Re: kvm network latency, higher with virtio ?

[Dor Laor <dlaor@redhat.com>]

On 09/16/2009 10:27 AM, Michael S. Tsirkin wrote:
> On Tue, Sep 15, 2009 at 05:15:09PM +0200, Luca Bigliardi wrote:
>> Hi,
>> I'm running some tests between two linux instances bridged together.
>>
>> If I try to ping 10 times I obtain the following results:
>>
>> -net nic,model=virtio -net tap :
>>      rtt min/avg/max/mdev = 0.756/0.967/2.115/0.389 ms
>>
>> -net nic,model=rtl8139 -net tap :
>>      rtt min/avg/max/mdev = 0.301/0.449/1.173/0.248 ms
>>
>> So it seems with virtio the latency is higher. Is it normal?
>
> Yes, the main reason is the TX timer it uses for interrupt/vm exit mitigation.

Originally we used the tx mitigation timer in order to provide better 
throughput on the expense of latency.
Measurements of older versions of virtio proved that we can cancel this 
timer and achieve better latency while not hurting throughput.

Vhost wouldn't use it. For the time being until be get vhost, we should 
probably remove it from qemu.

>
>> The results I'm reporting were obtained with
>> - host
>>    qemu-kvm 0.11-rc2
>>    kvm-kmod-2.6.30.1
>>    kernel: 2.6.30.5 (HIGH_RES_TIMERS=y as suggested in
>>                      http://www.linux-kvm.org/page/Virtio )
>> - guest
>>    kernel: 2.6.31
>>
>> but I also tested older versions always obtaining latency values at least two
>> times higher than rtl8139/e1000 .
>>
>> Thank you,
>> Luca
>>
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