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

[Laurent Vivier <Laurent.Vivier@bull.net>]

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 ?
Perhaps I can swap server and client between guest and host ?

> 
> Sorry (if it's true :)
>

Thank you for your help,
Laurent
-- 
------------- Laurent.Vivier@bull.net ---------------
"The best way to predict the future is to invent it."
- Alan Kay


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