17ce1ca48b
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
34 lines
1.0 KiB
C++
34 lines
1.0 KiB
C++
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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#pragma once
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#include <cstddef>
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#include <cstdint>
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#include "rocksdb/rocksdb_namespace.h"
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namespace ROCKSDB_NAMESPACE {
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// struct ReadaheadFileInfo contains readahead information that is passed from
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// one file to another file per level during iterations. This information helps
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// iterators to carry forward the internal automatic prefetching readahead value
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// to next file during sequential reads instead of starting from the scratch.
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struct ReadaheadFileInfo {
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struct ReadaheadInfo {
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size_t readahead_size = 0;
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int64_t num_file_reads = 0;
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};
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// Used by Data block iterators to update readahead info.
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ReadaheadInfo data_block_readahead_info;
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// Used by Index block iterators to update readahead info.
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ReadaheadInfo index_block_readahead_info;
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};
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} // namespace ROCKSDB_NAMESPACE
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