Re: [Qemu-devel] [libvirt] Performance about x-data-plane

[Weiwei Jia]

On Thu, Dec 22, 2016 at 4:30 AM, Daniel P. Berrange <address@hidden> wrote:
> On Thu, Dec 22, 2016 at 01:34:47AM -0500, Weiwei Jia wrote:
>> Hi,
>>
>> With QEMU x-data-plane, I find the performance has not been improved
>> very much. Please see following two settings.
>>
>> Setting 1: I/O thread in host OS (VMM) reads 4KB each time from disk
>> (8GB in total). Pin the I/O thread to pCPU 5 which will serve I/O
>> thread dedicatedly. I find the performance is around 250 MB/s.
>>
>> Setting 2: I/O thread in guest OS (VMM) reads 4KB each time from
>> virtual disk (8GB in total). Pin the I/O thread to vCPU 5 and pin vCPU
>> 5 thread to pCPU5 so that vCPU 5 handles this I/O thread dedicatedly
>> and pCPU5 serve vCPU5 dedicatedly. In order to keep vCPU5 not to be
>> idle, I also pin one cpu intensive thread (while (1) {i++}) on vCPU 5
>> so that the I/O thread on it can be served without delay. For this
>> setting, I find the performance for this I/O thread is around 190
>> MB/s.
>>
>> NOTE: For setting 2, I also pin the QEMU dedicated IOthread
>> (x-data-plane) in host OS to pCPU to handle I/O requests from guest OS
>> dedicatedly.
>>
>> I think for setting 2, the performance of I/O thread should be almost
>> the same as setting 1. I cannot understand why it is 60 MB/s lower
>> than setting 1. I am wondering whether there are something wrong with
>> my x-data-plane setting or virtio setting for VM.  Would you please
>> give me some hints? Thank you.
>
> The x-data-plane option is obsolete and should not be used. You should
> use the modern iothread option instead, which is explicitly supported
> by libvirt XML.
>
> http://libvirt.org/formatdomain.html#elementsIOThreadsAllocation

Thanks for your reply. However, I think this is not the main points of
my question. Let me see whether Stefan has some comments. Thank you.


Best,
Weiwei Jia