Re: [PATCH 1/2] ref-filter: hacky "streaming" mode

[Jeff King <peff@peff.net>]

On Sun, Sep 05, 2021 at 04:20:02PM +0800, ZheNing Hu wrote:

> > +       if (ref_cbdata->filter->streaming_format) {
> > +               pretty_print_ref(refname, oid, ref_cbdata->filter->streaming_format);
> 
> So we directly use pretty_print_ref() in streaming mode, OK.
> 
> > +       } else {
> > +               /*
> > +                * We do not open the object yet; sort may only need refname
> > +                * to do its job and the resulting list may yet to be pruned
> > +                * by maxcount logic.
> > +                */
> > +               ref = ref_array_push(ref_cbdata->array, refname, oid);
> > +               ref->commit = commit;
> > +               ref->flag = flag;
> > +               ref->kind = kind;
> > +       }
> >
> >         return 0;
> >  }
> 
> Therefore, in streaming mode, there is no need to push ref to
> ref_array, which can
> reduce the overhead of malloc(), free(), which makes sense.

By the way, one thing I wondered here: how much of the benefit is from
avoiding the ref_array, and how much is from skipping the sort entirely.

It turns out that most of it is from the latter. If I do this:

diff --git a/builtin/for-each-ref.c b/builtin/for-each-ref.c
index 89cb6307d4..037d5db814 100644
--- a/builtin/for-each-ref.c
+++ b/builtin/for-each-ref.c
@@ -78,7 +78,11 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 	filter.name_patterns = argv;
 	filter.match_as_path = 1;
 	filter_refs(&array, &filter, FILTER_REFS_ALL | FILTER_REFS_INCLUDE_BROKEN);
-	ref_array_sort(sorting, &array);
+	/*
+	 * we should skip this only when we are using the default refname
+	 * sorting, but as an experimental hack, we'll just comment it out.
+	 */
+	// ref_array_sort(sorting, &array);
 
 	if (!maxcount || array.nr < maxcount)
 		maxcount = array.nr;

then the timings I get are:

  Benchmark #1: ./git.old for-each-ref --format='%(objectname) %(refname)'
    Time (mean ± σ):     341.4 ms ±   7.4 ms    [User: 299.8 ms, System: 41.6 ms]
    Range (min … max):   333.5 ms … 355.1 ms    10 runs
   
  Benchmark #2: ./git.new for-each-ref --format='%(objectname) %(refname)'
    Time (mean ± σ):     249.1 ms ±   5.7 ms    [User: 211.8 ms, System: 37.2 ms]
    Range (min … max):   245.9 ms … 267.0 ms    12 runs
   
  Summary
    './git.new for-each-ref --format='%(objectname) %(refname)'' ran
      1.37 ± 0.04 times faster than './git.old for-each-ref --format='%(objectname) %(refname)''

So of the 1.5x improvement that the original patch showed, 1.37x is from
skipping the sort of the already-sorted data. I suspect that has less to
do with sorting at all, and more to do with the fact that even just
formatting "%(refname)" for each entry takes a non-trivial amount of
time.

-Peff