Re: [linux-usb-devel] Re: highmem and usb [was:"sr: unalignedtransfer" in 2.5.2-pre1]

[David Brownell <david-b@pacbell.net>]

> > > I'd rather eliminate as much overhead as possible -- I already get
> > > complaints from performance fanatics about the inability of usb-storage to
> > > get past 92% bus saturation (sustained), and the problem will only get
> > > worse on USB 2.0
> >
> > Well then you'll  be glad to see a patch from me, soonish, that teaches
> > the usb-storage "transport" code to use bulk queueing.  That'll get the
> > bandwidth utilization up as high as it can get.  It won't address any of
> > these highmem issues though.
> 
> And there's the overhead of sleeping and waking a kernel thread. Larger io
> requests might help, but I am not sure.

Yes, it's that sleep/wake between scatterlist segments that's creating
that 92% (at 12 Mbit/sec) or about 20% (at 480 Mbit/sec :) bottleneck ...
Convert those calls to use bulk queuing, and those delays vanish.

- Dave