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xen-devel
RE: [Xen-devel] Does Xen detect busy-spinning VCPUs?
Is your mini-OS pinned and you're sure dom0 or other
domains are not getting a piece of the pcpu? If so...
I've seen anecdotal evidence of long pauses that led me
to wonder about interrupt latency here:
http://lists.xensource.com/archives/html/xen-devel/2008-08/msg00232.html
I don't recall the situation or the length of the pause
but perhaps you are seeing something similar. Unfortunately,
I never pursued the answer to the interrupt latency question.
> -----Original Message-----
> From: Robert Kaiser [mailto:kaiser@xxxxxxxxxxxxxxxxxxxxxxxxxx]
> Sent: Friday, September 19, 2008 6:00 AM
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] Does Xen detect busy-spinning VCPUs?
>
>
> Hi all,
>
> I'm currently developing/testing a new scheduler for Xen and
> I am seeing some
> very strange behaviour which I can't seem to pinpoint: For
> benchmarking
> purposes, I am running a task inside Mini-OS in a tight,
> busy-spinning loop
> for some time. The loop repeatedly polls NOW() until it
> exceeds a certain
> time limit. What I am observing is that NOW() seems to "jump"
> sometimes: two
> subsequent reads return values which differ by tens of
> milliseconds! I notice
> that my scheduler gets invoked a couple of times, but it does
> *not* switch to
> another VCPU and I doubt that the scheduler invocations alone
> take that long.
> So the loop should indeed be contiuously spinning with sporadic
> interruptions in the range of a few microseconds, but not tens of
> milliseconds. Yet, this is not what I am seeing. I wonder
> where the (P)CPU
> goes during those time intervals and so this possibly weird
> idea came up that
> Xen might use some trickery trying to detect and pause
> busy-spinning VCPUs.
> Is there anything like that in Xen (BTW: This is xen-3.2.1) ,
> and, if there
> is, can it be disabled for a given domain?
>
> (Sorry if this is a silly question. Since my code is
> experimental and not well
> tested yet, there is of course the possibility that I made
> some stupid
> mistake. However, I've been staring at code, debug logs, etc.
> for several
> days now without much success and I am slowly getting
> desperate. If Xen
> really does pause spinning VCPUs it would explain everything.)
>
> Thanks for any help
>
> Rob
>
> --
> Robert Kaiser
> http://wwwvs.informatik.fh-wiesbaden.de
> Labor für Verteilte Systeme
> kaiser@xxxxxxxxxxxxxxxxxxxxxxxxxx
> FH Wiesbaden - University of Applied Sciences tel:
> (+49)611-9495-1294
> Kurt-Schumacher-Ring 18, 65197 Wiesbaden, Germany fax:
> (+49)611-9495-1289
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel
>
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