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SubjectRe: [PATCH 0/6] KVM support for TSC scaling
On 02/21/2011 07:28 PM, Roedel, Joerg wrote:
> > - what's the cost of wrmsr(TSC_MULT)?
>
> Hard to tell by now because I only have numbers for pre-production
> hardware.

Can you ask your hardware people what the cost will likely be? msrs are
often expensive, and here we have two in the lightweight exit path.

> > There are really two ways to implement this feature. One is fully
> > generic, like you did. The other is to implement it at the host level -
> > have a sysfs file and/or kernel parameter for the desired tsc frequency,
> > write it once, and forget about it. Trust management to set the host
> > tsc frequency to the same value on all hosts in a migration cluster.
>
> The motivation here is mostly the flexibility. Scale the TSC for the
> whole migration cluster only makes sense if all hosts there support the
> feature. But the most likely scenario is that existing migration
> clusters will be extended by new machines and guests will be migrated
> there. And these guests should be able to see the same TSC frequency on
> the new host as the had on the old one. The older machines in the
> cluster may even have different TSC frequencys. With this flexible
> implementation those scenarios are possible. A host-wide setting for the
> scaling will make the feature useless in those (common) scenarios.

This doesn't really work, since we don't know on what host the TSC
calibration loop ran:

- start guest on host H1
- migrate it around, now it's on host H2
- guest reboots, reruns calibration loop
- migrate it around some more, now it's on host H3
- migrate to host with tsc multiplier Hnew

So, what should we set the multiplier to? H1, H2, or H3's tsc rate?

--
error compiling committee.c: too many arguments to function



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