Re: [Qemu-devel] Performance of USB2.0

[Til Obes <til@obes.name>]

Hi.

Am 01.11.2011 10:37, schrieb Gerd Hoffmann:
> I think the main issue here is that we don't do buffering / pipelining
> for bulk transfers at the moment.  We grab a single transfer request
> from the guest, pass it to the kernel, when it is done pass it back to
> the guest, then look look for the next one.  Instead we could queue up
> all transfer requests from the guest to the kernel, which would give a
> noticable better throughput.  The qemu usb subsystem can't handle that
> (yet).  Fixing that is one the TODO list though.
>
> Additionally all bulk xfer processing is done in the 1000 Hz frame
> timer, which combined with the above limits the number of packets to
> 1000 packets per second (and direction), i.e. with a MTU of 1500 you'll
> get 1500 * 1000 = 1.5 MB/s max.  This you can expect to get with the
> current code.
>
> Add some protocol and other overhead to the 7 MBit/s you are actually
> seeing and the numbers are pretty close, so there isn't much room to
> improve things.  The only option I see is to operate the device with an
> larger MTU if the usb device and your network setup can handle that.

That helps and explains why there is no speed difference between 1.1
and 2.0 at my tests. How can i help to prioritize this issue? ;)

Regards Til