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rocksdb/memtable/skiplistrep.cc

345 lines
12 KiB

// 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).
//
Memtable sampling for mempurge heuristic. (#8628) Summary: Changes the API of the MemPurge process: the `bool experimental_allow_mempurge` and `experimental_mempurge_policy` flags have been replaced by a `double experimental_mempurge_threshold` option. This change of API reflects another major change introduced in this PR: the MemPurgeDecider() function now works by sampling the memtables being flushed to estimate the overall amount of useful payload (payload minus the garbage), and then compare this useful payload estimate with the `double experimental_mempurge_threshold` value. Therefore, when the value of this flag is `0.0` (default value), mempurge is simply deactivated. On the other hand, a value of `DBL_MAX` would be equivalent to always going through a mempurge regardless of the garbage ratio estimate. At the moment, a `double experimental_mempurge_threshold` value else than 0.0 or `DBL_MAX` is opnly supported`with the `SkipList` memtable representation. Regarding the sampling, this PR includes the introduction of a `MemTable::UniqueRandomSample` function that collects (approximately) random entries from the memtable by using the new `SkipList::Iterator::RandomSeek()` under the hood, or by iterating through each memtable entry, depending on the target sample size and the total number of entries. The unit tests have been readapted to support this new API. Pull Request resolved: https://github.com/facebook/rocksdb/pull/8628 Reviewed By: pdillinger Differential Revision: D30149315 Pulled By: bjlemaire fbshipit-source-id: 1feef5390c95db6f4480ab4434716533d3947f27
3 years ago
#include <random>
#include "db/memtable.h"
#include "memory/arena.h"
#include "memtable/inlineskiplist.h"
9 years ago
#include "rocksdb/memtablerep.h"
namespace ROCKSDB_NAMESPACE {
namespace {
class SkipListRep : public MemTableRep {
9 years ago
InlineSkipList<const MemTableRep::KeyComparator&> skip_list_;
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
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);
}
9 years ago
// 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));
}
support for concurrent adds to memtable Summary: This diff adds support for concurrent adds to the skiplist memtable implementations. Memory allocation is made thread-safe by the addition of a spinlock, with small per-core buffers to avoid contention. Concurrent memtable writes are made via an additional method and don't impose a performance overhead on the non-concurrent case, so parallelism can be selected on a per-batch basis. Write thread synchronization is an increasing bottleneck for higher levels of concurrency, so this diff adds --enable_write_thread_adaptive_yield (default off). This feature causes threads joining a write batch group to spin for a short time (default 100 usec) using sched_yield, rather than going to sleep on a mutex. If the timing of the yield calls indicates that another thread has actually run during the yield then spinning is avoided. This option improves performance for concurrent situations even without parallel adds, although it has the potential to increase CPU usage (and the heuristic adaptation is not yet mature). Parallel writes are not currently compatible with inplace updates, update callbacks, or delete filtering. Enable it with --allow_concurrent_memtable_write (and --enable_write_thread_adaptive_yield). Parallel memtable writes are performance neutral when there is no actual parallelism, and in my experiments (SSD server-class Linux and varying contention and key sizes for fillrandom) they are always a performance win when there is more than one thread. Statistics are updated earlier in the write path, dropping the number of DB mutex acquisitions from 2 to 1 for almost all cases. This diff was motivated and inspired by Yahoo's cLSM work. It is more conservative than cLSM: RocksDB's write batch group leader role is preserved (along with all of the existing flush and write throttling logic) and concurrent writers are blocked until all memtable insertions have completed and the sequence number has been advanced, to preserve linearizability. My test config is "db_bench -benchmarks=fillrandom -threads=$T -batch_size=1 -memtablerep=skip_list -value_size=100 --num=1000000/$T -level0_slowdown_writes_trigger=9999 -level0_stop_writes_trigger=9999 -disable_auto_compactions --max_write_buffer_number=8 -max_background_flushes=8 --disable_wal --write_buffer_size=160000000 --block_size=16384 --allow_concurrent_memtable_write" on a two-socket Xeon E5-2660 @ 2.2Ghz with lots of memory and an SSD hard drive. With 1 thread I get ~440Kops/sec. Peak performance for 1 socket (numactl -N1) is slightly more than 1Mops/sec, at 16 threads. Peak performance across both sockets happens at 30 threads, and is ~900Kops/sec, although with fewer threads there is less performance loss when the system has background work. Test Plan: 1. concurrent stress tests for InlineSkipList and DynamicBloom 2. make clean; make check 3. make clean; DISABLE_JEMALLOC=1 make valgrind_check; valgrind db_bench 4. make clean; COMPILE_WITH_TSAN=1 make all check; db_bench 5. make clean; COMPILE_WITH_ASAN=1 make all check; db_bench 6. make clean; OPT=-DROCKSDB_LITE make check 7. verify no perf regressions when disabled Reviewers: igor, sdong Reviewed By: sdong Subscribers: MarkCallaghan, IslamAbdelRahman, anthony, yhchiang, rven, sdong, guyg8, kradhakrishnan, dhruba Differential Revision: https://reviews.facebook.net/D50589
9 years ago
// 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,
Memtable sampling for mempurge heuristic. (#8628) Summary: Changes the API of the MemPurge process: the `bool experimental_allow_mempurge` and `experimental_mempurge_policy` flags have been replaced by a `double experimental_mempurge_threshold` option. This change of API reflects another major change introduced in this PR: the MemPurgeDecider() function now works by sampling the memtables being flushed to estimate the overall amount of useful payload (payload minus the garbage), and then compare this useful payload estimate with the `double experimental_mempurge_threshold` value. Therefore, when the value of this flag is `0.0` (default value), mempurge is simply deactivated. On the other hand, a value of `DBL_MAX` would be equivalent to always going through a mempurge regardless of the garbage ratio estimate. At the moment, a `double experimental_mempurge_threshold` value else than 0.0 or `DBL_MAX` is opnly supported`with the `SkipList` memtable representation. Regarding the sampling, this PR includes the introduction of a `MemTable::UniqueRandomSample` function that collects (approximately) random entries from the memtable by using the new `SkipList::Iterator::RandomSeek()` under the hood, or by iterating through each memtable entry, depending on the target sample size and the total number of entries. The unit tests have been readapted to support this new API. Pull Request resolved: https://github.com/facebook/rocksdb/pull/8628 Reviewed By: pdillinger Differential Revision: D30149315 Pulled By: bjlemaire fbshipit-source-id: 1feef5390c95db6f4480ab4434716533d3947f27
3 years ago
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 {
9 years ago
InlineSkipList<const MemTableRep::KeyComparator&>::Iterator iter_;
public:
// Initialize an iterator over the specified list.
// The returned iterator is not valid.
explicit Iterator(
9 years ago
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));
}
}
Memtable sampling for mempurge heuristic. (#8628) Summary: Changes the API of the MemPurge process: the `bool experimental_allow_mempurge` and `experimental_mempurge_policy` flags have been replaced by a `double experimental_mempurge_threshold` option. This change of API reflects another major change introduced in this PR: the MemPurgeDecider() function now works by sampling the memtables being flushed to estimate the overall amount of useful payload (payload minus the garbage), and then compare this useful payload estimate with the `double experimental_mempurge_threshold` value. Therefore, when the value of this flag is `0.0` (default value), mempurge is simply deactivated. On the other hand, a value of `DBL_MAX` would be equivalent to always going through a mempurge regardless of the garbage ratio estimate. At the moment, a `double experimental_mempurge_threshold` value else than 0.0 or `DBL_MAX` is opnly supported`with the `SkipList` memtable representation. Regarding the sampling, this PR includes the introduction of a `MemTable::UniqueRandomSample` function that collects (approximately) random entries from the memtable by using the new `SkipList::Iterator::RandomSeek()` under the hood, or by iterating through each memtable entry, depending on the target sample size and the total number of entries. The unit tests have been readapted to support this new API. Pull Request resolved: https://github.com/facebook/rocksdb/pull/8628 Reviewed By: pdillinger Differential Revision: D30149315 Pulled By: bjlemaire fbshipit-source-id: 1feef5390c95db6f4480ab4434716533d3947f27
3 years ago
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
};
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
// 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 {}
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
bool Valid() const override { return iter_.Valid(); }
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
const char* key() const override {
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
assert(Valid());
return iter_.key();
}
void Next() override {
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
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 {
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
assert(Valid());
iter_.Prev();
prev_ = iter_;
}
void Seek(const Slice& internal_key, const char* memtable_key) override {
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
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 {
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
iter_.SeekToFirst();
prev_ = iter_;
}
void SeekToLast() override {
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
iter_.SeekToLast();
prev_ = iter_;
}
protected:
std::string tmp_; // For passing to EncodeKey
private:
const SkipListRep& rep_;
9 years ago
InlineSkipList<const MemTableRep::KeyComparator&>::Iterator iter_;
InlineSkipList<const MemTableRep::KeyComparator&>::Iterator prev_;
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
};
MemTableRep::Iterator* GetIterator(Arena* arena = nullptr) override {
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
if (lookahead_ > 0) {
void *mem =
arena ? arena->AllocateAligned(sizeof(SkipListRep::LookaheadIterator))
: operator new(sizeof(SkipListRep::LookaheadIterator));
return new (mem) SkipListRep::LookaheadIterator(*this);
} else {
SkipListRep::LookaheadIterator Summary: This diff introduces the `lookahead` argument to `SkipListFactory()`. This is an optimization for the tailing use case which includes many seeks. E.g. consider the following operations on a skip list iterator: Seek(x), Next(), Next(), Seek(x+2), Next(), Seek(x+3), Next(), Next(), ... If `lookahead` is positive, `SkipListRep` will return an iterator which also keeps track of the previously visited node. Seek() then first does a linear search starting from that node (up to `lookahead` steps). As in the tailing example above, this may require fewer than ~log(n) comparisons as with regular skip list search. Test Plan: Added a new benchmark (`fillseekseq`) which simulates the usage pattern. It first writes N records (with consecutive keys), then measures how much time it takes to read them by calling `Seek()` and `Next()`. $ time ./db_bench -num 10000000 -benchmarks fillseekseq -prefix_size 1 \ -key_size 8 -write_buffer_size $[1024*1024*1024] -value_size 50 \ -seekseq_next 2 -skip_list_lookahead=0 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.389 micros/op 2569047 ops/sec; real 0m21.806s user 0m12.106s sys 0m9.672s $ time ./db_bench [...] -skip_list_lookahead=2 [...] DB path: [/dev/shm/rocksdbtest/dbbench] fillseekseq : 0.153 micros/op 6540684 ops/sec; real 0m19.469s user 0m10.192s sys 0m9.252s Reviewers: ljin, sdong, igor Reviewed By: igor Subscribers: dhruba, leveldb, march, lovro Differential Revision: https://reviews.facebook.net/D23997
10 years ago
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