Messages in this thread | | | Subject | Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV spinlock | From | Oscar Salvador <> | Date | Tue, 5 Sep 2017 08:57:16 +0200 |
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On 09/05/2017 08:28 AM, Juergen Gross wrote: > On 05/09/17 00:21, Davidlohr Bueso wrote: >> On Mon, 04 Sep 2017, Peter Zijlstra wrote: >> >>> For testing its trivial to hack your kernel and I don't feel this is >>> something an Admin can make reasonable decisions about. >>> >>> So why? In general less knobs is better. >> +1. >> >> Also, note how b8fa70b51aa (xen, pvticketlocks: Add xen_nopvspin parameter >> to disable xen pv ticketlocks) has no justification as to why its wanted >> in the first place. The only thing I could find was from 15a3eac0784 >> (xen/spinlock: Document the xen_nopvspin parameter): >> >> "Useful for diagnosing issues and comparing benchmarks in over-commit >> CPU scenarios." > Hmm, I think I should clarify the Xen knob, as I was the one requesting > it: > > In my previous employment we had a configuration where dom0 ran > exclusively on a dedicated set of physical cpus. We experienced > scalability problems when doing I/O performance tests: with a decent > number of dom0 cpus we achieved throughput of 700 MB/s with only 20% > cpu load in dom0. A higher dom0 cpu count let the throughput drop to > about 150 MB/s and cpu load was up to 100%. Reason was the additional > load due to hypervisor interactions on a high frequency lock. > > So in special configurations at least for Xen the knob is useful for > production environment.
It may be that the original patch was just to keep consistency between Xen and KVM, and also only for testing purposes. But we find a case when a customer of ours is running some workloads with 1<->1 mapping between physical cores and virtual cores, and we realized that with the pv spinlocks disabled there is a 4-5% of performance gain.
A perf analysis showed that the application was very lock intensive with a lot of time spent in __raw_callee_save___pv_queued_spin_unlock.
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