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
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@xxxxxxxxxxxx>
---
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realized that this comment was severely out-of-date.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(-)
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
