Taylor BlauThe 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 -f1In 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 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> --- Noticed this while reverting an old topic out of GitHub's fork, and realized that this comment was severely out-of-date.
pack-objects.h | 10 ---------- 1 file changed, 10 deletions(-)
Show changes to pack-objects.h +0 −11
diff --git a/pack-objects.h b/pack-objects.h index 393b9db546..579476687c 100644 --- a/pack-objects.h +++ b/pack-objects.h @@ -116,16 +116,6 @@ struct object_entry { unsigned dfs_state:OE_DFS_STATE_BITS; unsigned depth:OE_DEPTH_BITS; unsigned ext_base:1; /* delta_idx points outside packlist */ - - /* - * pahole results on 64-bit linux (gcc and clang) - * - * size: 80, bit_padding: 9 bits - * - * and on 32-bit (gcc) - * - * size: 76, bit_padding: 9 bits - */ }; struct packing_data { -- 2.37.0.1.g1379af2e9d
