Re: pvUSB backend performance

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Jun 24, 2015 at 02:06:02PM +0200, Juergen Gross wrote:
> Hi,
> 
> my qemu integrated pvUSB backend is now running stable enough to do
> some basic performance measurements. I've passed a memory-stick with
> about 90MB of data on it to a pv-domU. Then I read all the data on
> it with tar and looked how long this would take (elapsed time):
> 
> in dom0:                     5.2s
> in domU with kernel backend: 6.1s
> in domU with qemu backend:   8.2s
> 
> So the qemu backend is about 30% slower than the kernel backend. Is
> this acceptable?

That sounds about right. I remember seeing (two years ago?) an demonstration
by kraxel where he was doing USB camera in a guest. He pointed out
that USB1 (which is default one in QEMU) implementation sucks as QEMU
has to poll for the commands every time. The XHCI implementation is
better as it has some better API, or such (-EFORGOT).

Oh, this has more details:
https://www.kraxel.org/blog/2014/03/qemu-and-usb-tablet-cpu-consumtion/

> 
> BTW: I managed not to have to copy the I/O-data in the backend, it is
> just mapped into user space and then given to the USB framework of qemu.
> I think it will be copied there, but changing this will require a lot
> of work...
> 
> 
> Juergen
> 
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