Xen users - Aug 2013 - HVM vLAPIC timer interrupts intermittently disappearing

Hello,

I''m trying to port one of our existing appliances (running on 64-bit
Red Hat Enterprise Linux 6.4) to a Xen HVM guest .  We''re seeing some
very odd behaviour that doesn''t manifest on other platforms.

The guest experiences intermittent lockups of a few seconds- shell sessions
become unresponsive, and various software healthchecking in our application
triggers, even when the application is basically idle.  Our applications is
time-critical (it handles networking packets) so these delays are a major
problem.  There''s no pattern to when these delays happen that we can
see.

Some extensive probing of the guest kernel has revealed that during the delays,
the guest sees no APIC timer interrupts.  This then means that kernel timers are
not scheduled, causing system havoc.

I think this means that either:

*         Xen is not generating the emulated APIC timer interrupts for several
seconds

*         domU has disabled the timer interrupt (for whatever reason) for
several seconds.

Neither seems very likely.  We do not see this problem on a bare-metal system on
the same hardware, and we don''t see it when running on other
hypervisors (VMware or KVM).   We have seen this on guests with single or
multiple vCPUs.

Is it possible that Xen has erroneously disabled the local APIC timer?  Are
there any useful diagnostics we could get out of Xen?

We''re running Xen 4.2.2, dom0 is Fedora Core18 (kernel
3.6.10-4.fc18.x86_64).  Guest is RHEL6.4, kernel 2.6.32-358.0.1.el6.x86_64. 
Hardware is a Dell PowerEdge R620 server with 2 Intel Xeon E5-2690 6-core CPUs.

Thanks,
Mark

Mark Thebridge
Software Engineer
Metaswitch Networks

mark.thebridge@metaswitch.com<mailto:mark.thebridge@metaswitch.com>
+44 (0)2083661177
www.metaswitch.com<http://www.metaswitch.com/>



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