Re: [PATCH v2 2/4] pack-objects: support reachability bitmaps with `--path-walk`

[Derrick Stolee <stolee@gmail.com>]

On 6/19/2026 10:52 AM, Taylor Blau wrote:
> On Fri, Jun 19, 2026 at 10:40:51AM -0400, Derrick Stolee wrote:
>>> [...]
>>> , which gives us:
>>>
>>>     Test                                            HEAD^             HEAD
>>>     ----------------------------------------------------------------------------------------
>>>     5311.3: size of bitmapped pack                           278.8M            278.8M -0.0%
>>>     5311.38: size of bitmapped pack (--path-walk)            278.7M            278.7M +0.0%
>>>
>>> (eliding other tests). I considered whether there are other interesting
>>> tests, but I think "repack" is the right layer to run perf tests, since
>>> you're always writing a closed pack. We could try different subsets of
>>> the repository's objects (which would also have to be closed), but I
>>> don't think this is that interesting.
>>
>> This sort of thing does help to show that we're getting different
>> behavior when repacking with and without --path-walk. And this test
>> is showing the slightest change for git.git, but is likely more
>> impactful for the other repos I've used to demonstrate the benefits.
>>
>> So this is the kind of data I'm hoping to see, but also with data
>> from other repos whose data shapes benefit from --path-walk more
>> than git.git and repos where name-hash v1 is sufficient to give a
>> similar result.
> 
> I'm glad this is the sort of data you're looking for. I'm happy to run
> this on other repositories.
> 
>> I'd also like to see if the repack _time_ changes with this, but
>> these direct size comparisons are the biggest indicator I'd like to
>> see.
> 
> Unfortunately a timing comparison is kind of a pain here. We'd have to
> use test_perf, which will perform the same repack multiple times. We
> could do that, though it's wasteful, and changes like bf4a60874af
> (p5326: generate pack bitmaps before writing the MIDX bitmap,
> 2021-09-17) move us in the opposite direction.
> 
> I'm not opposed to changing this to test_perf if you feel strongly about
> it.
Repacking is expensive and time-consuming. I care a bit about it,
but not as much as I care about the size difference. Feel free to
skip the time performance impact for now.

Thanks,
-Stolee