On Thu, Oct 25, 2018 at 10:09:22PM +0800, Junchang Wang wrote: > On Thu, Oct 25, 2018 at 5:45 PM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> wrote: > > > > On Thu, Oct 25, 2018 at 10:11:18AM +0800, Junchang Wang wrote: > > > Hi Akira, > > > > > > Thanks for the mail. My understanding is that PPC uses LL/SC to > > > emulate CAS by using a tiny loop. Unfortunately, the LL/SC loop itself > > > could fail (due to, for example, context switches) even if *ptr equals > > > to old. In such a case, a CAS instruction in actually should return a > > > success. I think this is what the term "spurious fail" describes. Here > > > is a reference: > > > http://liblfds.org/mediawiki/index.php?title=Article:CAS_and_LL/SC_Implementation_Details_by_Processor_family > > > > First, thank you both for your work on this! And yes, my cmpxchg() code > > is clearly quite broken. > > > > > It seems that __atomic_compare_exchange_n() provides option "weak" for > > > performance. I tested these two solutions and got the following > > > results: > > > > > > 1 4 8 16 32 64 > > > my patch (ns) 35 34 37 73 142 281 > > > strong (ns) 39 39 41 79 158 313 > > > > So strong is a bit slower, correct? > > > > > I tested the performance of count_lim_atomic by varying the number of > > > updaters (./count_lim_atomic N uperf) on a 8-core PPC server. The > > > first row in the table is the result when my patch is used, and the > > > second row is the result when the 4th argument of the function is set > > > to false(0). It seems performance improves slightly if option "weak" > > > is used. However, there is no performance boost as we expected. So > > > your solution sounds good if safety is one major concern because > > > option "weak" seems risky to me :-) > > > > > > Another interesting observation is that the performance of LL/SC-based > > > CAS instruction deteriorates dramatically when the number of working > > > threads exceeds the number of CPU cores. > > > > If weak is faster, would it make sense to return (~o), that is, > > the bitwise complement of the expected arguement, when the weak > > __atomic_compare_exchange_n() fails? This would get the improved > > performance (if I understand your results above) while correctly handling > > the strange (but possible) case where o==n. > > > > Does that make sense, or am I missing something? > > Hi Paul and Akira, > > Yes, the weak version is faster. The solution looks good. But when I > tried to use the following patch > > #define cmpxchg(ptr, o, n) \ > ({ \ > typeof(*ptr) old = (o); \ > (__atomic_compare_exchange_n(ptr, (void *)&old, (n), 1, > __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST))? \ > (o) : (~o); \ > }) > > gcc complains of my use of complement symbol > > ../api.h:769:12: error: wrong type argument to bit-complement > (o) : (~o); \ > ^ > > Any suggestions? You might need to do this for the macro argument: "(~(o))". Another possibility is ((o) + 1), which would work for pointers as well as for integers. Thanx, Paul
