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diff --git a/README.md b/README.md new file mode 100644 index 0000000..658e099 --- /dev/null +++ b/README.md @@ -0,0 +1,27 @@ +# vcpu_steal + +measures actual vcpu steal time from a host that's running as a +virtual guest inside of kvm, like a vps from most common providers. +we do this by busy-looping a cycle counter on a singular pinned vcpu, +trying to find gaps between any iteration that count as a preemption +by the hypervisor. + +usually the actual steal time is not exposed on cheap vps providers, +and even more expensive providers, and the value in `/proc/stat` +which should represent it is often just empty. +since we fully pin a single cpu and run a busy loop, the only events +that can interrupt us are guest-kernel preemption (timer ticks, irqs) +and the host stealing our vcpu. the guest-kernel events are short and +usually sub-microsecond or just a little higher, so our 5us threshold +for found gaps filters them out as noise, leaving the host preemption +as the primary thing producing the longer gaps we measure. +this results in an error of +1-2% at the absolute most. + +reports the cycle counter gap value, alongside the kernel wall vs. +cpu time for comparison. +note that the guest kernel doesn't really know what's going on and +thinks that any time stolen from a task by the hypervisor belongs +to the running task, thus this is only here to contrast the tested +value we compute. + +usage: `./vcpu_steal [seconds] [cpu]` |