On 08/17/2009 11:04 AM, Yu Jiang (yujia) wrote:
Hi Dor,
Thank you very much for your reply!
Our kernel of host os was based on 2.6.18, so cgroup is not avaialbe for
us. And our Intel chip supports constant_tsc.
We will run RHEL 5.4 as guest os. Will the clock of guest os still be a
issue if we use kvm-clock in guest os?
The first release of rhel5.4 guest won't support guest pv clock. It does
support the host side of the clock. Further updates should enable this
feature in the guest.
And does it mean guest os will always has clock issue, if vcpu thread
was not scheduled on time?
The stable tsc is good, but the problem is that it works in conjunction
with the pit clock. The OS tries to compensate for lost ticks that it
automatically identifies (also on real hardware). This is why we
recommend to run RHEL guest with -no-kvm-pit-reinjection flag.
My fear is that too large pauses will drive the OS crazy.
You can test if it can cope with it.
Why not use nice instead? The effect is similar, but it might be a bit
more smooth.
Thanks,
Yu
On 08/17/2009 08:09 AM, Yu Jiang (yujia) wrote:
Hi KVM experts,
Our user case needs to run KVM and application on host together. To
reserve some CPU resource for application, we want to limit the CPU
usage of KVM. Without KVM CPU usage limitation, the idle CPU of host
OS becomes 0% in peak time.
I have searched this topic on internet, but didn't find much comments.
One possible solution could be managing KVM process as regular process
on host OS, and use tool like http://cpulimit.sourceforge.net/ to
limit maximum CPU usage of VM. Basically, the cpulimit tool use SIGSTP
and SIGCONT signals to stop and resume the execution of KVM process.
It works fine for us at moment. But, I feel there may be some risk to
do this, because the signal will cause whole process of KVM paused(not
only vcpu thread). Do you think it's safe to use cpulimit kinds of
tool to SIGSTP/SIGCONT kvm?
Another possible solution was:
Enhance QEMU user space to monitor the CPU usage of itself, and use
existing way(pause_all_vcpus?) to pause vcpu thread of KVM in case KVM
reaches CPU usage limitation. Is this solution possible?
A mgmt daemon can control qemu using the monitor and stop/cont it on
these cases.
The main problem with the two solutions above is that the guest clock
might drift. Moreover, you increase the latency for the guest
OS/applications.
You can use the 'nice' command to priorities the host applications.
For newer kernels you should use cgroups that solves this specific issue
exactly.
Any idea?
Thanks,
Yu
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