rocksdb/table/iterator_wrapper.h
Akanksha Mahajan 17ce1ca48b Reuse internal auto readhead_size at each Level (expect L0) for Iterations (#9056)
Summary:
RocksDB does auto-readahead for iterators on noticing more than two sequential reads for a table file if user doesn't provide readahead_size. The readahead starts at 8KB and doubles on every additional read up to max_auto_readahead_size. However at each level, if iterator moves over next file, readahead_size starts again from 8KB.

This PR introduces a new ReadOption "adaptive_readahead" which when set true will maintain readahead_size  at each level. So when iterator moves from one file to another, new file's readahead_size will continue from previous file's readahead_size instead of scratch. However if reads are not sequential it will fall back to 8KB (default) with no prefetching for that block.

1. If block is found in cache but it was eligible for prefetch (block wasn't in Rocksdb's prefetch buffer),  readahead_size will decrease by 8KB.
2. It maintains readahead_size for L1 - Ln levels.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9056

Test Plan:
Added new unit tests
Ran db_bench for "readseq, seekrandom, seekrandomwhilewriting, readrandom" with --adaptive_readahead=true and there was no regression if new feature is enabled.

Reviewed By: anand1976

Differential Revision: D31773640

Pulled By: akankshamahajan15

fbshipit-source-id: 7332d16258b846ae5cea773009195a5af58f8f98
2021-11-10 16:20:04 -08:00

189 lines
4.5 KiB
C++

// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#pragma once
#include <set>
#include "table/internal_iterator.h"
#include "test_util/sync_point.h"
namespace ROCKSDB_NAMESPACE {
// A internal wrapper class with an interface similar to Iterator that caches
// the valid() and key() results for an underlying iterator.
// This can help avoid virtual function calls and also gives better
// cache locality.
template <class TValue = Slice>
class IteratorWrapperBase {
public:
IteratorWrapperBase() : iter_(nullptr), valid_(false) {}
explicit IteratorWrapperBase(InternalIteratorBase<TValue>* _iter)
: iter_(nullptr) {
Set(_iter);
}
~IteratorWrapperBase() {}
InternalIteratorBase<TValue>* iter() const { return iter_; }
// Set the underlying Iterator to _iter and return
// previous underlying Iterator.
InternalIteratorBase<TValue>* Set(InternalIteratorBase<TValue>* _iter) {
InternalIteratorBase<TValue>* old_iter = iter_;
iter_ = _iter;
if (iter_ == nullptr) {
valid_ = false;
} else {
Update();
}
return old_iter;
}
void DeleteIter(bool is_arena_mode) {
if (iter_) {
if (!is_arena_mode) {
delete iter_;
} else {
iter_->~InternalIteratorBase<TValue>();
}
}
}
// Iterator interface methods
bool Valid() const { return valid_; }
Slice key() const {
assert(Valid());
return result_.key;
}
TValue value() const {
assert(Valid());
return iter_->value();
}
// Methods below require iter() != nullptr
Status status() const {
assert(iter_);
return iter_->status();
}
bool PrepareValue() {
assert(Valid());
if (result_.value_prepared) {
return true;
}
if (iter_->PrepareValue()) {
result_.value_prepared = true;
return true;
}
assert(!iter_->Valid());
valid_ = false;
return false;
}
void Next() {
assert(iter_);
valid_ = iter_->NextAndGetResult(&result_);
assert(!valid_ || iter_->status().ok());
}
bool NextAndGetResult(IterateResult* result) {
assert(iter_);
valid_ = iter_->NextAndGetResult(&result_);
*result = result_;
assert(!valid_ || iter_->status().ok());
return valid_;
}
void Prev() {
assert(iter_);
iter_->Prev();
Update();
}
void Seek(const Slice& k) {
assert(iter_);
iter_->Seek(k);
Update();
}
void SeekForPrev(const Slice& k) {
assert(iter_);
iter_->SeekForPrev(k);
Update();
}
void SeekToFirst() {
assert(iter_);
iter_->SeekToFirst();
Update();
}
void SeekToLast() {
assert(iter_);
iter_->SeekToLast();
Update();
}
bool MayBeOutOfLowerBound() {
assert(Valid());
return iter_->MayBeOutOfLowerBound();
}
IterBoundCheck UpperBoundCheckResult() {
assert(Valid());
return result_.bound_check_result;
}
void SetPinnedItersMgr(PinnedIteratorsManager* pinned_iters_mgr) {
assert(iter_);
iter_->SetPinnedItersMgr(pinned_iters_mgr);
}
bool IsKeyPinned() const {
assert(Valid());
return iter_->IsKeyPinned();
}
bool IsValuePinned() const {
assert(Valid());
return iter_->IsValuePinned();
}
bool IsValuePrepared() const {
return result_.value_prepared;
}
Slice user_key() const {
assert(Valid());
return iter_->user_key();
}
void UpdateReadaheadState(InternalIteratorBase<TValue>* old_iter) {
if (old_iter && iter_) {
ReadaheadFileInfo readahead_file_info;
old_iter->GetReadaheadState(&readahead_file_info);
iter_->SetReadaheadState(&readahead_file_info);
}
}
private:
void Update() {
valid_ = iter_->Valid();
if (valid_) {
assert(iter_->status().ok());
result_.key = iter_->key();
result_.bound_check_result = IterBoundCheck::kUnknown;
result_.value_prepared = false;
}
}
InternalIteratorBase<TValue>* iter_;
IterateResult result_;
bool valid_;
};
using IteratorWrapper = IteratorWrapperBase<Slice>;
class Arena;
// Return an empty iterator (yields nothing) allocated from arena.
template <class TValue = Slice>
extern InternalIteratorBase<TValue>* NewEmptyInternalIterator(Arena* arena);
} // namespace ROCKSDB_NAMESPACE