Messages in this thread | | | Date | Fri, 19 Apr 2019 09:56:10 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4 12/16] locking/rwsem: Enable time-based spinning on reader-owned rwsem |
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On Thu, Apr 18, 2019 at 11:15:33AM -0400, Waiman Long wrote: > On 04/18/2019 09:06 AM, Peter Zijlstra wrote:
> >> + /* > >> + * Check time threshold every 16 iterations to > >> + * avoid calling sched_clock() too frequently. > >> + * This will make the actual spinning time a > >> + * bit more than that specified in the threshold. > >> + */ > >> + else if (!(++loop & 0xf) && > >> + (sched_clock() > rspin_threshold)) { > > Why is calling sched_clock() lots a problem? > > Actually I am more concern about the latency introduced by the > sched_clock() call. BTW, I haven't done any measurement myself. Do you > know how much cost the sched_clock() call is? > > If the cost is relatively high, the average latency period after the > lock is free and the spinner is ready to do a trylock will increase.
Totally depends on the arch or course :/ For 'sane' x86 it is: RDTSC, MUL; SHRD; SHR; ADD, which is plenty fast.
I know we have poll loops with sched_clock/local_clock in them, I just can't seem to find any atm.
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