Re: The performance and behaviour of the anti-fragmentation related patches

Andrew Morton
2007-03-02T18:25:40Z

On Fri, 2 Mar 2007 10:15:36 -0800 (PST)
Christoph Lameter <clameter@engr.sgi.com> wrote:

> On Fri, 2 Mar 2007, Andrew Morton wrote:
>
> > > One particular case is a 32GB system with a database that takes most
> > > of memory. The amount of actually freeable page cache memory is in
> > > the hundreds of MB.
> >
> > Where's the rest of the memory? tmpfs? mlocked? hugetlb?
>
> The memory is likely in use but there is enough memory free in unmapped
> clean pagecache pages so that we occasionally are able to free pages. Then
> the app is reading more from disk replenishing that ...
> Thus we are forever cycling through the LRU lists moving pages between
> the lists aging etc etc. Can lead to a livelock.

Guys, with this level of detail thses problems will never be fixed.

> > > A third scenario is where a system has way more RAM than swap, and not
> > > a whole lot of freeable page cache. In this case, the VM ends up
> > > spending WAY too much CPU time scanning and shuffling around essentially
> > > unswappable anonymous memory and tmpfs files.
> >
> > Well we've allegedly fixed that, but it isn't going anywhere without
> > testing.
>
> We have fixed the case in which we compile the kernel without swap. Then
> anonymous pages behave like mlocked pages. Did we do more than that?

oh yeah, we took the ran-out-of-swapcache code out. But if we're going to
do this thing, we should find some way to bring it back.