From: Jonathan Nieder <jrnieder@gmail.com>
To: Jeff King <peff@peff.net>
Cc: Daniel Koverman <dkoverman@predictiveTechnologies.com>,
Junio C Hamano <gitster@pobox.com>,
"git@vger.kernel.org" <git@vger.kernel.org>,
Nasser Grainawi <nasser@codeaurora.org>
Subject: Re: Why does send-pack call pack-objects for all remote refs?
Date: Mon, 14 Dec 2015 14:31:55 -0800 [thread overview]
Message-ID: <20151214223155.GA6594@google.com> (raw)
In-Reply-To: <20151214210429.GC14788@sigill.intra.peff.net>
Jeff King wrote:
> Hmm. I guess that makes sense. The bitmap we want is the set difference
> between the objects we are sending, and the tips the other side has. If
> we have a bitmap at each ref tip, that's very fast. But if you have a
> very large number of refs, we don't make one for each ref, and it has to
> fallback to walking to the nearest one (and it ends up worse than a
> regular walk, because it's filling in the bitmap for each tree, rather
> than just doing the "good enough" commit walk that we usually do).
>
> I suspect there's room for improvement in the way we select commits to
> store bitmaps for (so that the average walk is smaller). But it's rather
> tricky; there's not a single constant to change to make it work better.
Git gc and JGit GC differ here. JGit partitions the commits being
packed by branch and then runs a selection algorithm on each part.
Git runs a selection once on a list of all commits.
Some effects:
- JGit selects more bitmaps, so the gc takes longer and the resulting
bitmap file is larger (bad)
- JGit is more likely to have bitmaps for the commits involved in
pushes and fetches (good)
The commit selection code, for reference:
https://eclipse.googlesource.com/jgit/jgit/+/86af34e1/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriterBitmapPreparer.java#151
https://kernel.googlesource.com/pub/scm/git/git/+/ed1c9977/pack-bitmap-write.c#383
Thoughts?
Jonathan
next prev parent reply other threads:[~2015-12-14 22:32 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-12-07 21:02 Why does send-pack call pack-objects for all remote refs? Daniel Koverman
2015-12-07 22:41 ` Junio C Hamano
2015-12-07 22:57 ` Jeff King
2015-12-08 17:34 ` Daniel Koverman
2015-12-10 4:19 ` Jeff King
2015-12-12 4:15 ` Nasser Grainawi
2015-12-14 13:47 ` Daniel Koverman
2015-12-14 21:04 ` Jeff King
2015-12-14 22:31 ` Jonathan Nieder [this message]
2015-12-14 22:37 ` Jeff King
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