Re: diffcore-rename performance mode

[Jeff King <peff@peff.net>]

On Tue, Sep 25, 2007 at 09:32:31PM +0200, David Kastrup wrote:

> >                  | stock | nofree | old somefree | fixed somefree
> > -----------------|-----------------------------------------------
> > user time (s)    | 76.78 | 16.96  | 46.26        | 16.99
> > peak memory (Kb) | 52300 | 66796  | 59156        | 57328
> >
> > So now we're at a 4.5x speedup for about 10% extra memory usage. Patch
> > will follow.
> 
> Sounds good except when we happen to just hit the "memory working set
> exceeds physical memory" sweet spot.  But the odds are much better
> than for "nofree".

Of course, there is the possibility that it is the 10% that pushes you
into swap (or kills your ability to cache the entire pack in RAM).
However, this is probably not a big deal for two reasons:
  1. this is a 50M process handling the linux-2.6 repository. Other
     operations such as repacking already consume significantly more
     memory (git-repack -a allocates 290M on the same repo).
  2. I specifically turned off rename limiting (-l0) to do rename
     detection on these large-ish diffs.  If you have a machine which is
     on the cusp of swapping, then don't do that. Jumping from -l100
     (the default) to -l0 in my tests is _already_ moving you from 28M
     to 52M, a much larger jump.

-Peff