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

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Message-ID: <84FF21A720B0874AA94B46D76DB98269045F7C35@008-AM1MPN1-004.mgdnok.nokia.com>
Date:	Fri, 8 Jun 2012 11:03:29 +0000
From:	<leonid.moiseichuk@...ia.com>
To:	<cbouatmailru@...il.com>
CC:	<kosaki.motohiro@...il.com>, <penberg@...nel.org>,
	<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:cbouatmailru@...il.com]
> Sent: 08 June, 2012 13:35
...
> > Context switches, parsing, activity in userspace even memory situation is
> not changed.
> 
> Sure, there is some additional overhead. I'm just saying that it is not drastic. It
> would be like 100 sprintfs + 100 sscanfs + 2 context switches? Well, it is
> unfortunate... but come on, today's phones are running X11 and Java. :-)

Vmstat generation is not so trivial. Meminfo has even higher overhead. I just checked generation time using idling device and open/read test:
- vmstat min 30, avg 94 max 2746 uSeconds
- meminfo min 30, average 65 max 15961 uSeconds

In comparison /proc/version for the same conditions: min 30, average 41, max 1505 uSeconds
 
> > In kernel space you can use sliding timer (increasing interval) + shinker.
> 
> Well, w/ Minchan's idea, we can get shrinker notifications into the userland,
> so the sliding timer thing would be still possible.

Only as a post-schrinker actions. In case of memory stressing or close-to-stressing conditions shrinkers called very often, I saw up to 50 times per second.