Re: pack operation is thrashing my server

From: Shawn O. Pearce <spearce@spearce.org>
Date: Aug 13, 2008, 15:04 UTC
Message-ID: <20080813150425.GC3782@spearce.org>
In-Reply-To: <m363q5t152.fsf@localhost.localdomain>

Jakub Narebski <jnareb@gmail.com> wrote:

Show 12 quoted lines

> Nicolas Pitre <nico@cam.org> writes:
> > On Tue, 12 Aug 2008, Geert Bosch wrote:
> > 
> > > One nice optimization we could do for those pesky binary large objects
> > > (like PDF, JPG and GZIP-ed data), is to detect such files and revert
> > > to compression level 0. This should be especially beneficial
> > > since already compressed data takes most time to compress again.
> > 
> > That would be a good thing indeed.
> 
> Perhaps take a sample of some given size and calculate entropy in it?
> Or just simply add gitattribute for per file compression ratio...

Estimating the entropy would make it "just magic". Most of Git is "just magic" so that's a good direction to take. I'm not familiar enough with the PDF/JPG/GZIP/ZIP stream formats to know what the first 4-8k looks like to know if it would give a good indication of being already compressed.

Though I'd imagine looking at the first 4k should be sufficient for any compressed file. Having a header composed of 4k of _text_ before binary compressed data would be nuts. Or a git-bundle with a large refs listing. ;-)

Using a gitattribute inside of pack-objects is not "simple". We currently only support reading attributes from the working directory if I recall correctly. pack-objects may not have a working directory.

Hence, "just magic" is probably the better route.

-- 
Shawn.