Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262300AbVC3Pz4 (ORCPT ); Wed, 30 Mar 2005 10:55:56 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262296AbVC3Pzz (ORCPT ); Wed, 30 Mar 2005 10:55:55 -0500 Received: from ida.rowland.org ([192.131.102.52]:7940 "HELO ida.rowland.org") by vger.kernel.org with SMTP id S262300AbVC3Pzi (ORCPT ); Wed, 30 Mar 2005 10:55:38 -0500 Date: Wed, 30 Mar 2005 10:55:36 -0500 (EST) From: Alan Stern X-X-Sender: stern@ida.rowland.org To: kus Kusche Klaus cc: linux-usb-users@lists.sourceforge.net, Subject: Re: 2.6.11, USB: High latency? In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1574 Lines: 35 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 majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/