Re: send data on "packet level"

On 8/8/07, Alan Stern <[email protected]> wrote:
> On Wed, 8 Aug 2007, Gabriel Maganis wrote:
>
> > My goal was to run a benchmark on how long the packets took to go from
> > my computer to the usb device. I tried to do it like "start =
> > timenow(); usb_control_msg(); end = timenow();" but the results I get
> > are not so stable and I don't think running the code for large n can
> > give me a good measure. Any idea on how else I might do it?
>
> Use a USB bus analyzer if you really want to see the low-level details.
> However the USB spec includes lots of information about timing.  You
> should be able to calculate the transfer time for a packet directly.
>
> Besides, it's still not clear what good such a benchmark would be.
> There are plenty of other variables besides the time required to
> transmit a packet:
>
>         Time to submit a request to the host controller driver;
>
>         Time for the host controller hardware to see the request
>         and begin transmitting the packets;
>
>         Time for the device to process and acknowledge the data;
>
>         Time for the host controller to raise an interrupt request;
>
>         Time for the interrupt to be processed.
>
> Out of all that, the time needed for actually sending a packet is
> pretty small.
>
> Alan Stern
>
>

Correct. I was hoping disabling preemption can keep those things
somewhat constant but looking at the list you gave, it seems the time
to travel over the cable is negligible. I am trying to see if a USB
extension cable can increase the time from my computer to the device.
Is that unlikely to be visible from a kernel module even if I use the
TSC?

Thanks,
Gabriel

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