Re: 2.6.11, USB: High latency?

From: Alan Stern (stern_at_rowland.harvard.edu)
Date: 03/30/05

  • Next message: Christoph Lameter: "Re: API changes to the slab allocator for NUMA memory allocation" 
    Date:	Wed, 30 Mar 2005 10:55:36 -0500 (EST)
To: kus Kusche Klaus <kus@keba.com>

    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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  • Next message: Christoph Lameter: "Re: API changes to the slab allocator for NUMA memory allocation"

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