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. Juergen
