On Mon, 2013-11-25 at 20:58 +0000, Thomas Gleixner wrote: > The patch set from Davidlohr [1] tried to attempt the same via an > atomic counter of waiters in a hash bucket. The atomic counter access > provided enough serialization for x86 so that a failure is not > observable in testing, but does not provide any guarantees. > > The same can be achieved with a smp_mb() pair including proper > guarantees for all architectures.
I am becoming hesitant about this approach. The following are some results, from my quad-core laptop, measuring the latency of nthread wakeups (1 at a time). In addition, failed wait calls never occur -- so we don't end up including the (otherwise minimal) overhead of the list queue+dequeue, only measuring the smp_mb() usage when !empty list never occurs. +---------+--------------------+--------+-------------------+--------+----------+ | threads | baseline time (ms) | stddev | patched time (ms) | stddev | overhead | +---------+--------------------+--------+-------------------+--------+----------+ | 512 | 4.2410 | 0.9762 | 12.3660 | 5.1020 | +191.58% | | 256 | 2.7750 | 0.3997 | 7.0220 | 2.9436 | +153.04% | | 128 | 1.4910 | 0.4188 | 3.7430 | 0.8223 | +151.03% | | 64 | 0.8970 | 0.3455 | 2.5570 | 0.3710 | +185.06% | | 32 | 0.3620 | 0.2242 | 1.1300 | 0.4716 | +212.15% | +---------+--------------------+--------+-------------------+--------+----------+ While the variation is quite a bit in the patched version for higher nthreads, the overhead is significant in all cases. Now, this is a very specific program and far from what occurs in the real world, but I believe it's good data to have to make a future decision about this kind of approach. Thanks, Davidlohr -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
