Re: [Qemu-devel] [PATCH 1/7] usb: remove old usb-host code

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 11/11/2013 09:47 AM, Gerd Hoffmann wrote:
> On Fr, 2013-11-08 at 17:51 +0100, Jan Kiszka wrote:
>> On 2013-11-08 16:39, Gerd Hoffmann wrote:
>>>    Hi,
>>>
>>>> OK, then here is the first issue I ran into while trying libusbx (git
>>>> head, i.e. 1.0.17+: The new stack causes significant latency issues that
>>>> makes it almost unusable for pass-through of USB audio devices (some
>>>> headset in my case). Reverting usb-linux and disabling libusb over QEMU
>>>> git head makes things work again. I'll have to stick with this for now
>>>> as it is affecting my work environment.
>>>>
>>>> Any spontaneous ideas how to analyse or even resolve this?
>>>
>>> Try setting isobsize property to something smaller than 32 (which is the
>>> default).
>>
>> OK, isobsize=2 and isobufs=32 helped, possibly other combinations as
>> well - but not just reducing isobsize or increasing isobufs. Any theory
>> about this? How can we find better defaults?
>
> isobsize is the size of a single buffer (in MaxPacketSize units).
> isobufs is the number of buffers in the ring.
>
> So the total ring buffer size is MaxPacketSize * isobsize * isobufs.
>
> isobsize basically trades overhead for latency.  Larger numbers reduce
> the overhead, smaller numbers reduce latency.
>
> isobufs should be as small as possible.  Start with 4 (default).  If you
> get overruns/underruns increase.
>
> We should probably look at the endpoint interval, then calculate how
> many packets we should expect within a certain time range and use that
> as additional factor for the buffer size.  That should get the defaults
> closer to the actual needs of the device.

hw/usb/redirect.c actually has some code for that, you could use that as
a start for the host redirection code...

Regards,

Hans