rocksdb/file/file_prefetch_buffer.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 "file/file_prefetch_buffer.h"
#include <algorithm>
#include <mutex>
#include "file/random_access_file_reader.h"
#include "monitoring/histogram.h"
#include "monitoring/iostats_context_imp.h"
#include "port/port.h"
#include "test_util/sync_point.h"
#include "util/random.h"
#include "util/rate_limiter.h"
namespace ROCKSDB_NAMESPACE {
Status FilePrefetchBuffer::Prefetch(const IOOptions& opts,
RandomAccessFileReader* reader,
uint64_t offset, size_t n,
bool for_compaction) {
if (!enable_ || reader == nullptr) {
return Status::OK();
}
TEST_SYNC_POINT("FilePrefetchBuffer::Prefetch:Start");
size_t alignment = reader->file()->GetRequiredBufferAlignment();
size_t offset_ = static_cast<size_t>(offset);
uint64_t rounddown_offset = Rounddown(offset_, alignment);
uint64_t roundup_end = Roundup(offset_ + n, alignment);
uint64_t roundup_len = roundup_end - rounddown_offset;
assert(roundup_len >= alignment);
assert(roundup_len % alignment == 0);
// Check if requested bytes are in the existing buffer_.
// If all bytes exist -- return.
// If only a few bytes exist -- reuse them & read only what is really needed.
// This is typically the case of incremental reading of data.
// If no bytes exist in buffer -- full pread.
Status s;
uint64_t chunk_offset_in_buffer = 0;
uint64_t chunk_len = 0;
bool copy_data_to_new_buffer = false;
if (buffer_.CurrentSize() > 0 && offset >= buffer_offset_ &&
offset <= buffer_offset_ + buffer_.CurrentSize()) {
if (offset + n <= buffer_offset_ + buffer_.CurrentSize()) {
// All requested bytes are already in the buffer. So no need to Read
// again.
return s;
} else {
// Only a few requested bytes are in the buffer. memmove those chunk of
// bytes to the beginning, and memcpy them back into the new buffer if a
// new buffer is created.
chunk_offset_in_buffer =
Rounddown(static_cast<size_t>(offset - buffer_offset_), alignment);
chunk_len = buffer_.CurrentSize() - chunk_offset_in_buffer;
assert(chunk_offset_in_buffer % alignment == 0);
assert(chunk_len % alignment == 0);
assert(chunk_offset_in_buffer + chunk_len <=
buffer_offset_ + buffer_.CurrentSize());
if (chunk_len > 0) {
copy_data_to_new_buffer = true;
} else {
// this reset is not necessary, but just to be safe.
chunk_offset_in_buffer = 0;
}
}
}
// Create a new buffer only if current capacity is not sufficient, and memcopy
// bytes from old buffer if needed (i.e., if chunk_len is greater than 0).
if (buffer_.Capacity() < roundup_len) {
buffer_.Alignment(alignment);
buffer_.AllocateNewBuffer(static_cast<size_t>(roundup_len),
copy_data_to_new_buffer, chunk_offset_in_buffer,
static_cast<size_t>(chunk_len));
} else if (chunk_len > 0) {
// New buffer not needed. But memmove bytes from tail to the beginning since
// chunk_len is greater than 0.
buffer_.RefitTail(static_cast<size_t>(chunk_offset_in_buffer),
static_cast<size_t>(chunk_len));
}
Slice result;
size_t read_len = static_cast<size_t>(roundup_len - chunk_len);
s = reader->Read(opts, rounddown_offset + chunk_len, read_len, &result,
buffer_.BufferStart() + chunk_len, nullptr, for_compaction);
#ifndef NDEBUG
if (!s.ok() || result.size() < read_len) {
// Fake an IO error to force db_stress fault injection to ignore
// truncated read errors
IGNORE_STATUS_IF_ERROR(Status::IOError());
}
#endif
if (s.ok()) {
buffer_offset_ = rounddown_offset;
buffer_.Size(static_cast<size_t>(chunk_len) + result.size());
}
return s;
}
bool FilePrefetchBuffer::TryReadFromCache(const IOOptions& opts,
uint64_t offset, size_t n,
Slice* result, bool for_compaction) {
if (track_min_offset_ && offset < min_offset_read_) {
min_offset_read_ = static_cast<size_t>(offset);
}
if (!enable_ || offset < buffer_offset_) {
return false;
}
// If the buffer contains only a few of the requested bytes:
// If readahead is enabled: prefetch the remaining bytes + readadhead bytes
// and satisfy the request.
// If readahead is not enabled: return false.
if (offset + n > buffer_offset_ + buffer_.CurrentSize()) {
if (readahead_size_ > 0) {
assert(file_reader_ != nullptr);
assert(max_readahead_size_ >= readahead_size_);
Status s;
if (for_compaction) {
s = Prefetch(opts, file_reader_, offset, std::max(n, readahead_size_),
for_compaction);
} else {
s = Prefetch(opts, file_reader_, offset, n + readahead_size_,
for_compaction);
}
if (!s.ok()) {
return false;
}
readahead_size_ = std::min(max_readahead_size_, readahead_size_ * 2);
} else {
return false;
}
}
uint64_t offset_in_buffer = offset - buffer_offset_;
*result = Slice(buffer_.BufferStart() + offset_in_buffer, n);
return true;
}
} // namespace ROCKSDB_NAMESPACE