From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: A clocksource question Date: Tue, 09 Mar 2010 15:36:09 -0800 Message-ID: <4B96DB69.10101@goop.org> References: <4B962748.90609@invisiblethingslab.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4B962748.90609@invisiblethingslab.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Joanna Rutkowska Cc: "xen-devel@lists.xensource.com" List-Id: xen-devel@lists.xenproject.org On 03/09/2010 02:47 AM, Joanna Rutkowska wrote: > I would appreciate if somebody could shed some light on the currently > preferred setting of the clocksource for the pvops0 kernels, used both > in Dom0 and DomU (I'm using pvops0 in DomU because it provides backends, > e.g. network backend). > clocksource=xen is supposed to be the ideal clocksource, with no downsides. > I have a bad experience with running the Dom0 kernel > (xen/stabale-2.6.31.6-based) with the default clocksource=xen, as it > results in the system getting temporary "hangs", very short ones, but > annoying. E.g. when one types fast on a keyboard, then every 10 seconds > or so, the keystroke processing seems to be slowing down significantly > for a second o so. > There have been a number of reports about hiccups of varying degrees of severity when using clocksource=xen (and, interestingly, in KVM when using its very similar pv clock interface). I haven't seen anyone mention keyboard interactive performance; the failure mode I've seen is disk IO hiccuping or even getting completely wedged. Unfortunately I haven't made much headway in debugging it (or even reliably reproducing it). What is the hardware platform? How many CPUs are you using? Does pinning the vcpus to pcpus help? > Running the kernel with clocksource=jiffies eliminates the above problem > but has a disadvantage of the clock drift in Dom0. This is not > acceptable on my setup, where I don't have any networking in Dom0, which > means I cannot correct it via NTP. > > Interestingly the above problem didn't seem to affect the Dom0 kernel > based on xen/stable-2.6.32. That's interesting; there's no difference in the Xen-specific timekeeping parts of the kernel, but perhaps something else has changed in the way time is handled. I don't remember if 3.4.2 ended up with tsc emulation, but if it does you might try enabling it. > Unfortunately I assume I'm no longer able to > run this kernel when using Xen 3.4.2. > Yes. 3.4.3 will support the newer kernels, but that isn't released yet. J