Re: [PATCH 2/2] pack-objects: prefetch objects to be packed

From: Junio C Hamano <gitster@pobox.com>
Date: Jul 21, 2020, 23:51 UTC
Message-ID: <xmqqzh7s5sv4.fsf@gitster.c.googlers.com>
In-Reply-To: <20200721232036.1358644-1-jonathantanmy@google.com>

Jonathan Tan <jonathantanmy@google.com> writes:

> My argument is that typically *no* objects are missing, so we should
> delay the prefetch as much as possible in the hope that we don't need it
> at all. I admit that if some objects are missing, I don't know where
> they will be in the to_pack list.

OK, if the common case we optimize for is where there is no object missing, then of course "prefetch missing ones upfront" will spend cycles to ensure all objects we will pack exist before going into the main loop, so the "delay as much as possible" optimization makes sense.

Show 15 quoted lines

>> In any case, the design choice needs to be justified in the log
>> message.  I am not sure if the lazy decision to prefetch at the last
>> moment is really worth the code smell.  Perhaps it is, if there is a
>> reason to believe that it would save extra work compared to the more
>> naive "if we have promissor remote, prefetch what is missing", but I
>> do not think I've heard that reason yet.
>
> I still think that there is a reason (the extra existence check), but if
> we think that the extra existence check is fast enough (compared to the
> other operations in pack-objects) or that there is a way to avoid
> calling oid_object_info_extended() twice for the same object (even with
> moving the prefetch loop to the beginning), then I agree that we don't
> need the lazy decision. (Or if we want to write the simpler code now and
> only improve the performance if we need it later, that's fine with me
> too.)

Now I finally have, I think, heard some of the reasoning behind the design decision made before this patch was written. That should be written in the proposed log message.

Thanks.