Re: qsbench, interesting results

[Daniel Phillips <phillips@arcor.de>]

On Tuesday 01 October 2002 18:52, Rik van Riel wrote:
> On Tue, 1 Oct 2002, Daniel Phillips wrote:
> > On Monday 30 September 2002 07:57, Andrew Morton wrote:
> > > I'll take a look at some preferential throttling later on.  But
> > > I must say that I'm not hugely worried about performance regression
> > > under wild swapstorms.  The correct fix is to go buy some more
> > > RAM, and the kernel should not be trying to cater for underprovisioned
> > > machines if that affects the usual case.
> >
> > The operative phrase here is "if that affects the usual case".
> > Actually, the quicksort bench is not that bad a model of a usual case,
> > i.e., a working set 50% bigger than RAM.
> 
> Having the working set of one process larger than RAM is
> a highly unusual case ...

No it's not, it's very similar to having several processes active whose
working sets add up to more than RAM.

> > The page replacement algorithm ought to do something sane with it,
> 
> ... page replacement ought to give this process less RAM
> because it isn't going to get enough to run anyway. No need
> to have a process like qsbench make other processes run
> slow, too.

It should run the process as efficiently as possible, given that there
isn't any competition.

> > and swap performance ought to be decent in general, since desktop users
> > typically have less than 1/2 GB.  With media apps, bloated desktops and
> > all, it doesn't go as far as it used to.
> 
> The difference there is that desktops don't have a working
> set larger than RAM. They've got a few (dozen?) of processes,
> each of which has a working set that easily fits in ram and
> a bunch of pages, or whole processes, which aren't currently
> in use.

Try loading a high res photo in gimp and running any kind of interesting
script-fu on it.  If it doesn't thrash, boot with half the memory and
repeat.

> In this situation the VM _can_ keep the whole working set in
> RAM, simply by evicting the stuff that's not in the working
> set.

We're not talking about that case.  Oh, by the way, since when did
it become ok to ignore corner cases?  I thought corner cases were what
users have been flaming us about for the last 2 or 3 years.

> > My impression is that page replacement just hasn't gotten a lot of
> > attention recently, and there is nothing wrong with that.  It's tuning,
> > not a feature.
> 
> As you write above, it's a pretty damn important feature,
> though.  One thing I'm experimenting with now for 2.4 rmap
> is to ignore the referenced bit and page age if a page is
> only in use by processes which haven't run recently, this
> should help the desktop (and university) workloads by
> swapping out memory from tasks which don't need it anyway
> at the moment.
> 
> There may be other modifications needed, too...

No doubt, and for the first time, we've got a solid base to build on.

-- 
Daniel