Re: [Qemu-devel] virtio-blk performance regression and qemu-kvm

[Dongsu Park]

Hi Martin,

On 05.03.2012 17:13, Martin Mailand wrote:
> Am 10.02.2012 15:36, schrieb Dongsu Park:
> >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