Messages in this thread | | | Date | Wed, 5 Aug 2009 12:41:50 +0800 | From | Wu Fengguang <> | Subject | Re: [RFC] respect the referenced bit of KVM guest pages? |
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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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