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

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Date:	Mon, 14 Jun 2010 00:01:45 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	kvm <kvm@...r.kernel.org>
Cc:	Avi Kivity <avi@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 1/2] Linux/Guest unmapped page cache control

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

> Selectively control Unmapped Page Cache (nospam version)
> 
> From: Balbir Singh <balbir@...ux.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
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