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

From: leonid.moiseichuk
Date: Tue May 08 2012 - 05:16:11 EST

> -----Original Message-----
> From: penberg@xxxxxxxxx [mailto:penberg@xxxxxxxxx] 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:
> >>
> >>
> >
> > 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.

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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