Re: [PATCH 1/6] fetch-pack: clear marks before everything_local()

[Jonathan Tan <jonathantanmy@google.com>]

On Tue, 5 Jun 2018 16:08:21 -0700
Jonathan Nieder <jrnieder@gmail.com> wrote:

> Hi,
> 
> Jonathan Tan wrote:
> 
> > If tag following is required when using a transport that does not
> > support tag following, fetch_pack() will be invoked twice in the same
> > process, necessitating a clearing of the object flags used by
> > fetch_pack() sometime during the second invocation. This is currently
> > done in find_common(), which means that the work done by
> > everything_local() in marking complete remote refs as COMMON_REF is
> > wasted.
> >
> > To avoid this wastage, move this clearing from find_common() to its
> > parent function do_fetch_pack(), right before it calls
> > everything_local().
> 
> I had to read this a few times and didn't end up understanding it.
> 
> Is the idea that this will speed something up?  Can you provide e.g.
> "perf stat" output (or even a new perf test in t/perf) demonstrating
> the improvement?  Or is it a cleanup?

Firstly, I don't know of a practical way to demonstrate this, because we
don't have an implementation of a transport that does not support tag
following. If we could demonstrate this, I think we can demonstrating it
by constructing a negotiation scenario in which COMMON_REF would have
been helpful, e.g. the following (untested) scenario:

 T C (T=tag, C=commit)
 |/
 O

We run "git fetch C" and on the second round (when we fetch T), if we
wiped the flags *after* everything_local() (as is currently done),
negotiation would send "have C" and "have O". But if we wiped the flags
*before* everything_local(), then C would have the COMMON_REF flag and
we will see that we only send "have C". So we have more efficient
negotiation.

> As an experiment, I tried applying the '-' part of the change without
> the '+' part to get confidence that tests cover this well.  To my
> chagrin, all tests still passed. :/

Yes, because we don't have tests against a transport which doesn't
support tag following.

> In the preimage, we call clear_marks in find_common.  This is right
> before we start setting up the revision walk, e.g. by inserting
> revisions for each ref.  In the postimage, we call clear_marks in
> do_fetch_pack.  This is right before we call everything_local.
> 
> I end up feeling that I don't understand the code well, neither before
> nor after the patch.  Ideas for making it clearer?

One idea is to first separate everything_local() into its side effect
parts and the "true" check that everything is local. I'll do that and
send it as part of v2 of this patch series.