[PATCH v2 5/7] reftable/block: swap buffers instead of copying
- From
Patrick Steinhardt <ps@pks.im>
- Date
- Feb 12, 2024, 08:32 UTC
- Message-ID
- <2317aa43b95fc5f418504e9cf24b01048c3dbf8f.1707726654.git.ps@pks.im>
- In-Reply-To
- <cover.1707726654.git.ps@pks.im>
When iterating towards the next record in a reftable block we need to keep track of the key that the last record had. This is required because reftable records use prefix compression, where subsequent records may reuse parts of their preceding record's key.
This key is stored in the `block_iter::last_key`, which we update after every call to `block_iter_next()`: we simply reset the buffer and then add the current key to it.
This is a bit inefficient though because it requires us to copy over the key on every iteration, which adds up when iterating over many records. Instead, we can make use of the fact that the `block_iter::key` buffer is basically only a scratch buffer. So instead of copying over contents, we can just swap both buffers.
The following benchmark prints a single ref matching a specific pattern out of 1 million refs via git-show-ref(1):
Benchmark 1: show-ref: single matching ref (revision = HEAD~)
Time (mean ± σ): 155.7 ms ± 5.0 ms [User: 152.1 ms, System: 3.4 ms]
Range (min … max): 150.8 ms … 185.7 ms 1000 runs Benchmark 2: show-ref: single matching ref (revision = HEAD)
Time (mean ± σ): 150.8 ms ± 4.2 ms [User: 147.1 ms, System: 3.5 ms]
Range (min … max): 145.1 ms … 180.7 ms 1000 runs Summary
show-ref: single matching ref (revision = HEAD) ran
1.03 ± 0.04 times faster than show-ref: single matching ref (revision = HEAD~)Signed-off-by: Patrick Steinhardt <ps@pks.im> --- reftable/block.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/reftable/block.c b/reftable/block.c index 1df3d8a0f0..44381ea6a3 100644 --- a/reftable/block.c +++ b/reftable/block.c @@ -342,8 +342,7 @@ int block_iter_next(struct block_iter *it, struct reftable_record *rec) return -1; string_view_consume(&in, n); - strbuf_reset(&it->last_key); - strbuf_addbuf(&it->last_key, &it->key); + strbuf_swap(&it->last_key, &it->key); it->next_off += start.len - in.len; return 0; }
-- 2.43.GIT