Re: [RFC/T/D][PATCH 2/2] Linux/Guest cooperative unmapped page cache control

Avi Kivity
2010-06-10T09:43:17Z

On 06/08/2010 06:51 PM, Balbir Singh wrote:
> Balloon unmapped page cache pages first
>
> From: Balbir Singh<balbir@linux.vnet.ibm.com>
>
> This patch builds on the ballooning infrastructure by ballooning unmapped
> page cache pages first. It looks for low hanging fruit first and tries
> to reclaim clean unmapped pages first.
>

I'm not sure victimizing unmapped cache pages is a good idea. Shouldn't
page selection use the LRU for recency information instead of the cost
of guest reclaim? Dropping a frequently used unmapped cache page can be
more expensive than dropping an unused text page that was loaded as part
of some executable's initialization and forgotten.

Many workloads have many unmapped cache pages, for example static web
serving and the all-important kernel build.

> The key advantage was that it resulted in lesser RSS usage in the host and
> more cached usage, indicating that the caching had been pushed towards
> the host. The guest cached memory usage was lower and free memory in
> the guest was also higher.
>

Caching in the host is only helpful if the cache can be shared,
otherwise it's better to cache in the guest.

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