On 05/09/17 08:58, Peter Zijlstra wrote: > On Tue, Sep 05, 2017 at 08:28:10AM +0200, 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. > > So the problem with qspinlock is that it will revert to a classic > test-and-set spinlock if you don't do paravirt but are running a HV. In the Xen case we just use the bare metal settings when xen_nopvspin has been specified. So paravirt, but without modifying any pv_lock_ops functions. Juergen > > And test-and-set is unfair and has all kinds of ugly starvation cases, > esp on slightly bigger hardware. > > So if we'd want to cater to the 1:1 virt case, we'll need to come up > with something else. _IF_ it is an issue of course. > -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
