Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes: > On Fri, 7 Apr 2017 14:49:01 +0800 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote: > >> To reduce the lock contention of swap_info_struct->lock when freeing >> swap entry. The freed swap entries will be collected in a per-CPU >> buffer firstly, and be really freed later in batch. During the batch >> freeing, if the consecutive swap entries in the per-CPU buffer belongs >> to same swap device, the swap_info_struct->lock needs to be >> acquired/released only once, so that the lock contention could be >> reduced greatly. But if there are multiple swap devices, it is >> possible that the lock may be unnecessarily released/acquired because >> the swap entries belong to the same swap device are non-consecutive in >> the per-CPU buffer. >> >> To solve the issue, the per-CPU buffer is sorted according to the swap >> device before freeing the swap entries. Test shows that the time >> spent by swapcache_free_entries() could be reduced after the patch. >> >> Test the patch via measuring the run time of swap_cache_free_entries() >> during the exit phase of the applications use much swap space. The >> results shows that the average run time of swap_cache_free_entries() >> reduced about 20% after applying the patch. > > "20%" is useful info, but it is much better to present the absolute > numbers, please. If it's "20% of one nanosecond" then the patch isn't > very interesting. If it's "20% of 35 seconds" then we know we have > more work to do. The average run time of swap_cache_free_entries() is reduced from about ~137us to ~111us. The total samples of swap_cache_free_entries() is about 200000, run on 16 CPUs, so the wall time is about 1.7s. I will revise the tests to get the total run time reduction. > If there is indeed still a significant problem here then perhaps it > would be better to move the percpu swp_entry_t buffer into the > per-device structure swap_info_struct, so it becomes "per cpu, per > device". That way we should be able to reduce contention further. > > Or maybe we do something else - it all depends upon the significance of > this problem, which is why a full description of your measurements is > useful. Yes. I will provide more and better measurement firstly. Best Regards, Huang, Ying -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
