Re: [Qemu-devel] qemu processes

[Alex Bennée]

Ryan Boyce <address@hidden> writes:

> Alex,
>
> I have another KVM/qemu related CPU question. I am hoping you will be so
> kind as to answer again. I have a Dell Poweredge T430 server running a
> single Intel Xeon E5-2603 v3 CPU with 6 cores. I have 6 VMs running CentOS
> 7 via KVM. I am noticing some terrible host CPU performance when a VM runs
> a simple process such as a recursive search on its file system. I see VM
> CPU acting normally but the host CPU skyrockets to well over 100%. I have a
> few ideas as to why this is but am hoping you might be able to point me in
> the right direction. Thank you so much for your help!

A lot will depend on caching and how much of your guest file-system is
already in the host page cache. Otherwise you have a round-trip to make
to load the blocks into RAM.

However usually my first step is to run something like:

  perf top

And see where the host is spending its time and go from there.

>
> Ryan
>
> On Wed, Jun 12, 2019 at 12:50 PM Ryan Boyce <address@hidden> wrote:
>
>> Thank you my friend!
>>
>> On Wed, Jun 12, 2019 at 12:48 PM Alex Bennée <address@hidden>
>> wrote:
>>
>>> You've actually contacted the development list but I can answer the
>>> question.
>>>
>>> QEMU is multi-threaded so there will be a thread per KVM based vCPU
>>> (and also for TCG based vCPUs where MTTCG is enabled). There will also
>>> be a number of additional threads created including at least one for
>>> the main monitor thread and potentially additional threads for
>>> servicing IO requests. So expect to see $SMP + 1 + n threads for each
>>> QEMU instance.
>>>
>>> On Wed, 12 Jun 2019 at 16:32, Ryan Boyce <address@hidden> wrote:
>>> >
>>> > Hi Stefan,
>>> >
>>> > I am a big fan of your blog! In relation to your blog post,
>>> >
>>> http://blog.vmsplice.net/2011/03/qemu-internals-big-picture-overview.html
>>> ,
>>> > I have a question I am really hoping you can answer:
>>> >
>>> > When I start a VM, I see that the qemu-kvm processes start and run as
>>> > "qemu-kvm..... -name "something1"...."
>>> > My issue, though, is I see multiple processes running as "qemu-kvm.....
>>> > -name "something1"...." and each of these processes has memory/cpu/disk
>>> > mapped to it.
>>> > Does qemu-kvm run VMs on a one-to-one VM-to-host process basis? Or does
>>> > each virtual cpu on the guest get its own process on the host?
>>> > For example, If I have a VM that has 4 virtual cpus, will I see 4
>>> processes
>>> > (PIDs) on the host or should I see just see one for the VM itself.
>>> >
>>> > Any help you can give would be GREATLY appreciated. Thank you!
>>> >
>>> > Ryan
>>>
>>>
>>>
>>> --
>>> Alex Bennée
>>> KVM/QEMU Hacker for Linaro
>>>
>>


--
Alex Bennée