Re: [Qemu-discuss] [problem]-starting a VM causes RT (latency) problem for application running on another VM

[Yadi]

swapper process? could I reproduce it on my side? what is operating 
Environment ?

Yadi
On 2016年07月26日 23:50, Yan Fridland wrote:

Looks like I saw the reason for that but I have no idea why this is 
happening.

Immediately after running "virsh start VM2" I saw on VM1 a swapper 
thread running for a lot of time causing this delay and another thing 
when I checked "top" before and after I saw a drop in a free RAM 
memory and increase in a SWAP available memory (both by ~100M). this 
explains why the swapper showed up.

BUT how can you explain such behavior if I have totally isolated VMs 
running with 4G RAM each when on the HOST I have ~60G of free RAM 
space? Is it possible that both QEMUs try to allocate a shared memory 
region in HOSTs RAM that causes such interference ?

Thanks,

Yan

*From:*Yadi [mailto:yadi...@windriver.com]
*Sent:* Tuesday, July 26, 2016 12:24 PM
*To:* qemu-discuss@nongnu.org; Yan Fridland
*Subject:* Re: [Qemu-discuss] [problem]-starting a VM causes RT 
(latency) problem for application running on another VM

On 2016年07月26日 15:48, Yan Fridland wrote:

    Hi Yadi,

    Thanks a lot for your comment.

    Actually I am doing emulator pin to the QEMU process in the XML:

    <vcpu placement='static'>3</vcpu>

    <cputune>

    <vcpupin vcpu='0' cpuset='1'/>

    <vcpupin vcpu='1' cpuset='2'/>

    <vcpupin vcpu='2' cpuset='3'/>

    <emulatorpin cpuset='8'/>

    <vcpusched vcpus='1-2' scheduler='fifo' priority='99'/>

    </cputune>

    *BUT*, looks like it doesn’t help. Is there any way to make ALL
    the related QEMU threads run within the scope of cores 1,2,3,8
    *ONLY* in the above example?


it is so wired, I have no better ways to handle it since I have no 
experienced the similar issue as your. so, just two suggestion for you:

1: check all qemu thread associated entries under cgroup filesystem,  
sure that  cpusets are correctly set.
2: if 1 fails, it may means there is nothing left to do, ftrace for 
schduler.

whatever result, let me know please. I am very interesting in the issue.

Yadi


    Thanks,

    Yan

    *From:*Yadi [mailto:yadi...@windriver.com]
    *Sent:* Tuesday, July 26, 2016 6:12 AM
    *To:* Yan Fridland; qemu-discuss@nongnu.org
    <mailto:qemu-discuss@nongnu.org>
    *Subject:* Re: [Qemu-discuss] [problem]-starting a VM causes RT
    (latency) problem for application running on another VM

    On 2016年07月25日 23:03, Yan Fridland wrote:

        Hello All,

        Here is a problem I saw when I ran virsh start to a VM.

        I have 2VMs on a server when one of them is running a RT
        application and the other is in shut down state. Then I
        executed "virsh start" to the second VM and saw 2-3 seconds
        later a big latency problem in my RT application running on
        the first VM.

        I want to emphasize that the VMs use dedicated isolated cores
        of the server and I have the relevant configuration on the
        server and the vcpu pinning section of virsh xml.

        Can anyone tell me why I experience such behavior and how it
        can be resolved?

        I will highly appreciate your help.

        Thank you,

        Yan

    I am not very sure, but I suggest you to add emulatorpin in xml
    file, then see results. actually,  all qemu associated process can
    use all processors via cgroup configure created by libvirt,

    https://libvirt.org/formatdomain.html#elementsCPUTuning