Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page

[Kirill A. Shutemov]

On Wed, Oct 24, 2012 at 01:25:52PM -0700, Andrew Morton wrote:
> On Wed, 24 Oct 2012 22:45:52 +0300
> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> 
> > On Wed, Oct 24, 2012 at 12:22:53PM -0700, Andrew Morton wrote:
> > > 
> > > I'm thinking that such a workload would be the above dd in parallel
> > > with a small app which touches the huge page and then exits, then gets
> > > executed again.  That "small app" sounds realistic to me.  Obviously
> > > one could exercise the zero page's refcount at higher frequency with a
> > > tight map/touch/unmap loop, but that sounds less realistic.  It's worth
> > > trying that exercise as well though.
> > > 
> > > Or do something else.  But we should try to probe this code's
> > > worst-case behaviour, get an understanding of its effects and then
> > > decide whether any such workload is realisic enough to worry about.
> > 
> > Okay, I'll try few memory pressure scenarios.

A test program:

        while (1) {
                posix_memalign((void **)&p, 2 * MB, 2 * MB);
                assert(*p == 0);
                free(p);
        }

With this code in background we have pretty good chance to have huge zero
page freeable (refcount == 1) when shrinker callback called - roughly one
of two.

Pagecache hog (dd if=hugefile of=/dev/null bs=1M) creates enough pressure
to get shrinker callback called, but it was only asked about cache size
(nr_to_scan == 0).
I was not able to get it called with nr_to_scan > 0 on this scenario, so
hzp never freed.

I also tried another scenario: usemem -n16 100M -r 1000. It creates real
memory pressure - no easy reclaimable memory. This time callback called
with nr_to_scan > 0 and we freed hzp. Under pressure we fails to allocate
hzp and code goes to fallback path as it supposed to.

Do I need to check any other scenario?

-- 
 Kirill A. Shutemov

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