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From: Weiwei Jia <harrynjit@gmail.com>
Date: Mon, 2 Jan 2017 00:36:45 -0500
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Subject: [Qemu-devel] Throughput confusion with data plane
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: qemu-devel@nongnu.org

Hi,

I run one I/O thread (sequential read 4 KB each time and read 8 GB in
total) in host OS, the throughput is around 420 MB/s. However, when I
run this I/O thread in one VM (no other VM is created and data-plane
is enabled) with dedicated hardware, the throughput will be around 350
MB/s. The VM's experiment setting is as follows.

In the VM, there are 15 vCPUs (vCPU0 - vCPU14); each vCPU is pinned to
corresponding dedicated pCPU (for example, vCPU 0 is pinned to pCPU0
... vCPU 14 is pinned to pCPU 14); "Idle=poll" is added in the VM's
boot grub so that the vCPU will not be idle; in the VM, all the
interrupts are pinned to vCPU0 to guarantee these interrupts can be
responded on time; the I/O thread is executed on one of the vCPU
except vCPU0.

In the host OS, there are 16 pCPUs (pCPU0 - pCPU15); pCPU 0 - pCPU 14
are used for vCPU0 - vCPU 14 in the VM dedicatedly; pCPU 15 is used by
QEMU IOthread (data-plane) to handle I/O read requests from VM
dedicatedly.

Kernel version: 3.16.39
QEMU version: 2.4.1


I don't know why there is 70 MB/s difference between host OS and guest
OS as above experiment. Does anyone have same experiences? Any
comments? Thank you in advance.

BTW, I have checked several times about my hardware configuration and
I think the throughput difference as above should be related to QEMU.
Maybe, I miss any configuration about QEMU.


Best,
Weiwei Jia