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

[Duy Nguyen <pclouds@gmail.com>]

On Fri, Jul 20, 2018 at 7:40 PM Jeff King <peff@peff.net> wrote:
>
> On Fri, Jul 20, 2018 at 05:39:43PM +0200, Nguyễn Thái Ngọc Duy wrote:
>
> > Let's start with some background about oe_delta_size() and
> > oe_set_delta_size(). If you already know, skip the next paragraph.
> >
> > These two are added in 0aca34e826 (pack-objects: shrink delta_size
> > field in struct object_entry - 2018-04-14) to help reduce 'struct
> > object_entry' size. The delta size field in this struct is reduced to
> > only contain max 2MB. So if any new delta is produced and larger than
> > 2MB, it's dropped because we can't really save such a large size
> > anywhere. Fallback is provided in case existingpackfiles already have
> > large deltas, then we can retrieve it from the pack.
>
> Minor nit, but isn't this 1MB (it was 2MB after one of your patches, but
> I think v2.18.0 has 20 bits)?

Argh.. I think I thought "2 ** 20" in my mind then typed "2 << 20" in
python. And I thought I made a mistake in my previous commit message
because it does mention 1MB...

> > With this, we do not have to drop deltas in try_delta() anymore. Of
> > course the downside is we use slightly more memory, even compared to
> > 2.17.0. But since this is considered an uncommon case, a bit more
> > memory consumption should not be a problem.
>
> I wondered how common this might be. The easiest way to see the largest
> delta sizes is:
>
>   git cat-file --batch-all-objects \
>                --batch-check='%(objectsize:disk) %(deltabase)' |
>   grep -v 0000000000000000000000000000000000000000 |
>   sort -rn | head
>
> The biggest one in the kernel is ~300k. Which is about what I'd expect
> for a normal source code repo. Even some private repos I have with a lot
> of binary artifacts top out at about 3MB. So the new 32MB is probably

I'll keep these numbers in v2 commit message, easier to find later.

> > [1] With a small tweak. 2.17.0 on 64-bit linux can hold 2^64 byte
> >     deltas, which is absolutely insane. But windows builds, even
> >     64-bit version, only hold 2^32. So reducing it to 2^32 should be
> >     quite safe.
>
> I'm not sure I completely agree with this. While 4GB deltas should be
> pretty rare, the nice thing about 64-bit is that you never have to even
> think about whether the variable is large enough. I think the 2^32
> limitations on Windows are something we should be fixing in the long
> term (though there it is even worse because it is not just deltas, but
> entire objects).

I guess that means we stick to uint64_t then? It does increase more
memory on 32-bit architecture (and probably processing cost as well
because 64-bit uses up 2 registers).

> > @@ -2278,6 +2274,8 @@ static void init_threaded_search(void)
> >       pthread_mutex_init(&cache_mutex, NULL);
> >       pthread_mutex_init(&progress_mutex, NULL);
> >       pthread_cond_init(&progress_cond, NULL);
> > +     pthread_mutex_init(&to_pack.lock, NULL);
> > +     to_pack.lock_initialized = 1;
> >       old_try_to_free_routine = set_try_to_free_routine(try_to_free_from_threads);
> >  }
>
> This is new in this iteration. I guess this is to cover the case where
> we are built with pthread support, but --threads=1?

If you mean the "lock_initialized" variable, not really. the other
_lock() macros in builtin/ call pthread_mutex_lock() unconditionally,
which is fine. But I feel a bit uncomfortable doing the same in
pack-objects.h which technically is library code (but yes practically
is a long arm of builtin/pack-objects.c), so lock_initialized is there
to make sure we don't touch uninitialized locks if somebody forgets to
init them first.

> Given that we no longer have to touch this lock during the realloc, is
> it worth actually putting it into to_pack? Instead, we could keep it
> local to pack-objects, alongside all the other locks (and use the
> lock_mutex() helper which handles the single-thread case).

You probably notice the lock name is not "delta_size_lock". I intended
to reuse this for locking other fields in struct packing_data as well.
But that might be a bad idea.

I have no strong opinion about this, so if we still end up locking the
whole functions, I'll just move the lock back close to the others in
builtin/pack-objects.c

> Your original patch had to copy the oe_* helpers into the file to handle
> that. But I think we're essentially just locking the whole functions:

I'll try to avoid this lock when deltas are small and see if it helps
the linux.git case on Elijah's machine. So we may end up locking just
a part of these functions.
-- 
Duy