Low perfomance when read from usb flash drive

[M. Warner Losh]

In message: <200903040922.48163.hselasky at c2i.net>
            Hans Petter Selasky <hselasky at c2i.net> writes:
: Hi Steve,
: 
: On Tuesday 03 March 2009, Steve Calfee wrote:
: > > I think the reduced performance can be explained by a clamp on the
: > > interrupt rate around 1000 interrupts per second instead of 8000. Maybe
: > > someone has an explanation for this?
: > >
: > > The EHCI is being programmed to interrupt at 125us intervals, but there
: > > seems to be limits other places.
: > >
: > > It is possible to workaround this in the umass driver by doing the cmd +
: > > read in one operation.
: >
: > Hi Hans,
: >
: > I am looking at using FreeBSD in an embedded product. I have not
: > examined your ehci software, but I am aware of how Linux and other
: > OSes run the controller.
: >
: > Why are you taking an interrupt every uFrame SOF?
: 
: If the transaction completes before 125us we take the interrupt before 125us. 
: The problem is that the interrupt delay becomes critical to performance when 
: the interrupt rate is close to the interrupt limitation.
: 
: For example:
: 
: Transferring 13Mbyte/sec at blocksize equal to 65536 bytes generates 600 
: interrupts. Hence the Mass Storage state machine has three steps the 
: throughput is computed like (600/3)*65536 bytes. If we on the average have to 
: wait 0.5ms for an interrupt we loose throughput. 

Shouldn't you be using filters and such to make this less relevant?  A
filter runs on the order of 5us after the interrupt on fast machines,
and 20us on slower (400MHz) ones.  You can feed the pipeline better,
and handle higher interrupt rates...

Warner