Re: A testing for KVM

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Dong, Eddie wrote:
> In Xen, we virtualize TSC too to make sure the guest TSC time is
> synchronized with 
> guest PIT time, so guest can see an accurate virtual time. (refer my
> presentation
> doc on Xen September summit 06.) It is good but time to time
> we see bugs due to the complicated time virtualization mechanism. As far
> as I know,
> Vmware doesn't solve this problem either, it depends on guest
> application to sync
> guest time with real time (network or host). 
>
>   

Do you mean that rdtsc is trapped?  Or that you play with TSC_OFFSET so 
that time is smoother?

> We can use PV time of linux, another way is to persuade community to
> give up 
> the cross reference of PIT and TSC to give up picking the lost ticks
> given that 
> today's OS is smart enough and won't disable IRQ for that long time
> before
>  another PIT expires (1-4ms), so those kind of picking up lost ticks is
> quit 
> unnecessary, especially for X86-64 (faster processor).
>
> Another big issue is that guest Linux will eventually fail back to PIT
> time source after
> hundreds of "lost ticks" and thus give up TSC time source. This is OK
> for normal application
> but dead set for database server where gettimeofday is frequently used.
> We saw
>  30-40% performance degradation due to this only. (gettimeofday  will
> read port 40
>  multiple times which is extremely slow under virtualization).
>
> I will ask Jun, Avi to bring this issue to virtualization mini summit to
> persuade 
> communitty to do some changes in guest timer IRQ handler.
>   

For newer kernels, we can supply a paravirt clocksource (as in Anthony's 
patchset) which will  remove the need for changing the bare hardware 
time code.

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


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