Re: [PATCH] pack-objects.h: remove outdated pahole results

["brian m. carlson" <sandals@crustytoothpaste.net>]

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On 2022-06-28 at 18:30:20, Taylor Blau wrote:
> The size and padding of `struct object_entry` is an important factor in
> determining the memory usage of `pack-objects`. For this reason,
> 3b13a5f263 (pack-objects: reorder members to shrink struct object_entry,
> 2018-04-14) added a comment containing some information from pahole
> indicating the size and padding of that struct.
> 
> Unfortunately, this comment hasn't been updated since 9ac3f0e5b3
> (pack-objects: fix performance issues on packing large deltas,
> 2018-07-22), despite the size of this struct changing many times since
> that commit.
> 
> To see just how often the size of object_entry changes, I skimmed the
> first-parent history with this script:
> 
>     for sha in $(git rev-list --first-parent --reverse 9ac3f0e..)
>     do
>       echo -n "$sha "
>       git checkout -q $sha
>       make -s pack-objects.o 2>/dev/null
>       pahole -C object_entry pack-objects.o | sed -n \
>         -e 's/\/\* size: \([0-9]*\).*/size \1/p' \
>         -e 's/\/\*.*padding: \([0-9]*\).*/padding \1/p' | xargs
>     done | uniq -f1
> 
> In between each merge, the size of object_entry changes too often to
> record every instance here. But the important merges (along with their
> corresponding sizes and bit paddings) in chronological order are:
> 
>     ad635e82d6 (Merge branch 'nd/pack-objects-pack-struct', 2018-05-23) size 80 padding 4
>     29d9e3e2c4 (Merge branch 'nd/pack-deltify-regression-fix', 2018-08-22) size 80 padding 9
>     3ebdef2e1b (Merge branch 'jk/pack-delta-reuse-with-bitmap', 2018-09-17) size 80 padding 8
>     33e4ae9c50 (Merge branch 'bc/sha-256', 2019-01-29) size 96 padding 8
> 
> (indicating that the current size of the struct is 96 bytes, with 8
> padding bits).
> 
> Even though this comment was written in a good spirit, it is updated
> infrequently enough that is serves to confuse rather than to encourage

I think you wanted to say, "that it serves:.

> contributors to update the appropriate values when the modify the
> definition of object_entry.
> 
> For that reason, eliminate the confusion by removing the comment
> altogether.
> 
> Signed-off-by: Taylor Blau <me@ttaylorr.com>

I agree with your rationale and that we should remove this.
-- 
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA

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