Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

[Randy Dunlap <randy.dunlap@oracle.com>]

On Mon, 15 Mar 2010 12:52:15 +0530 Balbir Singh wrote:

> 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!=none, 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.
> 
> The patch is applied against mmotm feb-11-2010.

Hi,
If you go ahead with this, please add the boot parameter & its description
to Documentation/kernel-parameters.txt.


> TODOS
> -----
> 1. Balance slab cache as well
> 2. Invoke the balance routines from the balloon driver

---
~Randy

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