linux-kernel - Re: [PATCH v7 7/7] zsmalloc: use shrinker to trigger auto-compaction

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Message-ID: <20150710004921.GA10230@bgram>
Date:	Fri, 10 Jul 2015 09:49:21 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH v7 7/7] zsmalloc: use shrinker to trigger auto-compaction

Hi Sergey,

On Wed, Jul 08, 2015 at 08:31:53PM +0900, Sergey Senozhatsky wrote:
> Perform automatic pool compaction by a shrinker when system
> is getting tight on memory.
> 
> User-space has a very little knowledge regarding zsmalloc fragmentation
> and basically has no mechanism to tell whether compaction will result
> in any memory gain. Another issue is that user space is not always
> aware of the fact that system is getting tight on memory. Which leads
> to very uncomfortable scenarios when user space may start issuing
> compaction 'randomly' or from crontab (for example). Fragmentation
> is not always necessarily bad, allocated and unused objects, after all,
> may be filled with the data later, w/o the need of allocating a new
> zspage. On the other hand, we obviously don't want to waste memory
> when the system needs it.
> 
> Compaction now has a relatively quick pool scan so we are able to
> estimate the number of pages that will be freed easily, which makes it
> possible to call this function from a shrinker->count_objects() callback.
> We also abort compaction as soon as we detect that we can't free any
> pages any more, preventing wasteful objects migrations.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Suggested-by: Minchan Kim <minchan@...nel.org>
Acked-by: Minchan Kim <minchan@...nel.org>

Thanks for great work!

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