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When reading a record from a block, we need to decode the record's key.
As reftable keys are prefix-compressed, meaning they reuse a prefix from
the preceding record's key, this is a bit more involved than just having
to copy the relevant bytes: we need to figure out the prefix and suffix
lengths, copy the prefix from the preceding record and finally copy the
suffix from the current record.
This is done by passing three buffers to `reftable_decode_key()`: one
buffer that holds the result, one buffer that holds the last key, and
one buffer that points to the current record. The final key is then
assembled by calling `strbuf_add()` twice to copy over the prefix and
suffix.
Performing two memory copies is inefficient though. And we can indeed do
better by decoding keys in place. Instead of providing two buffers, the
caller may only call a single buffer that is already pre-populated with
the last key. Like this, we only have to call `strbuf_setlen()` to trim
the record to its prefix and then `strbuf_add()` to add the suffix.
This refactoring leads to a noticeable performance bump when iterating
over 1 million refs:
Benchmark 1: show-ref: single matching ref (revision = HEAD~)
Time (mean ± σ): 112.2 ms ± 3.9 ms [User: 109.3 ms, System: 2.8 ms]
Range (min … max): 109.2 ms … 149.6 ms 1000 runs
Benchmark 2: show-ref: single matching ref (revision = HEAD)
Time (mean ± σ): 106.0 ms ± 3.5 ms [User: 103.2 ms, System: 2.7 ms]
Range (min … max): 103.2 ms … 133.7 ms 1000 runs
Summary
show-ref: single matching ref (revision = HEAD) ran
1.06 ± 0.05 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>
131 lines
3.6 KiB
C
131 lines
3.6 KiB
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 BLOCK_H
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#define BLOCK_H
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#include "basics.h"
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#include "record.h"
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#include "reftable-blocksource.h"
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/*
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* Writes reftable blocks. The block_writer is reused across blocks to minimize
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* allocation overhead.
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*/
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struct block_writer {
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uint8_t *buf;
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uint32_t block_size;
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/* Offset of the global header. Nonzero in the first block only. */
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uint32_t header_off;
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/* How often to restart keys. */
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int restart_interval;
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int hash_size;
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/* Offset of next uint8_t to write. */
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uint32_t next;
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uint32_t *restarts;
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uint32_t restart_len;
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uint32_t restart_cap;
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struct strbuf last_key;
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int entries;
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};
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/*
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* initializes the blockwriter to write `typ` entries, using `buf` as temporary
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* storage. `buf` is not owned by the block_writer. */
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void block_writer_init(struct block_writer *bw, uint8_t typ, uint8_t *buf,
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uint32_t block_size, uint32_t header_off, int hash_size);
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/* returns the block type (eg. 'r' for ref records. */
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uint8_t block_writer_type(struct block_writer *bw);
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/* appends the record, or -1 if it doesn't fit. */
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int block_writer_add(struct block_writer *w, struct reftable_record *rec);
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/* appends the key restarts, and compress the block if necessary. */
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int block_writer_finish(struct block_writer *w);
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/* clears out internally allocated block_writer members. */
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void block_writer_release(struct block_writer *bw);
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/* Read a block. */
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struct block_reader {
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/* offset of the block header; nonzero for the first block in a
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* reftable. */
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uint32_t header_off;
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/* the memory block */
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struct reftable_block block;
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int hash_size;
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/* size of the data, excluding restart data. */
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uint32_t block_len;
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uint8_t *restart_bytes;
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uint16_t restart_count;
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/* size of the data in the file. For log blocks, this is the compressed
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* size. */
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uint32_t full_block_size;
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};
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/* Iterate over entries in a block */
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struct block_iter {
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/* offset within the block of the next entry to read. */
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uint32_t next_off;
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struct block_reader *br;
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/* key for last entry we read. */
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struct strbuf last_key;
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};
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#define BLOCK_ITER_INIT { \
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.last_key = STRBUF_INIT, \
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}
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/* initializes a block reader. */
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int block_reader_init(struct block_reader *br, struct reftable_block *bl,
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uint32_t header_off, uint32_t table_block_size,
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int hash_size);
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/* Position `it` at start of the block */
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void block_reader_start(struct block_reader *br, struct block_iter *it);
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/* Position `it` to the `want` key in the block */
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int block_reader_seek(struct block_reader *br, struct block_iter *it,
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struct strbuf *want);
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/* Returns the block type (eg. 'r' for refs) */
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uint8_t block_reader_type(struct block_reader *r);
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/* Decodes the first key in the block */
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int block_reader_first_key(struct block_reader *br, struct strbuf *key);
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void block_iter_copy_from(struct block_iter *dest, struct block_iter *src);
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/* return < 0 for error, 0 for OK, > 0 for EOF. */
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int block_iter_next(struct block_iter *it, struct reftable_record *rec);
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/* Seek to `want` with in the block pointed to by `it` */
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int block_iter_seek(struct block_iter *it, struct strbuf *want);
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/* deallocate memory for `it`. The block reader and its block is left intact. */
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void block_iter_close(struct block_iter *it);
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/* size of file header, depending on format version */
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int header_size(int version);
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/* size of file footer, depending on format version */
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int footer_size(int version);
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/* returns a block to its source. */
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void reftable_block_done(struct reftable_block *ret);
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#endif
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