Re: [PATCH 1/5] upload-pack: look up "want" lines via commit-graph

[Derrick Stolee <derrickstolee@github.com>]

On 2/23/2022 7:35 AM, Patrick Steinhardt wrote:

> The following benchmark is executed in a repository with a huge number
> of references. It uses cached request from git-fetch(1) as input and
> contains about 876,000 "want" lines:
> 
>     Benchmark 1: git-upload-pack (HEAD~)
>       Time (mean ± σ):      7.113 s ±  0.028 s    [User: 6.900 s, System: 0.662 s]
>       Range (min … max):    7.072 s …  7.168 s    10 runs
> 
>     Benchmark 2: git-upload-pack (HEAD)
>       Time (mean ± σ):      6.622 s ±  0.061 s    [User: 6.452 s, System: 0.650 s]
>       Range (min … max):    6.535 s …  6.727 s    10 runs
> 
>     Summary
>       'git-upload-pack (HEAD)' ran
>         1.07 ± 0.01 times faster than 'git-upload-pack (HEAD~)'

Nice!

> -		o = parse_object(the_repository, &oid);
> +		commit = lookup_commit_in_graph(the_repository, &oid);
> +		if (commit)
> +			o = &commit->object;
> +		else
> +			o = parse_object(the_repository, &oid);
> +

This is a neat trick. I see that we've also done this trick in
revision.c:get_reference(). Perhaps it is worth creating a helper,
maybe named parse_probably_commit()?

>  		if (!o) {
>  			packet_writer_error(writer,
>  					    "upload-pack: not our ref %s",
> @@ -1434,7 +1440,7 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
>  	if (skip_prefix(line, "want-ref ", &refname_nons)) {
>  		struct object_id oid;
>  		struct string_list_item *item;
> -		struct object *o;
> +		struct object *o = NULL;
>  		struct strbuf refname = STRBUF_INIT;
>  
>  		strbuf_addf(&refname, "%s%s", get_git_namespace(), refname_nons);
> @@ -1448,7 +1454,15 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
>  		item = string_list_append(wanted_refs, refname_nons);
>  		item->util = oiddup(&oid);
>  
> -		o = parse_object_or_die(&oid, refname_nons);
> +		if (!starts_with(refname_nons, "refs/tags/")) {
> +			struct commit *commit = lookup_commit_in_graph(the_repository, &oid);
> +			if (commit)
> +				o = &commit->object;
> +		}
> +
> +		if (!o)
> +			o = parse_object_or_die(&oid, refname_nons);
> +

Even here, we _could_ use a parse_probably_commit() helper
inside the if (!starts_with(...)) block, even though we would
still need the if (!o) check later.

Thanks,
-Stolee