On Mon, 2011-05-30 at 14:36 +0200, Ingo Molnar wrote: > * 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. I've noticed the same issue when using a 2.6.39 kernel. Try doing the same with the 2.6.37 kernel we have in the tree. -- Sasha. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
