On Thu, Dec 22, 2016 at 4:30 AM, Daniel P. Berrange <berrange@redhat.com> 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The x-data-plane option is obsolete and should not be used.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