libusb performance on 8.1

[Daniel O'Connor]

On 23/01/2011, at 24:03, Hans Petter Selasky wrote:
>> While doing some speed testing I find that FreeBSD does ~4Mb/sec, but Linux
>> does 17Mb/sec & OS X does 8-10Mb/sec.
> 
> You need to change the way you buffer the data. FreeBSD does not queue more 
> than 2 URB's at any time, and the turnaround time varies from 1ms to 125us due 
> to hardware IRQ restrictions. Linux queues up all it can get, which leads to 
> other kind of problems. The current internal buffer limit is 16Kbyte 8000 
> times per second which gives a MAX of 128 MByte/second.
> 
> In general during hardware design:
> 
> 1) Avoid short packets.
> 2) Make sure transfer buffers are matched between host and device.
> 
> From my experience it is very possible to reach +30Mbyte/second with LibUSB 
> under FreeBSD 8.2+, given the buffering is optimal for the kernel.
> 

Hmm... I am transferring fixed size packets of 512 bytes, so there shouldn't be any short packets (unless I misunderstand what you mean).

Ahh, I I see what you mean now.. I was transferring a single packet at a time because I didn't realise you could transfer more at once.

If I switch to transferring 2k at once it goes up to 16Mb/sec.

Thanks :)

--
Daniel O'Connor software and network engineer
for Genesis Software - http://www.gsoft.com.au
"The nice thing about standards is that there
are so many of them to choose from."
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