* Avi Kivity <avi@xxxxxxxxxx> wrote:
> On 05/30/2011 02:49 PM, Ingo Molnar wrote:
> >* Avi Kivity<avi@xxxxxxxxxx> wrote:
> >
> >> On 05/30/2011 02:26 PM, Takuya Yoshikawa wrote:
> >> >> >
> >> >> qemu also allows having more VCPUs than cores.
> >> >
> >> >I have to check again, then :) Thank you!
> >> >I will try both with many VCPUs.
> >>
> >> Note, with cpu overcommit the results are going to be bad.
> >
> >And that is good: if pushed hard enough it will trigger exciting (or
> >obscure) bugs in the guest kernel much faster than if there's no
> >overcommit, so it's rather useful for testing. (We made that
> >surprising experience with -rt.)
> >
> >Also, such simulation would be very obviously useful if you get
> >bugreports about 1024 or 4096 CPUs, like i do sometimes! :-) [*]
>
> I'll be surprised if 1024 cpus actually boot on a reasonable
> machine. Without PLE support, any busy wait (like
> smp_call_function_single()) turns into a delay the length scheduler
> time slice (or CFS's unfairness measure, I forget how it's called)
> - 3 or 4 orders of magnitude larger. Even with PLE, it's
> significantly slower, plus 1-2 orders of magnitude loss from the
> overcommit itself.
You are probably right about 1024 CPUs.
Right now i can produce something similar to it: 42 vcpus on a single
CPU:
$ taskse 1 kvm run --cpus 42
And that hangs early on during bootup, around:
[ 0.236000] Disabled fast string operations
[ 0.242000] #4
[ 0.270000] Disabled fast string operations
[ 0.275000] #5
[ 0.317000] Disabled fast string operations
[ 0.322000] #6
[ 0.352000] Disabled fast string operations
[ 0.358000] #7
[ 0.414000] Disabled fast string operations
The threads seem to be livelocked:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
22227 mingo 20 0 471g 95m 904 R 12.9 0.8 0:06.39 kvm
22230 mingo 20 0 471g 95m 904 R 12.9 0.8 0:06.36 kvm
22226 mingo 20 0 471g 95m 904 R 11.9 0.8 0:07.04 kvm
22228 mingo 20 0 471g 95m 904 R 11.9 0.8 0:06.38 kvm
22229 mingo 20 0 471g 95m 904 R 11.9 0.8 0:06.37 kvm
22231 mingo 20 0 471g 95m 904 R 11.9 0.8 0:06.37 kvm
22232 mingo 20 0 471g 95m 904 R 11.9 0.8 0:06.36 kvm
22233 mingo 20 0 471g 95m 904 R 11.9 0.8 0:06.33 kvm
7 root -2 19 0 0 0 S 2.0 0.0 1:12.53 rcuc0
with no apparent progress being made.
Thanks,
Ingo
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