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rocksdb/table/merging_iterator.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).
//
// 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 "table/merging_iterator.h"
#include <string>
#include <vector>
#include "db/dbformat.h"
#include "db/pinned_iterators_manager.h"
#include "memory/arena.h"
#include "monitoring/perf_context_imp.h"
#include "rocksdb/comparator.h"
#include "rocksdb/iterator.h"
#include "rocksdb/options.h"
#include "table/internal_iterator.h"
#include "table/iter_heap.h"
#include "table/iterator_wrapper.h"
#include "test_util/sync_point.h"
#include "util/autovector.h"
#include "util/heap.h"
#include "util/stop_watch.h"
namespace rocksdb {
// Without anonymous namespace here, we fail the warning -Wmissing-prototypes
namespace {
typedef BinaryHeap<IteratorWrapper*, MaxIteratorComparator> MergerMaxIterHeap;
typedef BinaryHeap<IteratorWrapper*, MinIteratorComparator> MergerMinIterHeap;
} // namespace
const size_t kNumIterReserve = 4;
class MergingIterator : public InternalIterator {
public:
MergingIterator(const InternalKeyComparator* comparator,
InternalIterator** children, int n, bool is_arena_mode,
bool prefix_seek_mode)
: is_arena_mode_(is_arena_mode),
comparator_(comparator),
current_(nullptr),
direction_(kForward),
minHeap_(comparator_),
prefix_seek_mode_(prefix_seek_mode),
pinned_iters_mgr_(nullptr) {
children_.resize(n);
for (int i = 0; i < n; i++) {
children_[i].Set(children[i]);
}
for (auto& child : children_) {
AddToMinHeapOrCheckStatus(&child);
}
current_ = CurrentForward();
}
void considerStatus(Status s) {
if (!s.ok() && status_.ok()) {
status_ = s;
}
}
virtual void AddIterator(InternalIterator* iter) {
assert(direction_ == kForward);
children_.emplace_back(iter);
if (pinned_iters_mgr_) {
iter->SetPinnedItersMgr(pinned_iters_mgr_);
}
auto new_wrapper = children_.back();
AddToMinHeapOrCheckStatus(&new_wrapper);
if (new_wrapper.Valid()) {
current_ = CurrentForward();
}
}
~MergingIterator() override {
for (auto& child : children_) {
child.DeleteIter(is_arena_mode_);
}
}
bool Valid() const override { return current_ != nullptr && status_.ok(); }
Status status() const override { return status_; }
void SeekToFirst() override {
ClearHeaps();
status_ = Status::OK();
for (auto& child : children_) {
child.SeekToFirst();
AddToMinHeapOrCheckStatus(&child);
}
direction_ = kForward;
current_ = CurrentForward();
}
void SeekToLast() override {
ClearHeaps();
InitMaxHeap();
status_ = Status::OK();
for (auto& child : children_) {
child.SeekToLast();
AddToMaxHeapOrCheckStatus(&child);
}
direction_ = kReverse;
current_ = CurrentReverse();
}
void Seek(const Slice& target) override {
ClearHeaps();
status_ = Status::OK();
for (auto& child : children_) {
{
PERF_TIMER_GUARD(seek_child_seek_time);
child.Seek(target);
}
PERF_COUNTER_ADD(seek_child_seek_count, 1);
{
// Strictly, we timed slightly more than min heap operation,
// but these operations are very cheap.
PERF_TIMER_GUARD(seek_min_heap_time);
AddToMinHeapOrCheckStatus(&child);
}
}
direction_ = kForward;
{
PERF_TIMER_GUARD(seek_min_heap_time);
current_ = CurrentForward();
}
}
void SeekForPrev(const Slice& target) override {
ClearHeaps();
InitMaxHeap();
status_ = Status::OK();
for (auto& child : children_) {
{
PERF_TIMER_GUARD(seek_child_seek_time);
child.SeekForPrev(target);
}
PERF_COUNTER_ADD(seek_child_seek_count, 1);
{
PERF_TIMER_GUARD(seek_max_heap_time);
AddToMaxHeapOrCheckStatus(&child);
}
}
direction_ = kReverse;
{
PERF_TIMER_GUARD(seek_max_heap_time);
current_ = CurrentReverse();
}
}
void Next() override {
assert(Valid());
// Ensure that all children are positioned after key().
// If we are moving in the forward direction, it is already
// true for all of the non-current children since current_ is
// the smallest child and key() == current_->key().
if (direction_ != kForward) {
SwitchToForward();
// The loop advanced all non-current children to be > key() so current_
// should still be strictly the smallest key.
assert(current_ == CurrentForward());
}
// For the heap modifications below to be correct, current_ must be the
// current top of the heap.
assert(current_ == CurrentForward());
// as the current points to the current record. move the iterator forward.
current_->Next();
if (current_->Valid()) {
// current is still valid after the Next() call above. Call
// replace_top() to restore the heap property. When the same child
// iterator yields a sequence of keys, this is cheap.
assert(current_->status().ok());
minHeap_.replace_top(current_);
} else {
// current stopped being valid, remove it from the heap.
considerStatus(current_->status());
minHeap_.pop();
}
current_ = CurrentForward();
}
bool NextAndGetResult(IterateResult* result) override {
Next();
bool is_valid = Valid();
if (is_valid) {
result->key = key();
result->may_be_out_of_upper_bound = MayBeOutOfUpperBound();
}
return is_valid;
}
void Prev() override {
assert(Valid());
// Ensure that all children are positioned before key().
// If we are moving in the reverse direction, it is already
// true for all of the non-current children since current_ is
// the largest child and key() == current_->key().
if (direction_ != kReverse) {
// Otherwise, retreat the non-current children. We retreat current_
// just after the if-block.
SwitchToBackward();
}
// For the heap modifications below to be correct, current_ must be the
// current top of the heap.
assert(current_ == CurrentReverse());
current_->Prev();
if (current_->Valid()) {
// current is still valid after the Prev() call above. Call
// replace_top() to restore the heap property. When the same child
// iterator yields a sequence of keys, this is cheap.
assert(current_->status().ok());
maxHeap_->replace_top(current_);
} else {
// current stopped being valid, remove it from the heap.
considerStatus(current_->status());
maxHeap_->pop();
}
current_ = CurrentReverse();
}
Slice key() const override {
assert(Valid());
return current_->key();
}
Slice value() const override {
assert(Valid());
return current_->value();
}
// Here we simply relay MayBeOutOfLowerBound/MayBeOutOfUpperBound result
// from current child iterator. Potentially as long as one of child iterator
// report out of bound is not possible, we know current key is within bound.
bool MayBeOutOfLowerBound() override {
assert(Valid());
return current_->MayBeOutOfLowerBound();
}
bool MayBeOutOfUpperBound() override {
assert(Valid());
return current_->MayBeOutOfUpperBound();
}
void SetPinnedItersMgr(PinnedIteratorsManager* pinned_iters_mgr) override {
pinned_iters_mgr_ = pinned_iters_mgr;
for (auto& child : children_) {
child.SetPinnedItersMgr(pinned_iters_mgr);
}
}
bool IsKeyPinned() const override {
assert(Valid());
return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
current_->IsKeyPinned();
}
bool IsValuePinned() const override {
assert(Valid());
return pinned_iters_mgr_ && pinned_iters_mgr_->PinningEnabled() &&
current_->IsValuePinned();
}
private:
// Clears heaps for both directions, used when changing direction or seeking
void ClearHeaps();
// Ensures that maxHeap_ is initialized when starting to go in the reverse
// direction
void InitMaxHeap();
bool is_arena_mode_;
const InternalKeyComparator* comparator_;
autovector<IteratorWrapper, kNumIterReserve> children_;
// Cached pointer to child iterator with the current key, or nullptr if no
// child iterators are valid. This is the top of minHeap_ or maxHeap_
// depending on the direction.
IteratorWrapper* current_;
// If any of the children have non-ok status, this is one of them.
Status status_;
// Which direction is the iterator moving?
enum Direction {
kForward,
kReverse
};
Direction direction_;
MergerMinIterHeap minHeap_;
bool prefix_seek_mode_;
// Max heap is used for reverse iteration, which is way less common than
// forward. Lazily initialize it to save memory.
std::unique_ptr<MergerMaxIterHeap> maxHeap_;
PinnedIteratorsManager* pinned_iters_mgr_;
// In forward direction, process a child that is not in the min heap.
// If valid, add to the min heap. Otherwise, check status.
void AddToMinHeapOrCheckStatus(IteratorWrapper*);
// In backward direction, process a child that is not in the max heap.
// If valid, add to the min heap. Otherwise, check status.
void AddToMaxHeapOrCheckStatus(IteratorWrapper*);
void SwitchToForward();
// Switch the direction from forward to backward without changing the
// position. Iterator should still be valid.
void SwitchToBackward();
IteratorWrapper* CurrentForward() const {
assert(direction_ == kForward);
return !minHeap_.empty() ? minHeap_.top() : nullptr;
}
IteratorWrapper* CurrentReverse() const {
assert(direction_ == kReverse);
assert(maxHeap_);
return !maxHeap_->empty() ? maxHeap_->top() : nullptr;
}
};
void MergingIterator::AddToMinHeapOrCheckStatus(IteratorWrapper* child) {
if (child->Valid()) {
assert(child->status().ok());
minHeap_.push(child);
} else {
considerStatus(child->status());
}
}
void MergingIterator::AddToMaxHeapOrCheckStatus(IteratorWrapper* child) {
if (child->Valid()) {
assert(child->status().ok());
maxHeap_->push(child);
} else {
considerStatus(child->status());
}
}
void MergingIterator::SwitchToForward() {
// Otherwise, advance the non-current children. We advance current_
// just after the if-block.
ClearHeaps();
Slice target = key();
for (auto& child : children_) {
if (&child != current_) {
child.Seek(target);
if (child.Valid() && comparator_->Equal(target, child.key())) {
assert(child.status().ok());
child.Next();
}
}
AddToMinHeapOrCheckStatus(&child);
}
direction_ = kForward;
}
void MergingIterator::SwitchToBackward() {
ClearHeaps();
InitMaxHeap();
Slice target = key();
for (auto& child : children_) {
if (&child != current_) {
child.SeekForPrev(target);
TEST_SYNC_POINT_CALLBACK("MergeIterator::Prev:BeforePrev", &child);
if (child.Valid() && comparator_->Equal(target, child.key())) {
assert(child.status().ok());
child.Prev();
}
}
AddToMaxHeapOrCheckStatus(&child);
}
direction_ = kReverse;
if (!prefix_seek_mode_) {
// Note that we don't do assert(current_ == CurrentReverse()) here
// because it is possible to have some keys larger than the seek-key
// inserted between Seek() and SeekToLast(), which makes current_ not
// equal to CurrentReverse().
current_ = CurrentReverse();
}
assert(current_ == CurrentReverse());
}
void MergingIterator::ClearHeaps() {
minHeap_.clear();
if (maxHeap_) {
maxHeap_->clear();
}
}
void MergingIterator::InitMaxHeap() {
if (!maxHeap_) {
maxHeap_.reset(new MergerMaxIterHeap(comparator_));
}
}
InternalIterator* NewMergingIterator(const InternalKeyComparator* cmp,
InternalIterator** list, int n,
Arena* arena, bool prefix_seek_mode) {
assert(n >= 0);
if (n == 0) {
return NewEmptyInternalIterator<Slice>(arena);
} else if (n == 1) {
return list[0];
} else {
if (arena == nullptr) {
return new MergingIterator(cmp, list, n, false, prefix_seek_mode);
} else {
auto mem = arena->AllocateAligned(sizeof(MergingIterator));
return new (mem) MergingIterator(cmp, list, n, true, prefix_seek_mode);
}
}
}
MergeIteratorBuilder::MergeIteratorBuilder(
const InternalKeyComparator* comparator, Arena* a, bool prefix_seek_mode)
: first_iter(nullptr), use_merging_iter(false), arena(a) {
auto mem = arena->AllocateAligned(sizeof(MergingIterator));
merge_iter =
new (mem) MergingIterator(comparator, nullptr, 0, true, prefix_seek_mode);
}
MergeIteratorBuilder::~MergeIteratorBuilder() {
if (first_iter != nullptr) {
first_iter->~InternalIterator();
}
if (merge_iter != nullptr) {
merge_iter->~MergingIterator();
}
}
void MergeIteratorBuilder::AddIterator(InternalIterator* iter) {
if (!use_merging_iter && first_iter != nullptr) {
merge_iter->AddIterator(first_iter);
use_merging_iter = true;
first_iter = nullptr;
}
if (use_merging_iter) {
merge_iter->AddIterator(iter);
} else {
first_iter = iter;
}
}
InternalIterator* MergeIteratorBuilder::Finish() {
InternalIterator* ret = nullptr;
if (!use_merging_iter) {
ret = first_iter;
first_iter = nullptr;
} else {
ret = merge_iter;
merge_iter = nullptr;
}
return ret;
}
} // namespace rocksdb