From: Derrick Stolee <derrickstolee@xxxxxxxxxx>
The 'skip_hash' option in 'struct hashfile' indicates that we want to
use the hashfile API as a buffered writer, and not use the hash function
to create a trailing hash. We still write a trailing null hash to
indicate that we do not have a checksum at the end. This feature is
enabled for index writes using the 'index.computeHash' config key.
Create a similar (currently hidden) option for the packed-refs v2 file
format: refs.hashPackedRefs. This defaults to false because performance
is compared to the packed-refs v1 file format which does have a checksum
anywhere.
This change results in improvements to p1401 when using a repository
with a 42 MB packed-refs file (600,000+ refs).
Test HEAD~1 HEAD
--------------------------------------------------------------------
1401.1: git pack-refs (v1) 0.38(0.31+0.52) 0.37(0.28+0.52) -2.6%
1401.5: git pack-refs (v2) 0.39(0.33+0.52) 0.30(0.28+0.46) -23.1%
Note that these tests update a ref and then repack the packed-refs file.
The following benchmarks are from a hyperfine experiment that only ran
the 'git pack-refs --all' command for the two formats, but also compared
the effect when refs.hashPackedRefs=true.
Benchmark 1: v1
Time (mean ± σ): 163.5 ms ± 18.1 ms [User: 117.8 ms, System: 38.1 ms]
Range (min … max): 131.3 ms … 190.4 ms 50 runs
Benchmark 2: v2-no-hash
Time (mean ± σ): 95.8 ms ± 15.1 ms [User: 72.5 ms, System: 23.0 ms]
Range (min … max): 82.9 ms … 131.2 ms 50 runs
Benchmark 3: v2-hashing
Time (mean ± σ): 100.8 ms ± 16.4 ms [User: 77.2 ms, System: 23.1 ms]
Range (min … max): 83.0 ms … 131.1 ms 50 runs
Summary
'v2-no-hash' ran
1.05 ± 0.24 times faster than 'v2-hashing'
1.71 ± 0.33 times faster than 'v1'
In this case of repeatedly rewriting the same refs seems to demonstrate
a smaller improvement than the p1401 test. However, the overall
reduction from v1 matches the expected reduction in file size. In my
tests, the 42 MB packed-refs (v1) file was compacted to 28 MB in the v2
format.
Signed-off-by: Derrick Stolee <derrickstolee@xxxxxxxxxx>
---
refs/packed-format-v2.c | 7 +++++++
t/perf/p1401-ref-operations.sh | 5 +++++
2 files changed, 12 insertions(+)
diff --git a/refs/packed-format-v2.c b/refs/packed-format-v2.c
index 2cd45a5987a..ada34bf9bf0 100644
--- a/refs/packed-format-v2.c
+++ b/refs/packed-format-v2.c
@@ -417,6 +417,7 @@ struct write_packed_refs_v2_context *create_v2_context(struct packed_ref_store *
struct strbuf *err)
{
struct write_packed_refs_v2_context *ctx;
+ int do_skip_hash;
CALLOC_ARRAY(ctx, 1);
ctx->refs = refs;
@@ -430,6 +431,12 @@ struct write_packed_refs_v2_context *create_v2_context(struct packed_ref_store *
}
ctx->f = hashfd(refs->tempfile->fd, refs->tempfile->filename.buf);
+
+ /* Default to true, so skip_hash if not set. */
+ if (git_config_get_maybe_bool("refs.hashpackedrefs", &do_skip_hash) ||
+ do_skip_hash)
+ ctx->f->skip_hash = 1;
+
ctx->cf = init_chunkfile(ctx->f);
return ctx;
diff --git a/t/perf/p1401-ref-operations.sh b/t/perf/p1401-ref-operations.sh
index 1c372ba0ee8..0b88a2f531a 100755
--- a/t/perf/p1401-ref-operations.sh
+++ b/t/perf/p1401-ref-operations.sh
@@ -36,6 +36,11 @@ test_perf 'git pack-refs (v2)' '
git pack-refs --all
'
+test_perf 'git pack-refs (v2;hashing)' '
+ git commit --allow-empty -m "change one ref" &&
+ git -c refs.hashPackedRefs=true pack-refs --all
+'
+
test_perf 'git for-each-ref (v2)' '
git for-each-ref --format="%(refname)" >/dev/null
'
--
gitgitgadget
