linux-kernel - RE: [PATCH 2/5] vmevent: Convert from deferred timer to deferred work

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Message-ID: <84FF21A720B0874AA94B46D76DB98269045F7890@008-AM1MPN1-004.mgdnok.nokia.com>
Date:	Fri, 8 Jun 2012 07:05:46 +0000
From:	<leonid.moiseichuk@...ia.com>
To:	<anton.vorontsov@...aro.org>, <kosaki.motohiro@...il.com>,
	<penberg@...nel.org>
CC:	<b.zolnierkie@...sung.com>, <john.stultz@...aro.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<linaro-kernel@...ts.linaro.org>, <patches@...aro.org>,
	<kernel-team@...roid.com>
Subject: RE: [PATCH 2/5] vmevent: Convert from deferred timer to deferred
 work

> -----Original Message-----
> From: ext Anton Vorontsov [mailto:anton.vorontsov@...aro.org]
> Sent: 08 June, 2012 09:58
...
> If you're saying that we should set up a timer in the userland and constantly
> read /proc/vmstat, then we will cause CPU wake up every 100ms, which is
> not acceptable. Well, we can try to introduce deferrable timers for the
> userspace. But then it would still add a lot more overhead for our task, as this
> solution adds other two context switches to read and parse /proc/vmstat. I
> guess this is not a show-stopper though, so we can discuss this.
> 
> Leonid, Pekka, what do you think about the idea?

Seems to me not nice solution. Generating/parsing vmstat every 100ms plus wakeups it is what exactly should be avoid to have sense to API.
It also will cause page trashing because user-space code could be pushed out from cache if VM decide. 

> 
> Thanks,
> 
> --
> Anton Vorontsov
> Email: cbouatmailru@...il.com