From: "brian m. carlson" <sandals@crustytoothpaste.net>
To: Taylor Blau <me@ttaylorr.com>
Cc: git@vger.kernel.org, gitster@pobox.com, pclouds@gmail.com
Subject: Re: [PATCH] pack-objects.h: remove outdated pahole results
Date: Tue, 28 Jun 2022 21:04:27 +0000 [thread overview]
Message-ID: <Yrts22oGOS/igqH/@tapette.crustytoothpaste.net> (raw)
In-Reply-To: <1379af2e9d271b501ef3942398e7f159a9c77973.1656440978.git.me@ttaylorr.com>
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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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next prev parent reply other threads:[~2022-06-28 21:04 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-28 18:30 [PATCH] pack-objects.h: remove outdated pahole results Taylor Blau
2022-06-28 20:03 ` Derrick Stolee
2022-06-28 21:04 ` brian m. carlson [this message]
2022-06-28 21:26 ` Taylor Blau
2022-07-01 18:16 ` Jeff King
2022-07-01 19:48 ` Taylor Blau
2022-07-01 21:05 ` Jeff King
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