Re: [PATCH v2 14/14] commit.h: delete 'util' field in struct commit

[Duy Nguyen <pclouds@gmail.com>]

On Mon, May 14, 2018 at 8:14 PM, Derrick Stolee <stolee@gmail.com> wrote:
> I disagree with the removal of "generation". My intention is to make the
> commit-graph feature a standard feature that most repos (of reasonable size)
> want to have. In that case, 'graph_pos' and 'generation' will be set during
> every parse and used by every commit walk. This is just my gut reaction.

My 'often used' is a poor choice of words. The problem here is memory
consumption in full-repo walk like 'git clone' or 'git gc', where more
memory use may equal more swapping and that will slow things down.

commit-slab also has a few advantage over packing everything in struct
commit besides the performance argument. It makes it easier to see
what field is used for what. And if an operation only uses a field in
a short time, then it's possible to free data after it's done
(impossible to free these struct commits without a lot more thinking
and checking; whatever you add here will stay until the end of the
process, which could be a long time for things like pack-objects)

> As I investigate these changes, I'll try to see what performance hit is
> caused by converting the graph_pos and/or generation to commit slabs.
> (Again, I'm assuming the slabs will make this slower. I may be wrong here.)

The slab allocation code is very similar to memory-pool, which was
made common code also by Microsoft to make reading cache faster so I
don't think it will slow things down for you (again, guessing, no real
measurement from my side). This does make me think it may be a good
idea to try unify commit-slab and memory-pool, let commit-slab be a
thin layer on top of memory-pool or something like that (the fewer
custom mem allocators we have, the better)
-- 
Duy