Re: [PATCH 2/9] ksm: let shared pages be swappable

[Rik van Riel]

On 12/01/2009 04:59 AM, Andrea Arcangeli wrote:
> On Tue, Dec 01, 2009 at 06:46:06PM +0900, KOSAKI Motohiro wrote:
>    
> > Ah, well. please wait a bit. I'm under reviewing Larry's patch. I don't
> > dislike your idea. last mail only pointed out implementation thing.
> >      
> Yep thanks for pointing it out. It's an implementation thing I don't
> like. The VM should not ever touch ptes when there's light VM pressure
> and plenty of unmapped clean cache available, but I'm ok if others
> disagree and want to keep it that way.
>    
The VM needs to touch a few (but only a few) PTEs in
that situation, to make sure that anonymous pages get
moved to the inactive anon list and get to a real chance
at being referenced before we try to evict anonymous
pages.

Without a small amount of pre-aging, we would end up
essentially doing FIFO replacement of anonymous memory,
which has been known to be disastrous to performance
for over 40 years now.

A two-handed clock mechanism needs to put some distance
between the front and the back hands of the clock.

Having said that - it may be beneficial to keep very heavily
shared pages on the active list, without ever trying to scan
the ptes associated with them.
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