Re: [PATCH v2] pack-objects: fix performance issues on packing large deltas

[Duy Nguyen <pclouds@gmail.com>]

On Mon, Jul 23, 2018 at 8:04 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> Nguyễn Thái Ngọc Duy  <pclouds@gmail.com> writes:
>
> > Access to e->delta_size_ (and by extension
> > pack->delta_size[e - pack->objects]) is unprotected as before, the
> > thread scheduler in pack-objects must make sure "e" is never updated
> > by two different threads.
>
> OK.  Do we need to worry about "e" (e.g. "e->delta_size_valid")
> being accessed while/before it is set by another thread?

I left some details out because I think it's getting to a gray area.
We already do this

        if (!DELTA(trg_entry)) {
                max_size = trg_size/2 - the_hash_algo->rawsz;
                ref_depth = 1;
        } else {
                max_size = DELTA_SIZE(trg_entry);
        ...
        SET_DELTA(trg_entry, src_entry);
        SET_DELTA_SIZE(trg_entry, delta_size);

if the bottom half is running on one thread and stopped in the middle
while the top half is running in another thread, we have a problem.
But perhaps max_size is not that big a deal because incorrect max_size
may produce a bad pack but can't corrupt it.

I will have to study the thread dispatch code more to have a better
answer, unfortunately.
--
Duy