7048e74609
Our Bencher dashboards [1] have recently alerted us about a bunch of
performance regressions when writing references, specifically with the
reftable backend. There is a 3x regression when writing many refs with
preexisting refs in the reftable format, and a 10x regression when
migrating refs between backends in either of the formats.
Bisecting the issue lands us at 6ec4c0b45b (refs: don't store peeled
object IDs for invalid tags, 2025-10-23). The gist of the commit is that
we may end up storing peeled objects in both reftables and packed-refs
for corrupted tags, where the claimed tagged object type is different
than the actual tagged object type. This will then cause us to create
the `struct object *` with a wrong type, as well, and obviously nothing
good comes out of that.
The fix for this issue was to introduce a new flag to `peel_object()`
that causes us to verify the tagged object's type before writing it into
the refdb -- if the tag is corrupt, we skip writing the peeled value.
To verify whether the peeled value is correct we have to look up the
object type via the ODB and compare the actual type with the claimed
type, and that additional object lookup is costly.
This also explains why we see the regression only when writing refs with
the reftable backend, but we see the regression with both backends when
migrating refs:
- The reftable backend knows to store peeled values in the new table
immediately, so it has to try and peel each ref it's about to write
to the transaction. So the performance regression is visible for all
writes.
- The files backend only stores peeled values when writing the
packed-refs file, so it wouldn't hit the performance regression for
normal writes. But on ref migrations we know to write all new values
into the packed-refs file immediately, and that's why we see the
regression for both backends there.
Taking a step back though reveals an oddity in the new verification
logic: we not only verify the _tagged_ object's type, but we also verify
the type of the tag itself. But this isn't really needed, as we wouldn't
hit the bug in such a case anyway, as we only hit the issue with corrupt
tags claiming an invalid type for the tagged object.
The consequence of this is that we now started to look up the target
object of every single reference we're about to write, regardless of
whether it even is a tag or not. And that is of course quite costly.
Fix the issue by only verifying the type of the tagged objects. This
means that we of course still have a performance hit for actual tags.
But this only happens for writes anyway, and I'd claim it's preferable
to not store corrupted data in the refdb than to be fast here. Rename
the flag accordingly to clarify that we only verify the tagged object's
type.
This fix brings performance back to previous levels:
Benchmark 1: baseline
Time (mean ± σ): 46.0 ms ± 0.4 ms [User: 40.0 ms, System: 5.7 ms]
Range (min … max): 45.0 ms … 47.1 ms 54 runs
Benchmark 2: regression
Time (mean ± σ): 140.2 ms ± 1.3 ms [User: 77.5 ms, System: 60.5 ms]
Range (min … max): 138.0 ms … 142.7 ms 20 runs
Benchmark 3: fix
Time (mean ± σ): 46.2 ms ± 0.4 ms [User: 40.2 ms, System: 5.7 ms]
Range (min … max): 45.0 ms … 47.3 ms 55 runs
Summary
update-ref: baseline
1.00 ± 0.01 times faster than fix
3.05 ± 0.04 times faster than regression
[1]: https://bencher.dev/perf/git/plots
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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