On 04/23/2010 10:39 AM, Brian Jackson wrote:
On Friday 23 April 2010 12:08:22 David S. Ahern wrote:
After a few days of debugging I think kvmclock is the source of lockups
for a RHEL5.5-based VM. The VM works fine on one host, but repeatedly
locks up on another.
Server 1 - VM locks up repeatedly
-- DL580 G5
-- 4 quad-core X7350 processors at 2.93GHz
-- 48GB RAM
Server 2 - VM works just fine
-- DL380 G6
-- 2 quad-core E5540 processors at 2.53GHz
-- 24GB RAM
Both host servers are running Fedora Core 12, 2.6.32.11-99.fc12.x86_64
kernel. I have tried various versions of qemu-kvm -- the version in
FC-12 and the version for FC-12 in virt-preview. In both cases the
qemu-kvm command line is identical.
VM
- RHEL5.5, PAE kernel (also tried standard 32-bit)
- 2 vcpus
- 3GB RAM
- virtio network and disk
When the VM locks up both vcpu threads are spinning at 100%. Changing
the clocksource to jiffies appears to have addressed the problem.
Does changing the guest to -smp 1 help?
Based on our current understanding of the problem, it should help, but
it may not prevent the problem entirely.
There are three issues with kvmclock due to sampling:
1) smp clock alignment may be slightly off due to timing conditions
2) kvmclock is resampled at each switch of vcpu to another pcpu
3) kvmclock granularity exceeds that of kernel timespec, which means
sampling errors may show even on UP
Recommend using a different clocksource (tsc is great if you have stable
TSC and don't migrate across different-speed machines) until we have all
the fixes in place.
Zach
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