Date:	Sat, 24 Jul 2010 00:53:21 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Ed Tomlinson <edt@....ca>
CC:	Minchan Kim <minchan.kim@...il.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Rik van Riel <riel@...hat.com>, Avi Kivity <avi@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] Shrink zcache based on memlimit

On 07/21/2010 05:02 PM, Ed Tomlinson wrote:
> On Wednesday 21 July 2010 00:52:40 Nitin Gupta wrote:
>> On 07/21/2010 04:33 AM, Minchan Kim wrote:
>>> On Fri, Jul 16, 2010 at 9:37 PM, Nitin Gupta <ngupta@...are.org> wrote:
>>>> User can change (per-pool) memlimit using sysfs node:
>>>> /sys/kernel/mm/zcache/pool<id>/memlimit
>>>>
>>>> When memlimit is set to a value smaller than current
>>>> number of pages allocated for that pool, excess pages
>>>> are now freed immediately instead of waiting for get/
>>>> flush for these pages.
>>>>
>>>> Currently, victim page selection is essentially random.
>>>> Automatic cache resizing and better page replacement
>>>> policies will be implemented later.
>>>
>>> Okay. I know this isn't end. I just want to give a concern before you end up.
>>> I don't know how you implement reclaim policy.
>>> In current implementation, you use memlimit for determining when reclaim happen.
>>> But i think we also should follow global reclaim policy of VM.
>>> I means although memlimit doen't meet, we should reclaim zcache if
>>> system has a trouble to reclaim memory.
>>
>> Yes, we should have a way to do reclaim depending on system memory pressure
>> and also when user explicitly wants so i.e. when memlimit is lowered manually.
>>
>>> AFAIK, cleancache doesn't give any hint for that. so we should
>>> implement it in zcache itself.
>>
>> I think cleancache should be kept minimal so yes, all reclaim policies should
>> go in zcache layer only.
>>
>>> At first glance, we can use shrink_slab or oom_notifier. But both
>>> doesn't give any information of zone although global reclaim do it by
>>> per-zone.
>>> AFAIK, Nick try to implement zone-aware shrink slab. Also if we need
>>> it, we can change oom_notifier with zone-aware oom_notifier. Now it
>>> seems anyone doesn't use oom_notifier so I am not sure it's useful.
>>>
>>
>> I don't think we need these notifiers as we can simply create a thread
>> to monitor cache hit rate, system memory pressure etc. and shrink/expand
>> the cache accordingly.
> 
> Nitin,
> 
> Based on experience gained when adding the shrinker callbacks, I would 
> strongly recommend you use them.  I tried several hacks along the lines of
> what you are proposing before moving settling on the callbacks.  They 
> are effective and make sure that memory is released when its required.
> What would happen with the other methods is that memory would either 
> not be released or would be released when it was not needed.
> 


I had similar experience with "swap notify callback" -- yes, things
don't seem to work without a proper callback. I will check if some
callback already exists for OOM like condition or if new one can
be added easily.

Thanks,
Nitin
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