Re: [PATCH v2] rev-list: exclude promisor objects at walk time

[Jeff King <peff@peff.net>]

On Thu, Apr 04, 2019 at 04:47:26PM -0700, Josh Steadmon wrote:

> > Did you (or anybody else) have any thoughts on the case where a given
> > object is referred to both by a promisor and a non-promisor (and we
> > don't have it)? That's the "shortcut" I think we're taking here: we
> > would no longer realize that it's available via the promisor when we
> > traverse to it from the non-promisor. I'm just not clear on whether that
> > can ever happen.
> 
> I am not sure either. In process_blob() and process_tree() there are
> additional checks for whether missing blobs/trees are promisor objects
> using is_promisor_object()...  but if we call that we undo the
> performance gains from this change.

Hmm. That might be a good outcome, though. If it never happens, we're
fast. If it does happen, then our worst case is that we fall back to the
current slower-but-more-thorough check. (And I think that happens with
your patch, without us having to do anything further).

> > One other possible small optimization: we don't look up the object
> > unless the caller asked to exclude promisors, which is good. But we
> > could also keep a single flag for "is there a promisor pack at all?".
> > When there isn't, we know there's no point in looking for the object.
> [...]
> I'm not necessarily opposed, but I'm leaning towards the "won't matter
> much" side.
> 
> Where would such a flag live, in this case, and who would be responsible
> for initializing it? I guess it would only matter for rev-list, so we
> could initialize it in cmd_rev_list() if --exclude-promisor-objects is
> passed?

The check is really something like:

  int have_promisor_pack() {
	for (p = packed_git; p; p = p->next) {
		if (p->pack_promisor)
			return 1;
	}
	return 0;
  }

That could be lazily cached as a single bit, but it would need to be
reset whenever we call reprepare_packed_git().

Let's just punt on it for now. I'm not convinced it would actually yield
any benefit, unless we have a partial-clone repo that doesn't have any
promisor packs (but then, I suspect whatever un-partial'd it should
probably be resetting the partial flag in the config).

> > I didn't see any tweaks to the callers, which makes sense; we're already
> > passing --exclude-promisor-objects as necessary. Which means by itself,
> > this patch should be making things faster, right? Do you have timings to
> > show that off?
> 
> Yeah, for a partial clone of a large-ish Android repo [1], we see the
> connectivity check go from >180s to ~7s.

Those are nice numbers. :) Worth mentioning in the commit message, I
think. How does it compare to your earlier patch? I'd hope they're about
the same.

-Peff