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rocksdb/table/multiget_context.h

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13 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).
#pragma once
#include <algorithm>
#include <array>
#include <string>
#include "db/dbformat.h"
#include "db/lookup_key.h"
#include "db/merge_context.h"
#include "rocksdb/env.h"
#include "rocksdb/statistics.h"
#include "rocksdb/types.h"
#include "util/async_file_reader.h"
#include "util/autovector.h"
#include "util/math.h"
#include "util/single_thread_executor.h"
namespace ROCKSDB_NAMESPACE {
class GetContext;
struct KeyContext {
const Slice* key;
LookupKey* lkey;
Slice ukey_with_ts;
Slice ukey_without_ts;
Slice ikey;
ColumnFamilyHandle* column_family;
Status* s;
MergeContext merge_context;
SequenceNumber max_covering_tombstone_seq;
bool key_exists;
bool is_blob_index;
void* cb_arg;
PinnableSlice* value;
std::string* timestamp;
GetContext* get_context;
KeyContext(ColumnFamilyHandle* col_family, const Slice& user_key,
PinnableSlice* val, std::string* ts, Status* stat)
: key(&user_key),
lkey(nullptr),
column_family(col_family),
s(stat),
max_covering_tombstone_seq(0),
key_exists(false),
is_blob_index(false),
cb_arg(nullptr),
value(val),
timestamp(ts),
get_context(nullptr) {}
KeyContext() = default;
};
// The MultiGetContext class is a container for the sorted list of keys that
// we need to lookup in a batch. Its main purpose is to make batch execution
// easier by allowing various stages of the MultiGet lookups to operate on
// subsets of keys, potentially non-contiguous. In order to accomplish this,
// it defines the following classes -
//
// MultiGetContext::Range
// MultiGetContext::Range::Iterator
// MultiGetContext::Range::IteratorWrapper
//
// Here is an example of how this can be used -
//
// {
// MultiGetContext ctx(...);
// MultiGetContext::Range range = ctx.GetMultiGetRange();
//
// // Iterate to determine some subset of the keys
// MultiGetContext::Range::Iterator start = range.begin();
// MultiGetContext::Range::Iterator end = ...;
//
// // Make a new range with a subset of keys
// MultiGetContext::Range subrange(range, start, end);
//
// // Define an auxillary vector, if needed, to hold additional data for
// // each key
// std::array<Foo, MultiGetContext::MAX_BATCH_SIZE> aux;
//
// // Iterate over the subrange and the auxillary vector simultaneously
// MultiGetContext::Range::Iterator iter = subrange.begin();
// for (; iter != subrange.end(); ++iter) {
// KeyContext& key = *iter;
// Foo& aux_key = aux_iter[iter.index()];
// ...
// }
// }
class MultiGetContext {
public:
// Limit the number of keys in a batch to this number. Benchmarks show that
// there is negligible benefit for batches exceeding this. Keeping this < 32
// simplifies iteration, as well as reduces the amount of stack allocations
// that need to be performed
static const int MAX_BATCH_SIZE = 32;
// A bitmask of at least MAX_BATCH_SIZE - 1 bits, so that
// Mask{1} << MAX_BATCH_SIZE is well defined
using Mask = uint64_t;
static_assert(MAX_BATCH_SIZE < sizeof(Mask) * 8);
MultiGetContext(autovector<KeyContext*, MAX_BATCH_SIZE>* sorted_keys,
size_t begin, size_t num_keys, SequenceNumber snapshot,
const ReadOptions& read_opts, FileSystem* fs,
Statistics* stats)
: num_keys_(num_keys),
value_mask_(0),
value_size_(0),
lookup_key_ptr_(reinterpret_cast<LookupKey*>(lookup_key_stack_buf))
#if USE_COROUTINES
,
reader_(fs, stats),
executor_(reader_)
#endif // USE_COROUTINES
{
(void)fs;
(void)stats;
assert(num_keys <= MAX_BATCH_SIZE);
if (num_keys > MAX_LOOKUP_KEYS_ON_STACK) {
lookup_key_heap_buf.reset(new char[sizeof(LookupKey) * num_keys]);
lookup_key_ptr_ = reinterpret_cast<LookupKey*>(
lookup_key_heap_buf.get());
}
for (size_t iter = 0; iter != num_keys_; ++iter) {
// autovector may not be contiguous storage, so make a copy
sorted_keys_[iter] = (*sorted_keys)[begin + iter];
sorted_keys_[iter]->lkey = new (&lookup_key_ptr_[iter])
LookupKey(*sorted_keys_[iter]->key, snapshot, read_opts.timestamp);
sorted_keys_[iter]->ukey_with_ts = sorted_keys_[iter]->lkey->user_key();
sorted_keys_[iter]->ukey_without_ts = StripTimestampFromUserKey(
sorted_keys_[iter]->lkey->user_key(),
read_opts.timestamp == nullptr ? 0 : read_opts.timestamp->size());
sorted_keys_[iter]->ikey = sorted_keys_[iter]->lkey->internal_key();
sorted_keys_[iter]->timestamp = (*sorted_keys)[begin + iter]->timestamp;
sorted_keys_[iter]->get_context =
(*sorted_keys)[begin + iter]->get_context;
}
}
~MultiGetContext() {
for (size_t i = 0; i < num_keys_; ++i) {
lookup_key_ptr_[i].~LookupKey();
}
}
#if USE_COROUTINES
SingleThreadExecutor& executor() { return executor_; }
AsyncFileReader& reader() { return reader_; }
#endif // USE_COROUTINES
private:
static const int MAX_LOOKUP_KEYS_ON_STACK = 16;
alignas(alignof(LookupKey))
char lookup_key_stack_buf[sizeof(LookupKey) * MAX_LOOKUP_KEYS_ON_STACK];
std::array<KeyContext*, MAX_BATCH_SIZE> sorted_keys_;
size_t num_keys_;
Mask value_mask_;
uint64_t value_size_;
std::unique_ptr<char[]> lookup_key_heap_buf;
LookupKey* lookup_key_ptr_;
#if USE_COROUTINES
AsyncFileReader reader_;
SingleThreadExecutor executor_;
#endif // USE_COROUTINES
public:
// MultiGetContext::Range - Specifies a range of keys, by start and end index,
// from the parent MultiGetContext. Each range contains a bit vector that
// indicates whether the corresponding keys need to be processed or skipped.
// A Range object can be copy constructed, and the new object inherits the
// original Range's bit vector. This is useful for progressively skipping
// keys as the lookup goes through various stages. For example, when looking
// up keys in the same SST file, a Range is created excluding keys not
// belonging to that file. A new Range is then copy constructed and individual
// keys are skipped based on bloom filter lookup.
class Range {
public:
// MultiGetContext::Range::Iterator - A forward iterator that iterates over
// non-skippable keys in a Range, as well as keys whose final value has been
// found. The latter is tracked by MultiGetContext::value_mask_
class Iterator {
public:
// -- iterator traits
using self_type = Iterator;
using value_type = KeyContext;
using reference = KeyContext&;
using pointer = KeyContext*;
using difference_type = int;
using iterator_category = std::forward_iterator_tag;
Iterator(const Range* range, size_t idx)
: range_(range), ctx_(range->ctx_), index_(idx) {
while (index_ < range_->end_ &&
(Mask{1} << index_) &
(range_->ctx_->value_mask_ | range_->skip_mask_ |
range_->invalid_mask_))
index_++;
}
Iterator(const Iterator&) = default;
Iterator(const Iterator& other, const Range* range)
: range_(range), ctx_(other.ctx_), index_(other.index_) {
assert(range->ctx_ == other.ctx_);
}
Iterator& operator=(const Iterator&) = default;
Iterator& operator++() {
while (++index_ < range_->end_ &&
(Mask{1} << index_) &
(range_->ctx_->value_mask_ | range_->skip_mask_ |
range_->invalid_mask_))
;
return *this;
}
bool operator==(Iterator other) const {
assert(range_->ctx_ == other.range_->ctx_);
return index_ == other.index_;
}
bool operator!=(Iterator other) const {
assert(range_->ctx_ == other.range_->ctx_);
return index_ != other.index_;
}
KeyContext& operator*() {
assert(index_ < range_->end_ && index_ >= range_->start_);
return *(ctx_->sorted_keys_[index_]);
}
KeyContext* operator->() {
assert(index_ < range_->end_ && index_ >= range_->start_);
return ctx_->sorted_keys_[index_];
}
size_t index() { return index_; }
private:
friend Range;
const Range* range_;
const MultiGetContext* ctx_;
size_t index_;
};
Range(const Range& mget_range,
const Iterator& first,
const Iterator& last) {
ctx_ = mget_range.ctx_;
if (first == last) {
// This means create an empty range based on mget_range. So just
// set start_ and and_ to the same value
start_ = mget_range.start_;
end_ = start_;
} else {
start_ = first.index_;
end_ = last.index_;
}
skip_mask_ = mget_range.skip_mask_;
invalid_mask_ = mget_range.invalid_mask_;
assert(start_ < 64);
assert(end_ < 64);
}
Range() = default;
Iterator begin() const { return Iterator(this, start_); }
Iterator end() const { return Iterator(this, end_); }
bool empty() const { return RemainingMask() == 0; }
void SkipIndex(size_t index) { skip_mask_ |= Mask{1} << index; }
void SkipKey(const Iterator& iter) { SkipIndex(iter.index_); }
bool IsKeySkipped(const Iterator& iter) const {
return skip_mask_ & (Mask{1} << iter.index_);
}
// Update the value_mask_ in MultiGetContext so its
// immediately reflected in all the Range Iterators
void MarkKeyDone(Iterator& iter) {
ctx_->value_mask_ |= (Mask{1} << iter.index_);
}
bool CheckKeyDone(Iterator& iter) const {
return ctx_->value_mask_ & (Mask{1} << iter.index_);
}
uint64_t KeysLeft() const { return BitsSetToOne(RemainingMask()); }
void AddSkipsFrom(const Range& other) {
assert(ctx_ == other.ctx_);
skip_mask_ |= other.skip_mask_;
}
uint64_t GetValueSize() { return ctx_->value_size_; }
void AddValueSize(uint64_t value_size) { ctx_->value_size_ += value_size; }
MultiGetContext* context() const { return ctx_; }
Range Suffix(const Range& other) const {
size_t other_last = other.FindLastRemaining();
size_t my_last = FindLastRemaining();
if (my_last > other_last) {
return Range(*this, Iterator(this, other_last),
Iterator(this, my_last));
} else {
return Range(*this, begin(), begin());
}
}
// The += operator expands the number of keys in this range. The expansion
// is always to the right, i.e start of the additional range >= end of
// current range. There should be no overlap. Any skipped keys in rhs are
// marked as invalid in the invalid_mask_.
Range& operator+=(const Range& rhs) {
assert(rhs.start_ >= end_);
// Check for non-overlapping ranges and adjust invalid_mask_ accordingly
if (end_ < rhs.start_) {
invalid_mask_ |= RangeMask(end_, rhs.start_);
skip_mask_ |= RangeMask(end_, rhs.start_);
}
start_ = std::min<size_t>(start_, rhs.start_);
end_ = std::max<size_t>(end_, rhs.end_);
skip_mask_ |= rhs.skip_mask_ & RangeMask(rhs.start_, rhs.end_);
invalid_mask_ |= (rhs.invalid_mask_ | rhs.skip_mask_) &
RangeMask(rhs.start_, rhs.end_);
assert(start_ < 64);
assert(end_ < 64);
return *this;
}
// The -= operator removes keys from this range. The removed keys should
// come from a range completely overlapping the current range. The removed
// keys are marked invalid in the invalid_mask_.
Range& operator-=(const Range& rhs) {
assert(start_ <= rhs.start_ && end_ >= rhs.end_);
skip_mask_ |= (~rhs.skip_mask_ | rhs.invalid_mask_) &
RangeMask(rhs.start_, rhs.end_);
invalid_mask_ |= (~rhs.skip_mask_ | rhs.invalid_mask_) &
RangeMask(rhs.start_, rhs.end_);
return *this;
}
// Return a complement of the current range
Range operator~() {
Range res = *this;
res.skip_mask_ = ~skip_mask_ & RangeMask(start_, end_);
return res;
}
private:
friend MultiGetContext;
MultiGetContext* ctx_;
size_t start_;
size_t end_;
Mask skip_mask_;
Mask invalid_mask_;
Range(MultiGetContext* ctx, size_t num_keys)
: ctx_(ctx),
start_(0),
end_(num_keys),
skip_mask_(0),
invalid_mask_(0) {
assert(num_keys < 64);
}
static Mask RangeMask(size_t start, size_t end) {
return (((Mask{1} << (end - start)) - 1) << start);
}
Mask RemainingMask() const {
return (((Mask{1} << end_) - 1) & ~((Mask{1} << start_) - 1) &
~(ctx_->value_mask_ | skip_mask_));
}
size_t FindLastRemaining() const {
Mask mask = RemainingMask();
size_t index = (mask >>= start_) ? start_ : 0;
while (mask >>= 1) {
index++;
}
return index;
}
};
// Return the initial range that encompasses all the keys in the batch
Range GetMultiGetRange() { return Range(this, num_keys_); }
};
} // namespace ROCKSDB_NAMESPACE