Re: [PATCH] index-pack: reduce memory footprint a bit

Nguyễn Thái Ngọc Duy  [off-list ref] writes:

For each object in the input pack, we need one struct object_entry. On
x86-64, this struct is 64 bytes long. Although:

 - The 8 bytes for delta_depth and base_object_no are only useful when
   show_stat is set. And it's never set unless someone is debugging.

 - The three fields hdr_size, type and real_type take 4 bytes each
   even though they never use more than 4 bits.

By moving delta_depth and base_object_no out of struct object_entry
and make the other 3 fields one byte long instead of 4, we shrink 25%
of this struct.

On a 3.4M object repo that's about 53MB. The saving is less impressive
compared to index-pack total memory use (about 400MB before delta
resolving, so the saving is just 13%)

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
---
 I'm not sure if this patch is worth pursuing. It makes the code a
 little bit harder to read. I was just wondering how much memory could
 be saved..

I would say 13% is already impressive ;-).

I do not find the result all that harder to read.  I however think
that the change would make it a lot harder to maintain, especially
because the name "object-entry-extra" does not have any direct link
to "show-stat" to hint us that this must be allocated when show-stat
is in use and must never be looked at when show-stat is not in use.

Also it makes me wonder if the compilers are smart enough to notice
that the codepaths that access objects_extra[] are OK because they
are all inside "if (show_stat)".