Re: [PATCH 0/2] Batch writes to MMIO

[Anthony Liguori <anthony@codemonkey.ws>]

Laurent Vivier wrote:
> Le mercredi 23 avril 2008 à 19:25 +0300, Avi Kivity a écrit :
>   
>> Laurent Vivier wrote:
>>     
>>> Le mercredi 23 avril 2008 à 10:10 -0500, Anthony Liguori a écrit :
>>> [...]
>>>   
>>>       
>>>> The ne2k is pretty mmio heavy.  You should be able to observe a boost 
>>>> with something like iperf (guest=>host) I would think if this is a real 
>>>> savings.
>>>>     
>>>>         
>>> I like your advices :-D
>>>
>>> I use iperf with e1000 emulation and a slightly modified patch (to
>>> detect MMIO write in a loop), server is on the host, client on the
>>> guest, with default values.
>>>
>>> RESULT WITHOUT BATCHING:
>>>
>>> [  4]  0.0-10.0 sec    235 MBytes    197 Mbits/sec
>>> [  5]  0.0-10.0 sec    194 MBytes    163 Mbits/sec
>>> [  4]  0.0-10.0 sec    185 MBytes    155 Mbits/sec
>>> [  5]  0.0-10.0 sec    227 MBytes    190 Mbits/sec
>>> [  4]  0.0-10.0 sec    196 MBytes    164 Mbits/sec
>>> [  5]  0.0-10.0 sec    194 MBytes    163 Mbits/sec
>>> [  4]  0.0-10.0 sec    184 MBytes    154 Mbits/sec
>>>
>>> RESULT WITH BATCHING:
>>>
>>> ------------------------------------------------------------
>>> Server listening on TCP port 5001
>>> TCP window size: 85.3 KByte (default)
>>> ------------------------------------------------------------
>>> [  4]  0.0-10.0 sec    357 MBytes    299 Mbits/sec
>>> [  5]  0.0-10.1 sec    418 MBytes    347 Mbits/sec
>>> [  4]  0.0-10.0 sec    408 MBytes    342 Mbits/sec
>>> [  5]  0.0-10.0 sec    422 MBytes    353 Mbits/sec
>>> [  4]  0.0-10.1 sec    436 MBytes    362 Mbits/sec
>>> [  5]  0.0-10.0 sec    416 MBytes    348 Mbits/sec
>>> [  4]  0.0-10.0 sec    431 MBytes    361 Mbits/sec
>>>
>>> Well, it's nice ?
>>>   
>>>       
>> It's too good to be true.
>>
>> I think we're seeing two bugs cancel each other out, resulting in a 
>> performance gain.  Linux doesn't know how to queue outgoing packets, so 
>> it bangs on the mmio that starts the transmit after every packet.  mmio 
>> batching doesn't know that this mmio register is critical for latency, 
>> so it queues it up.  The result is that you you get not just mmio 
>> batching, but also packet batching!  Which dramatically improves 
>> performace at the expense of latency.
>>     
>
> How can I check that ? How can I measure latency ?
>   

ping (from guest to host)

Regards,

Anthony Liguori

> Perhaps I can swap server and client between guest and host ?
>
>   
>> Sorry (if it's true :)
>>
>>     
>
> Thank you for your help,
> Laurent
>   


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