From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: Jonathan Tan <jonathantanmy@google.com>,
christian.couder@gmail.com, git@vger.kernel.org,
chriscool@tuxfamily.org, ramsay@ramsayjones.plus.com
Subject: Re: [RFC PATCH 10/10] pack-objects: improve partial packfile reuse
Date: Sat, 12 Oct 2019 09:04:58 +0900 [thread overview]
Message-ID: <xmqqsgnyg76d.fsf@gitster-ct.c.googlers.com> (raw)
In-Reply-To: <20191011180125.GA20601@sigill.intra.peff.net> (Jeff King's message of "Fri, 11 Oct 2019 14:01:25 -0400")
Jeff King <peff@peff.net> writes:
> The current code does so by creating a new entry in the reused_chunks
> array. In the worst case that can grow to have the same number of
> entries as we have objects. So this code was an attempt to pad the
> header of a shrunken entry to keep it the same size. I don't remember
> all the problems we ran into with that, but in the end we found that it
> didn't actually help much, because in practice we don't end up with a
> lot of chunks anyway.
Hmm, I am kind of surprised that the decoding side allowed such a
padding.
> For instance, packing torvalds/linux on our servers just now reused 6.5M
> objects, but only needed ~50k chunks.
OK, that's a good argument for not trying to optimize.
> I think the original code may simply have been buggy and nobody noticed.
> Here's what I wrote when this line was added in our fork:
>
> pack-objects: check reused pack bitmap for duplicate objects
>
> If a client both asks for a tag by sha1 and specifies
> "include-tag", we may end up including the tag in the reuse
> bitmap (due to the first thing), and then later adding it to
> the packlist (due to the second). This results in duplicate
> objects in the pack, which git chokes on. We should notice
> that we are already including it when doing the include-tag
> portion, and avoid adding it to the packlist.
>
> The simplest place to fix this is right in add_ref_tag,
> where we could avoid peeling the tag at all if we know that
> we are already including it. However, I've pushed the check
> instead into have_duplicate_entry. This fixes not only this
> case, but also means that we cannot have any similar
> problems lurking in other code.
>
> No tests, because git does not actually exhibit this "ask
> for it and also include-tag" behavior. We do one or the
> other on clone, depending on whether --single-branch is set.
> However, libgit2 does both.
>
> I'm not sure why we didn't notice it with the older reuse code. It may
> simply have been that it hardly ever triggered on our servers.
Impressed by the careful thinking here.
Thanks.
next prev parent reply other threads:[~2019-10-12 0:07 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-13 13:02 [RFC PATCH 00/10] Rewrite packfile reuse code Christian Couder
2019-09-13 13:02 ` [RFC PATCH 01/10] builtin/pack-objects: report reused packfile objects Christian Couder
2019-09-13 13:02 ` [RFC PATCH 02/10] packfile: expose get_delta_base() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 03/10] ewah/bitmap: introduce bitmap_word_alloc() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 04/10] ewah/bitmap: always allocate 2 more words Christian Couder
2019-10-10 23:40 ` Jonathan Tan
2019-10-11 7:49 ` Christian Couder
2019-10-11 18:05 ` Jeff King
2019-09-13 13:02 ` [RFC PATCH 05/10] pack-bitmap: don't rely on bitmap_git->reuse_objects Christian Couder
2019-10-10 23:44 ` Jonathan Tan
2019-10-11 7:50 ` Christian Couder
2019-09-13 13:02 ` [RFC PATCH 06/10] pack-bitmap: introduce bitmap_walk_contains() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 07/10] csum-file: introduce hashfile_total() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 08/10] pack-objects: introduce pack.allowPackReuse Christian Couder
2019-09-13 21:37 ` Junio C Hamano
2019-09-13 13:02 ` [RFC PATCH 09/10] builtin/pack-objects: introduce obj_is_packed() Christian Couder
2019-09-13 13:02 ` [RFC PATCH 10/10] pack-objects: improve partial packfile reuse Christian Couder
2019-09-13 22:29 ` Junio C Hamano
2019-09-14 2:02 ` Jeff King
2019-09-14 3:06 ` Junio C Hamano
2019-10-02 15:57 ` Jeff King
2019-10-03 2:06 ` Junio C Hamano
2019-10-03 6:55 ` Christian Couder
2019-10-10 23:59 ` Jonathan Tan
2019-10-11 7:39 ` Christian Couder
2019-10-11 18:01 ` Jeff King
2019-10-11 21:04 ` Jonathan Tan
2019-10-12 0:04 ` Junio C Hamano [this message]
2019-10-13 7:38 ` Jeff King
2019-10-17 7:03 ` Junio C Hamano
2019-10-17 7:23 ` Jeff King
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