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Date: Wed, 02 Jan 2002 10:43:33 -0800
From: David Brownell <david-b@pacbell.net>
Subject: Re: [linux-usb-devel] Re: highmem and usb [was:"sr: unalignedtransfer"
 in 2.5.2-pre1]
To: Oliver.Neukum@lrz.uni-muenchen.de
Cc: Matthew Dharm <mdharm-kernel@one-eyed-alien.net>,
        Jens Axboe <axboe@suse.de>, linux-kernel@vger.kernel.org,
        linux-usb-devel@lists.sourceforge.net, Greg KH <greg@kroah.com>
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> > > 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


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