Subject: Re: rather slow 'git repack' in 'blob:none' partial clones

From: Rafael Silva <[email-masked]>
Date: Mon, 12 Apr 2021 09:09:46 +0200

Jeff King <[email-masked]> writes:

> On Mon, Apr 05, 2021 at 03:02:33AM +0200, Rafael Silva wrote:
>
>> I'm not entirely sure about this (not this late in the day), but it seems to
>> me that we should simply skip the "missing" (promisor) files when
>> operating on a partial clone.
>> 
>> Perhaps something like:
>> 
>> --- >8 ---
>> diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
>> index 525c2d8552..fedf58323d 100644
>> --- a/builtin/pack-objects.c
>> +++ b/builtin/pack-objects.c
>> @@ -3468,6 +3468,8 @@ static int loosened_object_can_be_discarded(const struct object_id *oid,
>>  {
>>         if (!unpack_unreachable_expiration)
>>                 return 0;
>> +       if (exclude_promisor_objects && is_promisor_object(oid))
>> +               return 1;
>>         if (mtime > unpack_unreachable_expiration)
>>                 return 0;
>>         if (oid_array_lookup(&recent_objects, oid) >= 0)
>> --- >8 ---
>
> you are avoiding writing out the file. But we should realize much
> earlier that it is not something we need to even consider loosening.
>
> In the loop in loosen_unused_packed_objects(), we skip packs that are
> marked as "keep", so we'd skip the new promisor pack entirely. But we'd
> still see all these objects in the _old_ promisor pack. However, for
> each object there, we call has_sha1_pack_kept_or_nonlocal(), so that
> would likewise realize that each object is already being kept in the
> other pack.
>

Agreed. Realizing sooner that we shouldn't even consider loosening the
objects from the packfile it's better solution.

> Something like this seems to work, but I only lightly tested it, and it
> could probably use some refactoring to make it less horrible:
>
> diff --git a/builtin/repack.c b/builtin/repack.c
> index fdee8e4578..457525953a 100644
> --- a/builtin/repack.c
> +++ b/builtin/repack.c
> @@ -574,6 +574,23 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
>  		repack_promisor_objects(&po_args, &names);
>  
>  		if (existing_packs.nr && delete_redundant) {
> +			/*
> +			 * tell pack-objects about our new promisor pack, which
> +			 * we will also be keeping
> +			 */
> +			for_each_string_list_item(item, &names) {
> +				/*
> +				 * yuck, we seem to only have the name with the
> +				 * packdir prefixed
> +				 */
> +				const char *prefix;
> +				if (!skip_prefix(packtmp, packdir, &prefix) ||
> +				    *prefix++ != '/')
> +					BUG("confused by packtmp");
> +				strvec_pushf(&cmd.args, "--keep-pack=%s-%s.pack",
> +					     prefix, item->string);
> +			}
> +
>  			if (unpack_unreachable) {
>  				strvec_pushf(&cmd.args,
>  					     "--unpack-unreachable=%s",
>
> Do you want to try to work with that?
>

Yes, I'll try to work with that, together with refactoring that you
mentioned in the code and the other replies.

Thanks for the suggestion.

>> A quick benchmark did show some promising result:
>> 
>>     # built from: 2e36527f23 (The sixth batch, 2021-04-02)
>>     Benchmark #1: ./bin-wrappers/git -C git.git gc
>>           Time (mean ± σ):     135.669 s ±  0.665 s    [User: 42.789 s, System: 91.332 s]
>>           Range (min … max):   134.905 s … 136.115 s    3 runs
>> 
>>     # built from: 2e36527f23 + minor patch (from above)
>>     Benchmark #2: ./bin-wrappers/git -C git.git gc
>>           Time (mean ± σ):     12.586 s ±  0.031 s    [User: 11.462 s, System: 1.365 s]
>>           Range (min … max):   12.553 s … 12.616 s    3 runs
>> 
>>     Summary:
>>           'Benchmark #2' ran 10.78 ± 0.06 times faster than 'Benchmark #1'
>
> It's still quite a bit slower than a non-partial clone because the
> traversal with --exclude-promisor-objects is slow. I think that's
> because it has to open up all of the objects in the promisor pack to see
> what they refer to. I don't know if we can do better (and it's largely
> an orthogonal problem to what you're solving here, so it probably makes
> sense to just punt on it for now).
>
> -Peff

Make sense.

-- 
Thanks
Rafael