Re: [PATCH] diff-delta: produce optimal pack data

From: Nicolas Pitre <nico@cam.org>
Date: Feb 24, 2006, 20:19 UTC
Message-ID: <Pine.LNX.4.64.0602241509050.31162@localhost.localdomain>
In-Reply-To: <Pine.LNX.4.64.0602241152290.22647@g5.osdl.org>

On Fri, 24 Feb 2006, Linus Torvalds wrote:

Show 9 quoted lines

> The other thing to look at is "max_size": right now it initializes that to 
> "size / 2 - 20", which just says that we don't ever want a delta that is 
> larger than about half the result (plus the 20 byte overhead for pointing 
> to the thing we delta against). Again, if you feel that normal compression 
> compresses better than half, you could try changing that to
> 
> 	..
> 	max_size = size / 4 - 20;
> 	..

Like I mentioned, max_size should also be caped with the deflated undeltified object size. This value is easy to get since plain objects are already deflated.

Show 6 quoted lines

> NOTE! Every SINGLE one of those heuristics are just totally made up by 
> yours truly, and have no testing behind them. They're more of the type 
> "that sounds about right" than "this is how it must be". As mentioned, 
> Nico has already been playing with the heuristics - but he wanted better 
> packs, not better CPU usage, so he went the other way from what you would 
> want to try..

Actually it's a good balance I'm after.

Using 30% more CPU for 10% smaller packs is OK I'd say.

Using 100 times the CPU for 50% saving on only one particular delta is not acceptable.

And using more than one hour for 200MB of data with the current window default is not acceptable either.

Nicolas