Re: [PATCH v5] pack-objects mru

From: Jeff King <peff@peff.net>
Date: Aug 11, 2016, 16:19 UTC
Message-ID: <20160811161903.zfam5ietxpa2yyqm@sigill.intra.peff.net>
In-Reply-To: <xmqqmvkj2vdm.fsf@gitster.mtv.corp.google.com>

On Thu, Aug 11, 2016 at 08:11:33AM -0700, Junio C Hamano wrote:

Show 22 quoted lines

> Jeff King <peff@peff.net> writes:
> 
> > So considering "--depth" as a space-saving measure for --aggressive does
> > not seem that effective. But it feels weird to quietly drop actions
> > people might have done with previous aggressive runs.
> 
> That argument cuts both ways, doesn't it?
> 
> If the user explicitly asks to use lower "--depth" from the command
> line when the second repack runs, the intention is clear: the
> existing pack may use delta chains that are too long and is
> detrimental to the run-time performance, and the user wants to
> correct it by repacking with shorter delta chain.
> 
> Should the act of letting "gc --auto" use lower "--depth", by not
> configuring to always use deeper chain, be interpreted the same way?
> I am not sure.  The old packing with large --depth is something the
> user did long time ago, and the decision the user made not to use
> large depth always is also something the user did long time ago.  I
> do not think it is so cut-and-dried which one of the two conflicting
> wishes we should honor when running the second repack, especially
> when it is run unattended like "gc --auto" does.

Good points. Explicitly saying "repack --depth=..." carries a lot more weight to me than "git gc --auto" randomly kicking in, as far as knowing that what the user actually wants. My patch doesn't differentiate, of course, but I think it could.

The other problem with my patch is the fact that we don't do a good job of finding new, in-limit deltas for the ones we discard. If you want to do that, you really need to "git repack -f" (at least with the current code). At which point we do not reuse the on-disk deltas at all, and the problem is moot (you could also interpret the fact that the user did _not_ pass "-f" as "you want to reuse deltas, which means you want to reuse even long chains", but as you've argued above, you can make a lot of guesses about the user's intention from what they did or did not say).

So if we were to go this route, I don't think my patch is quite sufficient; we'd want something else on top to do a better job of finding replacement deltas.

Regarding my "does not seem that effective" above, I think we should drop the aggressive depth to 50, and I just posted a patch with reasoning and numbers:

  http://public-inbox.org/git/20160811161309.ecmebaafcz6rkg6o@sigill.intra.peff.net/

That's maybe orthogonal, but it does remove the weird "gc --aggressive followed by gc --auto produces a bad pack" issue, because unless you are doing something clever, the depth will always be 50 (modulo people who did an aggressive pack with an older version of git :-/ ).

-Peff