linux-kernel - Re: [PATCH 4/8] Shrink zcache based on memlimit

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Date:	Wed, 21 Jul 2010 10:22:40 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Minchan Kim <minchan.kim@...il.com>
CC:	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 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.


Thanks for your comments.
Nitin
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