Re: [RFC][PATCH 1/2] Linux/Guest unmapped page cache control

Balbir Singh
2010-06-13T18:31:51Z

* Balbir Singh <balbir@linux.vnet.ibm.com> [2010-06-08 21:21:46]:

> Selectively control Unmapped Page Cache (nospam version)
>
> From: Balbir Singh <balbir@linux.vnet.ibm.com>
>
> This patch implements unmapped page cache control via preferred
> page cache reclaim. The current patch hooks into kswapd and reclaims
> page cache if the user has requested for unmapped page control.
> This is useful in the following scenario
>
> - In a virtualized environment with cache=writethrough, we see
> double caching - (one in the host and one in the guest). As
> we try to scale guests, cache usage across the system grows.
> The goal of this patch is to reclaim page cache when Linux is running
> as a guest and get the host to hold the page cache and manage it.
> There might be temporary duplication, but in the long run, memory
> in the guests would be used for mapped pages.
> - The option is controlled via a boot option and the administrator
> can selectively turn it on, on a need to use basis.
>
> A lot of the code is borrowed from zone_reclaim_mode logic for
> __zone_reclaim(). One might argue that the with ballooning and
> KSM this feature is not very useful, but even with ballooning,
> we need extra logic to balloon multiple VM machines and it is hard
> to figure out the correct amount of memory to balloon. With these
> patches applied, each guest has a sufficient amount of free memory
> available, that can be easily seen and reclaimed by the balloon driver.
> The additional memory in the guest can be reused for additional
> applications or used to start additional guests/balance memory in
> the host.
>
> KSM currently does not de-duplicate host and guest page cache. The goal
> of this patch is to help automatically balance unmapped page cache when
> instructed to do so.
>
> There are some magic numbers in use in the code, UNMAPPED_PAGE_RATIO
> and the number of pages to reclaim when unmapped_page_control argument
> is supplied. These numbers were chosen to avoid aggressiveness in
> reaping page cache ever so frequently, at the same time providing control.
>
> The sysctl for min_unmapped_ratio provides further control from
> within the guest on the amount of unmapped pages to reclaim.
>

Are there any major objections to this patch?

--
Three Cheers,
Balbir