Re: [PATCH 2/2] index-pack: prefetch missing REF_DELTA bases

From: Jonathan Tan <jonathantanmy@google.com>
To: peff@peff.net
Cc: jonathantanmy@google.com, git@vger.kernel.org
Subject: Re: [PATCH 2/2] index-pack: prefetch missing REF_DELTA bases
Date: Thu, 16 May 2019 16:15:09 -0700	[thread overview]
Message-ID: <20190516231509.253998-1-jonathantanmy@google.com> (raw)
In-Reply-To: <20190516214257.GD10787@sigill.intra.peff.net>

> > > Right, REF_DELTA is definitely correctly handled currently, and I don't
> > > think that would break with your patch. It's just that your patch would
> > > introduce a bunch of extra traffic as we request bases separately that
> > > are already in the pack.
> > 
> > Ah...I see. For this problem, I think that it can be solved with the
> > "if (objects[d->obj_no].real_type != OBJ_REF_DELTA)" check that the
> > existing code uses before calling read_object(). I'll include this in
> > the next reroll if any other issue comes up.
> 
> I'm confused about this. Aren't we pre-fetching before we've actually
> resolved deltas? The base could be in the pack as a true base, and we
> might have seen it already then. But it could itself be a delta, and we
> wouldn't know we have it until we resolve it (this gets into the
> lucky/unlucky ordering thing).

resolve_deltas(), invoked before any new code introduced in this patch,
has this comment:

> /*
>  * Second pass:
>  * - for all non-delta objects, look if it is used as a base for
>  *   deltas;
>  * - if used as a base, uncompress the object and apply all deltas,
>  *   recursively checking if the resulting object is used as a base
>  *   for some more deltas.
>  */

I haven't seen any code that contradicts this comment. And looking at
the code, for each non-delta object, I think that all deltas are checked
- regardless of whether they appear before or after that non-delta
object. (find_ref_delta() does a binary search from 0 to
nr_ref_deltas, calculated in parse_pack_objects() which happens before
any resolution of deltas.)

And find_unresolved_deltas_1() (called from resolve_deltas() indirectly)
sets the real_type when it resolves a delta, as far as I can tell.

So there is more than one "resolve deltas" step - resolve_deltas() and
then fix_unresolved_deltas(). The pre-fetching happens only during the
latter.