[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [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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