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Re: [Qemu-discuss] Latency degradation on VM due to several QEMU process
Re: [Qemu-discuss] Latency degradation on VM due to several QEMU processes on the host.
Mon, 20 Jun 2016 11:39:47 +0000
Thanks a lot for the information below.
It really may improve some RT capabilities of my applications running on VMs
and honestly I am doing 90% from what is mentioned there.
But, one of the problems I saw, to make long story short, if you want low
latencies, you have to isolate all the cores and run each VCPU on a dedicated
core and each VCPU thread on different core and each QEMU on different core.
This way I will run out of cores really fast :), which is extremely problematic
for me. For example in the ppt I saw:
"Run VCPU threads on dedicated CPUs"
"Run KVM guest VCPUs on isolated CPUs"
If I have 3 vcpus this means that I will need 6 dedicated cores on the host ??
Anyway, I will try running QEMUs with "mlock=on" and do some configuration
updates e.g. idle=poll and not mwait as I have now, but my main questions
1. How can I run several QEMUs on a single core and still keep low
latencies on the VMs ? Is "mlock=on" the solution for this?
2. QEMU generates different threads running on different cores (besides
VCPU threads running on the affinity cores, defined in a pinning section of the
virsh XML). May this harm latency as well? May and should I do some special
affinity to them or change priority, etc ... ?
From: Qemu-discuss [mailto:address@hidden On Behalf Of Alex
Sent: Sunday, June 19, 2016 3:40 PM
Subject: Re: [Qemu-discuss] Latency degradation on VM due to several QEMU
processes on the host.
Take a look at Qemu's realtime feature, might be what you need.
On 19/06/16 14:38, Yan Fridland wrote:
> Can you please comment on the overhead QEMU brings in terms of latency
> and CPU utilization?
> I am running a highly latency sensitive application on my VMs and I
> see some degradation in performance when running 2 QEMU processes on a
> single core, or even when I run 1 QEMU process on a single core
> without a dedicated affinity and RT priority to the QEMU process.
> Can I make the QEMU lighter to be able to run 2,3 or more QEMUs on a
> single core? Is it matter of XML configuration / affinity / priority
> or anything else?
> Thank you,