Re: [RFC] respect the referenced bit of KVM guest pages?

[Wu Fengguang]

On Wed, Aug 05, 2009 at 12:15:40PM +0800, KOSAKI Motohiro wrote:
> Hi
> 
> > Greetings,
> > 
> > Jeff Dike found that many KVM pages are being refaulted in 2.6.29:
> > 
> > "Lots of pages between discarded due to memory pressure only to be
> > faulted back in soon after. These pages are nearly all stack pages.
> > This is not consistent - sometimes there are relatively few such pages
> > and they are spread out between processes."
> 
> I suprise this result really.
> 
>   - Why this issue happened only on kvm?

Maybe because
- they take up a large portion of memory
- their access patterns/frequencies vary a lot

>   - Why shrink_inactive_list() can't find pte young bit?

It can, but I guess the grace period would be much shorter than with
this patch.

>     Is this really unused stack?

They were actually being refaulted.  So they should be kind of
not-too-hot as well as not-too-cold pages. 

Thanks,
Fengguang

> > 
> > The refaults can be drastically reduced by the following patch, which
> > respects the referenced bit of all anonymous pages (including the KVM
> > pages).
> > 
> > However it risks reintroducing the problem addressed by commit 7e9cd4842
> > (fix reclaim scalability problem by ignoring the referenced bit,
> > mainly the pte young bit). I wonder if there are better solutions?
> > 
> > Thanks,
> > Fengguang
> > 
> > ---
> >  mm/vmscan.c |   10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> > 
> > --- linux.orig/mm/vmscan.c
> > +++ linux/mm/vmscan.c
> > @@ -1288,12 +1288,12 @@ static void shrink_active_list(unsigned 
> >  			 * Identify referenced, file-backed active pages and
> >  			 * give them one more trip around the active list. So
> >  			 * that executable code get better chances to stay in
> > -			 * memory under moderate memory pressure.  Anon pages
> > -			 * are not likely to be evicted by use-once streaming
> > -			 * IO, plus JVM can create lots of anon VM_EXEC pages,
> > -			 * so we ignore them here.
> > +			 * memory under moderate memory pressure.
> > +			 *
> > +			 * Also protect anon pages: swapping could be costly,
> > +			 * and KVM guest's referenced bit is helpful.
> >  			 */
> > -			if ((vm_flags & VM_EXEC) && !PageAnon(page)) {
> > +			if ((vm_flags & VM_EXEC) || PageAnon(page)) {
> >  				list_add(&page->lru, &l_active);
> >  				continue;
> >  			}
> 
> 
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