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

[Laurent V. <Lau...@bu...>]

Le mercredi 23 avril 2008 à 11:48 -0500, Anthony Liguori a écrit :
> 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)

I have 40 ms instead of 0.09 ms, so Avi you are right.

Laurent
-- 
------------- Lau...@bu... ---------------
"The best way to predict the future is to invent it."
- Alan Kay