Re: [Xen-devel] Does Xen detect busy-spinning VCPUs?

["Geoffrey Lefebvre" <geoffrey@xxxxxxxxx>]

> I am seeing situations where two subsequent calls to NOW() in the Mini-OS
> context deliver time coordinates that differ by 95(!) milliseconds. If this
> were due to interrupt latencies, surely that would have been noticed by
> someone?

These are long latencies. Have you tried comparing the values returned
by NOW() in mini-os with reads from the tsc? NOW() is probably correct
but it's a good thing to rule this out.

geoffrey


>
>> 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
>
>
>
> --
> 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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