Re: [Libusb-win32-devel] Limit on Maximum Asynchronous URBs

[Gopal S. <go...@ne...>]

Thank you Stephan.  That information is useful.

One question though.  I wasn't aware that the maximum URB size was 64 kB in=
=20
Windows.  Is this true?  In Linux I believe the maximum is 16384 (and only=
=20
with Kernel versions 2.6 and up).

Gopal


On Tuesday 13 June 2006 10:18, Stephan Meyer wrote:
> Libusb's kernel driver doesn't have any built-in limitations because
> it doesn't internally queue any data. It just allocates an URB for each
> userspace request and passes that URB down the stack.
>
> But it should be obvious that the host controller driver has to queue
> these request and that the size of this queue is limited since kernel
> memory is a limited resource.
>
> I wrote a small test program (see attachment) to see how many requests
> Windows' USB stack can handle at once. And the limit on WinXP-SP2
> (with 1GB of RAM) seems to be exactly 2500 URBs regardless of the URB's
> data size.
>
> The maximum amount of data I was able to request at once with the test
> program was 2500 URB each with 64kB of data. That's more than 150MB!
>
> This limit may depend on the host controller driver and the Windows
> version.
>
>
> Stephan
>
> > Hi all,
> >
> > I did some poking around on the web but have had a hard time finding an
> > answer to this question.  Maybe someone here knows.
> >
> > Is there a maximum number of URBs that can be submitted to an endpoint /
> > interface / device at any given time?  For example, can I submit 1024
> > URBs each with 16 KB data to a read endpoint or will I be restricted by
> > the driver stack, host controller, or something else?  If there is such=
 a
> > restriction, does it apply to the device as a whole, just the interface,
> > or just the endpoint?
> >
> > Any insights would be much appreciated.
> >
> > Best regards,
> > Gopal
> >
> >
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