RE: [Xen-devel] HZ=1000 with Xen?

["Neugebauer, Rolf" <rolf.neugebauer@xxxxxxxxx>]

Xen is run with HZ=100 but this should only be used for time keeping.
Most of the timer functions are actually hung off a reprogrammed APIC
timer. For guests we ideally want them to tell xen their next timeout
value (which they should know) and then we'd program a timeout value for
them . IIRC a guest gets periodic timer interrupts when it actually has
the CPU and we fake out he missed ticks when scheduling a VM. 

As the Linux/370 people have pointed out a while ago (a few years)
sending a periodic timer to quiescent VMs is not scalable as the system
will spent an awful lot of time just processing pointless timer
interrupts for these VMs. 

For xenLinux 2.4 we therefore used the IBM patch (to the timer code) to
program the APIC timer (via Xen). I don't remember what the current 2.6
story is. I believe there is an effort to push these patches in main
stream Linux.

With all this in mind, why would you want to increase the HZ value?

Thanks
rolf

> -----Original Message-----
> From: xen-devel-admin@xxxxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> admin@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of Diwaker Gupta
> Sent: 10 March 2005 19:25
> To: xen-devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: [Xen-devel] HZ=1000 with Xen?
> 
> AFAIK, right now both the Xen hypervisor and the guests run with a HZ
> value of 100. Is there any fundamental problem running them at 1000
> instead? (well I am trying it now, so I'll know if it even runs at
> all, but I just wanted to know if there were any deeper issues)
> 
> --
> Diwaker Gupta
> http://resolute.ucsd.edu/diwaker
> 
> 
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