From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <marcelo@kvack.org>
Subject: Re: [PATCH] shrinker support for the mmu cache
Date: Mon, 17 Mar 2008 18:33:52 -0300
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Cc: Marcelo Tosatti <marcelo@kvack.org>,
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To: Avi Kivity <avi@qumranet.com>
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On Mon, Mar 17, 2008 at 04:41:18PM +0200, Avi Kivity wrote:
> Marcelo Tosatti wrote:
> >>
> >>While aging is not too hard to do, I don't think it would add much in 
> >>practice; we rarely observe mmu shadow pages being recycled due to 
> >>memory pressure.  So this is mostly helpful for preventing a VM from 
> >>pinning memory when under severe memory pressure, where we don't expect 
> >>good performance anyway.
> >>    
> >
> >Issue is that the shrinker callback will not be called only under
> >severe memory pressure, but for normal system pressure too.
> >
> >  
> 
> How much shrinkage goes on under normal pressure?

It depends on the number of LRU pages scanned and the size of the cache.

Roughly the number of LRU pages scanned divided by shrinker->seeks,
relative to cache size (mm/vmscan.c shrink_slab).

> Rebuilding a single shadow page costs a maximum of 512 faults (so about 
> 1 msec).  If the shrinker evicts one entry per second, this is a 
> performance hiy of 0.1%.
> 
> Perhaps if we set the cost high enough, the normal eviction rate will be 
> low enough.

I think its pretty easy to check for the referenced bit on pages to
avoid recently used ones from being zapped.

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