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

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Message-ID: <AANLkTinaX-huEMGP-k4mCSr0USQhJp68AUgOf4FHqr5Q@mail.gmail.com>
Date:	Wed, 21 Jul 2010 08:03:09 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Nitin Gupta <ngupta@...are.org>
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

Hi,

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.
AFAIK, cleancache doesn't give any hint for that. so we should
implement it in zcache itself.
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.

It's just my opinion.
Thanks for effort for good feature. Nitin.
-- 
Kind regards,
Minchan Kim
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