From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4741AFCF.7070103@domain.hid> Date: Mon, 19 Nov 2007 16:46:23 +0100 From: Theo Veenker MIME-Version: 1.0 References: <47415786.6040208@domain.hid> <2ff1a98a0711190725j78fbdc85j852cda9621a067be@domain.hid> In-Reply-To: <2ff1a98a0711190725j78fbdc85j852cda9621a067be@domain.hid> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-help] accuracy of system clock List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: Xenomai Gilles Chanteperdrix wrote: > On Nov 19, 2007 10:29 AM, Theo Veenker wrote: >> Hi all, >> >> I'm using clock_gettime(REALTIME,&t) in a kernel module (posix skin) to >> timestamp something. I know accessing this clock from user space yields >> values different from those returned by the xenomai posix skin due to >> ntpdate. > > Well, you can use the posix skin in user-space as well. > >> But my concern is that the difference in pace is too much. >> On one system it is about 90us/s wrong on another it is 160us/s. > >> I assume the speed of the ntp corrected clock is more or less correct. >> Is there some calibration procedure I can run to tweak xenomai to make >> sure in xenomai I get 1e9 ticks per *true* second? >> I have enabled CONFIG_X86_UP_APIC. > > There is currently no way to calibrate Xenomai, but there is a known > issue about the APIC frequency specifically. I do not know if some > quick fixes have been posted, but there is a (rather large) series of > patches fixing it. Can you suggest a different timing source? I thought using the APIC was preferred over using the TSC. What do other users do if they want 1 ms to last 1 ms? If the drift appears to be constant on each system I can problably account for it and apply a correction. I'll try that. Theo