Re: Something is broken in repack

[Nicolas Pitre <nico@cam.org>]

On Fri, 7 Dec 2007, Jon Smirl wrote:

> Using this config:
> [pack]
>         threads = 4
>         deltacachesize = 256M
>         deltacachelimit = 0

Since you have a different result according to the source pack used then 
those cache settings, even if there was a bug with them, are not 
significant.

> And the 330MB gcc pack for input
>  git repack -a -d -f  --depth=250 --window=250
> 
> complete seconds RAM
> 10%  47 1GB
> 20%  29 1Gb
> 30%  24 1Gb
> 40%  18 1GB
> 50%  110 1.2GB
> 60%  85 1.4GB
> 70%  195 1.5GB
> 80%  186 2.5GB
> 90%  489 3.8GB
> 95%  800 4.8GB
> I killed it because it started swapping
> 
> The mmaps are only about 400MB in this case.
> At the end the git process had 4.4GB of physical RAM allocated.

That's really bad.

> Starting from a highly compressed pack greatly aggravates the problem.

That is really interesting though.

> Starting with a 2GB pack of the same data my process size only grew to
> 3GB with 2GB of mmaps.

Which is quite reasonable, even if the same issue might still be there.

So the problem seems to be related to the pack access code and not the 
repack code.  And it must have something to do with the number of deltas 
being replayed.  And because the repack is attempting delta compression 
roughly from newest to oldest, and because old objects are typically in 
a deeper delta chain, then this might explain the logarithmic slowdown.

So something must be wrong with the delta cache in sha1_file.c somehow.


Nicolas