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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
