Hi Martin,

On 05.03.2012 17:13, Martin Mailand wrote:

Am 10.02.2012 15:36, schrieb Dongsu Park:

quoted

Recently I observed performance regression regarding virtio-blk,
especially different IO bandwidths between qemu-kvm 0.14.1 and 1.0.
So I want to share the benchmark results, and ask you what the reason
would be.

Hi,
I think I found the problem, there is no regression in the code.
I think the problem is, that qmeu-kvm with the IO-Thread enabled
doesn't produce enough cpu load to get the core to a higher cpu
frequency, because the load is distributed to two threads.
If I change the cpu governor to "performance" the result from the
master branch is better than from the v0.14.1 branch.
I get the same results on a serversystem without powermanagment activated.

@Dongsu Could you confirm those findings?

Yes, I can confirm that.
I just tested with different CPU governor configs, "ondemand" and 
"performance". (qemu-kvm 1.0, AMD Phenom II X4 955)

The result is more or less like yours.
Bandwidth with "performance" gets more than 100% better than that with
"ondemand".
That looks definitely one of the reasons of regressions I experienced.
Actually I had always tested with "ondemand", which I haven't noticed.
 
Good catch, thanks!
Dongsu

1. Test on i7 Laptop with Cpu governor "ondemand".

v0.14.1
bw=63492KB/s iops=15873
bw=63221KB/s iops=15805

v1.0
bw=36696KB/s iops=9173
bw=37404KB/s iops=9350

master
bw=36396KB/s iops=9099
bw=34182KB/s iops=8545

Change the Cpu governor to "performance"
master
bw=81756KB/s iops=20393
bw=81453KB/s iops=20257


2. Test on AMD Istanbul without powermanagement activated.

v0.14.1
bw=53167KB/s iops=13291
bw=61386KB/s iops=15346

v1.0
bw=43599KB/s iops=10899
bw=46288KB/s iops=11572

master
bw=60678KB/s iops=15169
bw=62733KB/s iops=15683

-martin