Subject: Re: [PATCH] commit-graph: don't show progress percentages while expanding reachable commits

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Fri, 22 Mar 2019 18:23:03 +0100

On Fri, Mar 22 2019, SZEDER Gábor wrote:

> On Fri, Mar 22, 2019 at 04:11:24PM +0100, Ævar Arnfjörð Bjarmason wrote:
>>
>> On Fri, Mar 22 2019, SZEDER Gábor wrote:
>>
>> > On Fri, Mar 22, 2019 at 03:28:34PM +0100, Ævar Arnfjörð Bjarmason wrote:
>> >>
>> >> On Fri, Mar 22 2019, SZEDER Gábor wrote:
>> >>
>> >> > On Fri, Mar 22, 2019 at 12:11:26PM +0100, Ævar Arnfjörð Bjarmason wrote:
>> >> >>
>> >> >> On Fri, Mar 22 2019, SZEDER Gábor wrote:
>> >> >>
>> >> >> > Commit 49bbc57a57 (commit-graph write: emit a percentage for all
>> >> >> > progress, 2019-01-19) was a bit overeager when it added progress
>> >> >> > percentages to the "Expanding reachable commits in commit graph" phase
>> >> >> > as well, because most of the time the number of commits that phase has
>> >> >> > to iterate over is not known in advance and grows significantly, and,
>> >> >> > consequently, we end up with nonsensical numbers:
>> >> >> >
>> >> >> >   $ git commit-graph write --reachable
>> >> >> >   Expanding reachable commits in commit graph: 138606% (824706/595), done.
>> >> >> >   [...]
>> >> >> >
>> >> >> >   $ git rev-parse v5.0 | git commit-graph write --stdin-commits
>> >> >> >   Expanding reachable commits in commit graph: 81264400% (812644/1), done.
>> >> >> >   [...]
>> >> >> >
>> >> >> > Therefore, don't show progress percentages in the "Expanding reachable
>> >> >> > commits in commit graph" phase.
>> >> >>
>> >> >> There's indeed a bug here as your examples show, but there *are* cases
>> >> >> where it's correct, as the commit message for my patch on "master" shows
>> >> >> there's cases where we correctly.
>> >> >>
>> >> >> So this "fixes" things by always removing the progress, why not instead
>> >> >> pass down the state to close_reachable() about what we're walking over,
>> >> >> so we can always show progress, or at least in some cases?
>> >> >
>> >> > The cases where it does display correct percentages are exceptional,
>> >> > and doesn't worth the effort to try to find out whether ther current
>> >> > operation happens to be such a case.
>> >>
>> >> It's the "write" entry point without arguments that displays the correct
>> >> progress. So not exceptional, but yeah, it's not what we use on "gc".
>> >
>> > Bit it displays the correct number only if all the reachable commits
>> > are in packfiles, which is not necessarily the case (e.g. unpacked
>> > small packs during 'git fetch').
>>
>> No, argument-less "write" only considers packed commits.
>
> No, it considers packed commits as starting points, and then expands
> to all reachable commits, that's what that loop in question is about.
>
>   $ git init
>   Initialized empty Git repository in /tmp/test/.git/
>   $ echo >file
>   $ git add file
>   $ git commit -q -m initial
>   $ echo 1 >file
>   $ git commit -q -m 1 file
>   $ git rev-parse HEAD | git pack-objects
>   .git/objects/pack/pack
>   Enumerating objects: 1, done.
>   Counting objects: 100% (1/1), done.
>   ece3ff72952af2b28e048fa5c58db88c44312876
>   Writing objects: 100% (1/1), done.
>   Total 1 (delta 0), reused 0 (delta 0)
>   $ git commit-graph write
>   Computing commit graph generation numbers: 100% (2/2), done.

Ah, indeed. I think in practice it'll be unlikely to happen in practice
except on servers due to receive.unpackLimit, and then it won't be off
by much.

So I think it's best to do something like my WIP patch upthread + the
"snap to 100%" at the end behavior. I'll try to clean that up / test it
/ submit that sometime soon.

>> >> In any case, the problem is that sometimes we've walked the full set of
>> >> commits already, and some other times we haven't.
>> >
>> > ... and that we can't really be sure whether we've walked the full set
>> > of commits until after this loop.
>>
>> I'm fairly sure we can when we start with a full walk. See my
>> explanation in <[email-masked]>, but I may have
>> missed something.
>>
>> >> So in cases where we have we can show progress, and as a TODO (I think
>> >> this came up in previous discussions), we could do better if we had a
>> >> approximate_commit_count().
>> >>
>> >> In any case, the below fix seems correct to me, but I haven't poked it
>> >> much. It *does* suffer from a theoretical race with the progress bar
>> >> similar to d9b1b309cf ("commit-graph write: show progress for object
>> >> search", 2019-01-19), but I work around it in the same way:
>> >>
>> >> diff --git a/commit-graph.c b/commit-graph.c
>> >> index 47e9be0a3a..0fab3d8b2b 100644
>> >> --- a/commit-graph.c
>> >> +++ b/commit-graph.c
>> >> @@ -693,7 +693,8 @@ static void add_missing_parents(struct packed_oid_list *oids, struct commit *com
>> >>  	}
>> >>  }
>> >>
>> >> -static void close_reachable(struct packed_oid_list *oids, int report_progress)
>> >> +static void close_reachable(struct packed_oid_list *oids, int report_progress,
>> >> +			    uint64_t oids_count_for_progress)
>> >>  {
>> >>  	int i;
>> >>  	struct commit *commit;
>> >> @@ -717,7 +718,8 @@ static void close_reachable(struct packed_oid_list *oids, int report_progress)
>> >>  	 */
>> >>  	if (report_progress)
>> >>  		progress = start_delayed_progress(
>> >> -			_("Expanding reachable commits in commit graph"), oids->nr);
>> >> +			_("Expanding reachable commits in commit graph"),
>> >> +			oids_count_for_progress);
>> >>  	for (i = 0; i < oids->nr; i++) {
>> >>  		display_progress(progress, i + 1);
>> >>  		commit = lookup_commit(the_repository, &oids->list[i]);
>> >> @@ -725,6 +727,8 @@ static void close_reachable(struct packed_oid_list *oids, int report_progress)
>> >>  		if (commit && !parse_commit(commit))
>> >>  			add_missing_parents(oids, commit);
>> >>  	}
>> >> +	if (oids->nr < oids_count_for_progress)
>> >> +		display_progress(progress, oids_count_for_progress);
>> >>  	stop_progress(&progress);
>> >>
>> >>  	if (report_progress)
>> >> @@ -829,6 +833,7 @@ void write_commit_graph(const char *obj_dir,
>> >>  	uint64_t progress_cnt = 0;
>> >>  	struct strbuf progress_title = STRBUF_INIT;
>> >>  	unsigned long approx_nr_objects;
>> >> +	uint64_t oids_count_for_progress = 0;
>> >>
>> >>  	if (!commit_graph_compatible(the_repository))
>> >>  		return;
>> >> @@ -934,9 +939,10 @@ void write_commit_graph(const char *obj_dir,
>> >>  		if (oids.progress_done < approx_nr_objects)
>> >>  			display_progress(oids.progress, approx_nr_objects);
>> >>  		stop_progress(&oids.progress);
>> >> +		oids_count_for_progress = oids.nr;
>> >>  	}
>> >>
>> >> -	close_reachable(&oids, report_progress);
>> >> +	close_reachable(&oids, report_progress, oids_count_for_progress);
>> >>
>> >>  	if (report_progress)
>> >>  		progress = start_delayed_progress(
>> >>