Re: [PATCH] mm: inactive-clean list

From: Peter Zijlstra
Date: Wed Jul 26 2006 - 10:44:27 EST

On Wed, 2006-07-26 at 15:04 +0200, Martin Schwidefsky wrote:
> On 7/26/06, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> > > Hmm, I wonder how the inactive clean list helps in regard to the fast
> > > host reclaim
> > > scheme. In particular since the memory pressure that triggers the
> > > reclaim is in the
> > > host, not in the guest. So all pages might be on the active list but
> > > the host still
> > > wants to be able to discard pages.
> > >
> >
> > I think Rik would want to set all the already unmapped pages to volatile
> > state in the hypervisor.
> >
> > These pages can be dropped without loss of information on the guest
> > system since they are all already on a backing-store, be it regular
> > files or swap.
> I guessed that as well. It isn't good enough. Consider a guest with a
> large (virtual) memory size and a host with a small physical memory
> size. The guest will never put any page on the inactive_clean list
> because it does not have memory pressure. vmscan will never run. The
> host wants to reclaim memory of the guest, but since the
> inactive_clean list is empty it will find only stable pages.

Wouldn't we typically have all free pages > min_free in state U?
Also wouldn't all R/O mapped pages not also be V, all R/W mapped pages
and unmapped page-cache pages P like you state in your paper.

This patch would just increase the number of V pages with the tail end
of the guest LRU, which are typically the pages you would want to evict
(perhaps even add 5th guest state to indicate that these V pages are
preferable over the others?)

But isn't it so that for the gross over-commit scenario you outline the
host OS will have to swap out S pages eventually?

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