Re: 2.6.11, USB: High latency?

[Alan Stern <stern@rowland.harvard.edu>]

On Wed, 30 Mar 2005, kus Kusche Klaus wrote:

> I'm performing realtime latency tests (for details about the hardware
> and software, see my mail "[BUG] 2.6.11: Random SCSI/USB errors when
> reading from USB memory stick" erlier today).
> 
> Even when the errors described in my previous mail does not occur,
> massive USB stick transfers cause latencies of 1 to 2 milliseconds,
> which is way too much for realtime control systems. 
> 
> I observe these latencies on a vanilla 2.6.11 at any rtprio (even 99),
> and on realtime-preempt-2.6.12-rc1-V0.7.41-11 at low rtprio (1). When
> running the program on realtime-preempt-2.6.12-rc1-V0.7.41-11 with
> rtprio 99, the latencies are gone, but using a rtprio higher than the
> interrupt handlers is not realistic.
> 
> Is there anything which can be done about it?

The latencies are almost certainly caused by the USB host controller 
driver.  I'm planning improvements to uhci-hcd which should help reduce 
the latency, but it will still be on the large side.  And I won't have 
time to write the changes to the driver for several months.

The best solution is to stop using uhci-hcd.  Get a PCI card with an OHCI 
or EHCI (high-speed) controller.  They do much more work in hardware, 
reducing the amount of time the driver needs to spend with interrupts 
disabled.

Alan Stern

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