = 2;
static const uint32_t kOneRef = 1 << kRefsOffset;
// Helper functions to extract cache handle flags and counters.
static bool InCache(uint32_t flags) { return flags & kInCacheBit; }
static bool HasUsage(uint32_t flags) { return flags & kUsageBit; }
static uint32_t CountRefs(uint32_t flags) { return flags >> kRefsOffset; }
// If the entry in in cache, increase reference count and return true.
// Return false otherwise.
//
// Not necessary to hold mutex_ before being called.
bool Ref(CacheHandle* handle);
// Decrease reference count of the entry. If this decreases the count to 0,
// recycle the entry. If set_usage is true, also set the usage bit.
//
// Not necessary to hold mutex_ before being called.
void Unref(CacheHandle* handle, bool set_usage, CleanupContext* context);
// Unset in-cache bit of the entry. Recycle the handle if necessary.
//
// Has to hold mutex_ before being called.
void UnsetInCache(CacheHandle* handle, CleanupContext* context);
// Put the handle back to recycle_ list, and put the value associated with
// it into to-be-deleted list. It doesn't cleanup the key as it might be
// reused by another handle.
//
// Has to hold mutex_ before being called.
void RecycleHandle(CacheHandle* handle, CleanupContext* context);
// Remove the key from hash map. Put the key associated with the entry into
// to be deleted list.
//
// Has to hold mutex_ before being called.
void EraseKey(CacheHandle* handle, CleanupContext* context);
// Delete keys and values in to-be-deleted list. Call the method without
// holding mutex, as destructors can be expensive.
void Cleanup(const CleanupContext& context);
// Examine the handle for eviction. If the handle is in cache, usage bit is
// not set, and referece count is 0, evict it from cache. Otherwise unset
// the usage bit.
//
// Has to hold mutex_ before being called.
bool TryEvict(CacheHandle* value, CleanupContext* context);
// Scan through the circular list, evict entries until we get enough capacity
// for new cache entry of specific size. Return true if success, false
// otherwise.
//
// Has to hold mutex_ before being called.
bool EvictFromCache(size_t charge, CleanupContext* context);
CacheHandle* Insert(const Slice& key, uint32_t hash, void* value,
size_t change,
void (*deleter)(const Slice& key, void* value),
bool hold_reference, CleanupContext* context);
// Guards list_, head_, and recycle_. In addition, updating table_ also has
// to hold the mutex, to avoid the cache being in inconsistent state.
mutable port::Mutex mutex_;
// The circular list of cache handles. Initially the list is empty. Once a
// handle is needed by insertion, and no more handles are available in
// recycle bin, one more handle is appended to the end.
//
// We use std::deque for the circular list because we want to make sure
// pointers to handles are valid through out the life-cycle of the cache
// (in contrast to std::vector), and be able to grow the list (in contrast
// to statically allocated arrays).
std::deque<CacheHandle> list_;
// Pointer to the next handle in the circular list to be examine for
// eviction.
size_t head_;
// Recycle bin of cache handles.
autovector<CacheHandle*> recycle_;
// Maximum cache size.
std::atomic<size_t> capacity_;
// Current total size of the cache.
std::atomic<size_t> usage_;
// Total un-released cache size.
std::atomic<size_t> pinned_usage_;
// Whether allow insert into cache if cache is full.
std::atomic<bool> strict_capacity_limit_;
// Hash table (tbb::concurrent_hash_map) for lookup.
HashTable table_;
};
ClockCacheShard::ClockCacheShard()
: head_(0), usage_(0), pinned_usage_(0), strict_capacity_limit_(false) {}
size_t ClockCacheShard::GetUsage() const {
return usage_.load(std::memory_order_relaxed);
}
size_t ClockCacheShard::GetPinnedUsage() const {
return pinned_usage_.load(std::memory_order_relaxed);
}
void ClockCacheShard::ApplyToAllCacheEntries(void (*callback)(void*, size_t),
bool thread_safe) {
if (thread_safe) {
mutex_.Lock();
}
for (auto& handle : list_) {
// Use relaxed semantics instead of acquire semantics since we are either
// holding mutex, or don't have thread safe requirement.
uint32_t flags = handle.flags.load(std::memory_order_relaxed);
if (InCache(flags)) {
callback(handle.value, handle.charge);
}
}
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

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// 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