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Re: [Xen-devel] A clocksource question

To: Joanna Rutkowska <joanna@xxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] A clocksource question
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Thu, 11 Mar 2010 16:49:30 -0800
Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
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On 03/11/2010 04:27 PM, Joanna Rutkowska wrote:
I think that the important clue is this message appearing in my dmesg
(in Dom0 and also in DomUs):

hrtimer: interrupt too slow, forcing clock min delta to 540150561 ns

This is almost 0.5s (!) and I think this might explain my kbd hiccup. I
wrote that I feel it every 10s or so, but when I was playing on my
system without jiffies setting today, I saw this hiccup occurring much
more often; in fact it was more of a "slow keyboard/system" feeling than
a hiccup".

Yes, its definitely a good clue. It could point to a Xen scheduler issue (I assume you don't have any other busy domains when this is going on), but that wouldn't explain why it works OK with jiffies. Perhaps there's a problem with programming the timer so that sometimes it gets missed and the system doesn't get a kick until some subsequent event (whereas with clocksource=jiffies there'll always be a timer in the next 100ms).


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