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Re: Delta directions (was Re: delta combiner sloooooooowwwwww)

From: Daniel Berlin <dberlin_at_dberlin.org>
Date: 2005-02-12 20:07:14 CET

On Sat, 2005-02-12 at 03:45 +0100, Branko ibej wrote:
> Greg Hudson wrote:
> >I hadn't read part of the "make blame even faster" thread when I sent
> >this. Having read it, I have one addendum:
> >
> >On Thu, 2005-02-10 at 13:08, Greg Hudson wrote:
> >
> >
> >>(Obviously not before 2.0, but I'm not sure if we can
> >>realize any of these benefits before 2.0.)
> >>
> >>
> >
> >As people pointed out in that other thread, we can realize the "faster
> >delta combiner" benefit before 2.0, and it may be worth changing to
> >xdelta in 1.2 in order to do so. My biggest concern is that FSFS
> >repositories with very shallow file histories may become significantly
> >larger, since we will lose all self-compression. Perhaps we can
> >continue using vdelta for self-compression in FSFS? I'm not sure how
> >much of the speed win (if any) we'd be giving up by doing that.
> >
> >
> If we use vdelta strictly for self-compression of the first revision of
> a file in FSFS, it shouldn't be a problem. That delta would always be
> the last in any chain of combinations, but because the self-compressed
> vdelta doesn't contain source copy ops, the combiner should behave nicely.

I actually tried this.
We still get a very large amount of copy_source_ops time according to
oprofile (though a factor of 5 or 10 less than you do with vdelta every

This was with vdelta only used when source_len == 0

I tested this with two repos. One where vdelta was enabled if source_len
== 0,and one where vdelta was not enabled for sotring to the repo, but
was for compressing fulltexts that went over the wire (through a small
hack to add an argument to compose_window, and disable vdelta for the
tpush_* call to compose_window, which is only used by fsfs when storing
to the repo).

I verified that vdelta was happening only during over-the-wire transmits
by aborting if it triggered vdelta, doing a repository load to make sure
it didn't happen during that, and then making sure it *did* abort when i
did a checkout or something that was going to send fulltexts.

No clue why the combiner still behaves badly but i can provide you with
repos and patches to reproduce.


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Received on Sat Feb 12 20:08:31 2005

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