linux-kernel - RE: [PATCH 3/3] vmevent: Implement special low-memory attribute

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Message-ID: <84FF21A720B0874AA94B46D76DB98269045D63B3@008-AM1MPN1-003.mgdnok.nokia.com>
Date:	Tue, 8 May 2012 09:15:46 +0000
From:	<leonid.moiseichuk@...ia.com>
To:	<penberg@...nel.org>, <kosaki.motohiro@...il.com>
CC:	<anton.vorontsov@...aro.org>, <minchan@...nel.org>,
	<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 3/3] vmevent: Implement special low-memory attribute

> -----Original Message-----
> From: penberg@...il.com [mailto:penberg@...il.com] On Behalf Of ext
> Pekka Enberg
> Sent: 08 May, 2012 11:03
> To: KOSAKI Motohiro
> Cc: Anton Vorontsov; Minchan Kim; Moiseichuk Leonid (Nokia-MP/Espoo); John
...
> >> That comes from a real-world requirement. See Leonid's email on the topic:
> >>
> >> https://lkml.org/lkml/2012/5/2/42
> >
> > I know, many embedded guys prefer such timer interval. I also have an
> > experience similar logic when I was TV box developer. but I must
> > disagree. Someone hope timer housekeeping complexity into kernel. but
> > I haven't seen any justification.
> 
> Leonid?

The "usleep(timeout); read(vmevent_fd)" will eliminate opportunity to use vmevent API for mobile devices.  
Developers already have to use heartbeat primitives to align/sync timers and update code which is not always simple to do.
But the idea is to have user-space wakeup only if we have something change in memory numbers, thus aligned timers will not help much in vmevent case due to memory situation may change a lot in short time.
Short depends from software stack but usually it below 1s.  To have use-time and wakeups on good level (below 50Hz by e.g. powertop) and allow cpu switch off timers of such short period like 1s are not allowed.

Leonid
PS: Sorry, meetings prevent to do interesting things :(  I am tracking conversation with quite low understanding how it will be useful for practical needs because user-space developers in 80% cases needs to track simply dirty memory changes i.e. modified pages which cannot be dropped.
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