Re: [PATCH] vmscan: synchronous lumpy reclaim don't call congestion_wait()

[KOSAKI Motohiro]

> Hi KOSAKI, 
> 
> On Sun, Aug 01, 2010 at 06:12:47PM +0900, KOSAKI Motohiro wrote:
> > rebased onto Wu's patch
> > 
> > ----------------------------------------------
> > From 35772ad03e202c1c9a2252de3a9d3715e30d180f Mon Sep 17 00:00:00 2001
> > From: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > Date: Sun, 1 Aug 2010 17:23:41 +0900
> > Subject: [PATCH] vmscan: synchronous lumpy reclaim don't call congestion_wait()
> > 
> > congestion_wait() mean "waiting for number of requests in IO queue is
> > under congestion threshold".
> > That said, if the system have plenty dirty pages, flusher thread push
> > new request to IO queue conteniously. So, IO queue are not cleared
> > congestion status for a long time. thus, congestion_wait(HZ/10) is
> > almostly equivalent schedule_timeout(HZ/10).
> Just a nitpick. 
> Why is it a problem?
> HZ/10 is upper bound we intended.  If is is rahter high, we can low it. 
> But totally I agree on this patch. It would be better to remove it 
> than lowing. 

because all of _unnecessary_ sleep is evil. the problem is, congestion_wait()
mean "wait until queue congestion will be cleared, iow, wait all of IO". 
but we want to wait until _my_ IO finished.

So, if flusher thread conteniously push new IO into the queue, that makes
big difference.

Thanks.


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