Date: Wed, 25 Sep 2013 22:56:56 +0800 From: Julian Elischer <julian@freebsd.org> To: virtualization@FreeBSD.org Subject: Xen (and others Hypervisors) how do they handle IPIs? Message-ID: <5242F9B8.9090300@freebsd.org>
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If CPUs are mapped around, how are IPIs handled? I assume they must be emulated? I've noticed that under Xen (on both Amazon EC2 and a Redhat server) whenever you schedule a thread it always sits on the run queue for 20 uSecs before it starts running. It looks to me like it's the IPI taking a long time to be emulated. We have some workloads where there is a lot of flipping back and forth between threads and they are slowed down by an order of magnitude due to this.. turning off NOADAPTIVE(mumble) seems to help a bit as some of the reschedules go away, but it's still a problem. Does anyone know if BHyVe or HyperV also have this problem? I have not yet gone to the source of Xen to see what it does but it would be educational to know what the other Hypervisors do.
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