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rocksdb/table/multiget_context.h

247 lines
7.8 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 <string>
#include "db/lookup_key.h"
#include "db/merge_context.h"
#include "rocksdb/env.h"
#include "rocksdb/statistics.h"
#include "rocksdb/types.h"
#include "util/autovector.h"
namespace rocksdb {
class GetContext;
struct KeyContext {
const Slice* key;
LookupKey* lkey;
Slice ukey;
Slice ikey;
Status* s;
MergeContext merge_context;
SequenceNumber max_covering_tombstone_seq;
bool key_exists;
void* cb_arg;
PinnableSlice* value;
GetContext* get_context;
KeyContext(const Slice& user_key, PinnableSlice* val, Status* stat)
: key(&user_key),
lkey(nullptr),
s(stat),
max_covering_tombstone_seq(0),
key_exists(false),
cb_arg(nullptr),
value(val),
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 < 64
// simplifies iteration, as well as reduces the amount of stack allocations
// htat need to be performed
static const int MAX_BATCH_SIZE = 32;
MultiGetContext(KeyContext** sorted_keys, size_t num_keys,
SequenceNumber snapshot)
: sorted_keys_(sorted_keys),
num_keys_(num_keys),
value_mask_(0),
lookup_key_ptr_(reinterpret_cast<LookupKey*>(lookup_key_stack_buf)) {
int index = 0;
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) {
sorted_keys_[iter]->lkey = new (&lookup_key_ptr_[index])
LookupKey(*sorted_keys_[iter]->key, snapshot);
sorted_keys_[iter]->ukey = sorted_keys_[iter]->lkey->user_key();
sorted_keys_[iter]->ikey = sorted_keys_[iter]->lkey->internal_key();
index++;
}
}
~MultiGetContext() {
for (size_t i = 0; i < num_keys_; ++i) {
lookup_key_ptr_[i].~LookupKey();
}
}
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];
KeyContext** sorted_keys_;
size_t num_keys_;
uint64_t value_mask_;
std::unique_ptr<char[]> lookup_key_heap_buf;
LookupKey* lookup_key_ptr_;
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
typedef Iterator self_type;
typedef KeyContext value_type;
typedef KeyContext& reference;
typedef KeyContext* pointer;
typedef int difference_type;
typedef std::forward_iterator_tag iterator_category;
Iterator(const Range* range, size_t idx)
: range_(range), ctx_(range->ctx_), index_(idx) {
while (index_ < range_->end_ &&
(1ull << index_) &
(range_->ctx_->value_mask_ | range_->skip_mask_))
index_++;
}
Iterator(const Iterator&) = default;
Iterator& operator=(const Iterator&) = default;
Iterator& operator++() {
while (++index_ < range_->end_ &&
(1ull << index_) &
(range_->ctx_->value_mask_ | range_->skip_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_;
start_ = first.index_;
end_ = last.index_;
skip_mask_ = mget_range.skip_mask_;
}
Range() = default;
Iterator begin() const { return Iterator(this, start_); }
Iterator end() const { return Iterator(this, end_); }
bool empty() {
return (((1ull << end_) - 1) & ~((1ull << start_) - 1) &
~(ctx_->value_mask_ | skip_mask_)) == 0;
}
void SkipKey(const Iterator& iter) { skip_mask_ |= 1ull << iter.index_; }
// Update the value_mask_ in MultiGetContext so its
// immediately reflected in all the Range Iterators
void MarkKeyDone(Iterator& iter) {
ctx_->value_mask_ |= (1ull << iter.index_);
}
bool CheckKeyDone(Iterator& iter) {
return ctx_->value_mask_ & (1ull << iter.index_);
}
private:
friend MultiGetContext;
MultiGetContext* ctx_;
size_t start_;
size_t end_;
uint64_t skip_mask_;
Range(MultiGetContext* ctx, size_t num_keys)
: ctx_(ctx), start_(0), end_(num_keys), skip_mask_(0) {}
};
// Return the initial range that encompasses all the keys in the batch
Range GetMultiGetRange() { return Range(this, num_keys_); }
};
} // namespace rocksdb