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The purpose of the merged reftable iterator is to iterate through all
entries of a set of tables in the correct order. This is implemented by
using a sub-iterator for each table, where the next entry of each of
these iterators gets put into a priority queue. For each iteration, we
do roughly the following steps:
1. Retrieve the top record of the priority queue. This is the entry we
want to return to the caller.
2. Retrieve the next record of the sub-iterator that this record came
from. If any, add it to the priority queue at the correct position.
The position is determined by comparing the record keys, which e.g.
corresponds to the refname for ref records.
3. Keep removing the top record of the priority queue until we hit the
first entry whose key is larger than the returned record's key.
This is required to drop "shadowed" records.
The last step will lead to at least one comparison to the next entry,
but may lead to many comparisons in case the reftable stack consists of
many tables with shadowed records. It is thus part of the hot code path
when iterating through records.
The code to compare the entries with each other is quite inefficient
though. Instead of comparing record keys with each other directly, we
first format them into `struct strbuf`s and only then compare them with
each other. While we already optimized this code path to reuse buffers
in 829231dc20 (reftable/merged: reuse buffer to compute record keys,
2023-12-11), the cost to format the keys into the buffers still adds up
quite significantly.
Refactor the code to use `reftable_record_cmp()` instead, which has been
introduced in the preceding commit. This function compares records with
each other directly without requiring any memory allocations or copying
and is thus way more efficient.
The following benchmark uses git-show-ref(1) to print a single ref
matching a pattern out of 1 million refs. This is the most direct way to
exercise ref iteration speed as we remove all overhead of having to show
the refs, too.
Benchmark 1: show-ref: single matching ref (revision = HEAD~)
Time (mean ± σ): 180.7 ms ± 4.7 ms [User: 177.1 ms, System: 3.4 ms]
Range (min … max): 174.9 ms … 211.7 ms 1000 runs
Benchmark 2: show-ref: single matching ref (revision = HEAD)
Time (mean ± σ): 162.1 ms ± 4.4 ms [User: 158.5 ms, System: 3.4 ms]
Range (min … max): 155.4 ms … 189.3 ms 1000 runs
Summary
show-ref: single matching ref (revision = HEAD) ran
1.11 ± 0.04 times faster than show-ref: single matching ref (revision = HEAD~)
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
39 lines
762 B
C
39 lines
762 B
C
/*
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Copyright 2020 Google LLC
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Use of this source code is governed by a BSD-style
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license that can be found in the LICENSE file or at
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https://developers.google.com/open-source/licenses/bsd
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*/
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#ifndef MERGED_H
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#define MERGED_H
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#include "pq.h"
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struct reftable_merged_table {
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struct reftable_table *stack;
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size_t stack_len;
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uint32_t hash_id;
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/* If unset, produce deletions. This is useful for compaction. For the
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* full stack, deletions should be produced. */
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int suppress_deletions;
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uint64_t min;
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uint64_t max;
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};
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struct merged_iter {
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struct reftable_iterator *stack;
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uint32_t hash_id;
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size_t stack_len;
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uint8_t typ;
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int suppress_deletions;
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struct merged_iter_pqueue pq;
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};
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void merged_table_release(struct reftable_merged_table *mt);
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#endif
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