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[Xen-users] clocksource - XenServer 7 - AKA xen 4.6.1

  • To: xen-users@xxxxxxxxxxxxx
  • From: WebDawg <webdawg@xxxxxxxxx>
  • Date: Tue, 25 Oct 2016 16:41:57 -0500
  • Delivery-date: Tue, 25 Oct 2016 21:43:38 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>


I was looking at my xen server clock source and I see:

xl dmesg | grep -i timer

(XEN) [ 3.306920] Platform timer is 14.318MHz HPET

I have a E5420 and I have ran into clocksource issues with proxmox/kvm
so I was wondering about some things while reading this documentation:


Quote:  "If TSC frequency is invariant across freq/voltage scaling
(true for all Intel processors supporting VTx) and all processors
within the box share one crystal, please give boot option

Is that really true?  There is no shutdown/slowdown/suspend of the tsc
clock with VT processors?  The only reason I ask is because of the
proxmox (kvm) issue I was having and I think I had to disable speed
step on the system to fix at the time.  It was an i5 proc w/ VT-x...no
VT-d though.

I actually get more and more confused the more I read about CPU
timers.  I have a CPU intensive domU python application that seems to
need a more high precision timer then it has right now.

It reads the current time, does something, and the reads again and if
both reads are the same (which I think they are) it crashes.  I always
thought that tsc was more precise but (as I said i was getting
confused) I just read something that says that HPET was more precise.

It could be that I do not need precision but instead the speed so it
increments.  But I cannot understand how a clock on a CPU (that does
not follow the speed step stuff or power save stuff) cannot be

Reading this:  

Quote:  "remember that Time stamp counter is incremented at every CPU
clock cycle."

CPU clock cycle > HPET

It does go on to talk about the tsc issues that have to do with idle
states, scaling, and power but none of this applies to me right?

Any suggestions on what I should do here or which is better?

Why is xen 4.6.1 choosing HPET as default?  Also even if I switch over
to tsc or constant_tsc is the xen clocksource in the domU able to
handle the higher clock rate of a tsc based timer?

Last question I have:  It looks like I have to select my clocksource
in xen with clocksource=hpet/acpi/pit

That is from 
but it does not mention tsc there.  It also does not mention a way to
figure out what clocksource methods that the hardware supports.  Is
there a way to figure out what the hardware supports without running a
live cd or something?

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