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

[Avi Kivity <avi@redhat.com>]

On 03/15/2010 10:07 AM, Balbir Singh wrote:
> * Avi Kivity<avi@redhat.com>  [2010-03-15 09:48:05]:
>
>    
>> On 03/15/2010 09:22 AM, 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.
>>>        
>> Well, for a guest, host page cache is a lot slower than guest page cache.
>>
>>      
> Yes, it is a virtio call away, but is the cost of paying twice in
> terms of memory acceptable?

Usually, it isn't, which is why I recommend cache=off.

> One of the reasons I created a boot
> parameter was to deal with selective enablement for cases where
> memory is the most important resource being managed.
>
> I do see a hit in performance with my results (please see the data
> below), but the savings are quite large. The other solution mentioned
> in the TODOs is to have the balloon driver invoke this path. The
> sysctl also allows the guest to tune the amount of unmapped page cache
> if needed.
>
> The knobs are for
>
> 1. Selective enablement
> 2. Selective control of the % of unmapped pages
>    

An alternative path is to enable KSM for page cache.  Then we have 
direct read-only guest access to host page cache, without any guest 
modifications required.  That will be pretty difficult to achieve though 
- will need a readonly bit in the page cache radix tree, and teach all 
paths to honour it.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.