Re: A clocksource question

[Jeremy Fitzhardinge <jeremy@goop.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