willy@xxxxxxxxxxxxxxxxxxxx writes: > On Mon, Jun 22, 2020 at 10:20:40AM -0500, Eric W. Biederman wrote: >> Junxiao Bi <junxiao.bi@xxxxxxxxxx> writes: >> > On 6/20/20 9:27 AM, Matthew Wilcox wrote: >> >> On Fri, Jun 19, 2020 at 05:42:45PM -0500, Eric W. Biederman wrote: >> >>> Junxiao Bi <junxiao.bi@xxxxxxxxxx> writes: >> >>>> Still high lock contention. Collect the following hot path. >> >>> A different location this time. >> >>> >> >>> I know of at least exit_signal and exit_notify that take thread wide >> >>> locks, and it looks like exit_mm is another. Those don't use the same >> >>> locks as flushing proc. >> >>> >> >>> >> >>> So I think you are simply seeing a result of the thundering herd of >> >>> threads shutting down at once. Given that thread shutdown is fundamentally >> >>> a slow path there is only so much that can be done. >> >>> >> >>> If you are up for a project to working through this thundering herd I >> >>> expect I can help some. It will be a long process of cleaning up >> >>> the entire thread exit process with an eye to performance. >> >> Wengang had some tests which produced wall-clock values for this problem, >> >> which I agree is more informative. >> >> >> >> I'm not entirely sure what the customer workload is that requires a >> >> highly threaded workload to also shut down quickly. To my mind, an >> >> overall workload is normally composed of highly-threaded tasks that run >> >> for a long time and only shut down rarely (thus performance of shutdown >> >> is not important) and single-threaded tasks that run for a short time. >> > >> > The real workload is a Java application working in server-agent mode, issue >> > happened in agent side, all it do is waiting works dispatching from server and >> > execute. To execute one work, agent will start lots of short live threads, there >> > could be a lot of threads exit same time if there were a lots of work to >> > execute, the contention on the exit path caused a high %sys time which impacted >> > other workload. >> >> If I understand correctly, the Java VM is not exiting. Just some of >> it's threads. >> >> That is a very different problem to deal with. That are many >> optimizations that are possible when _all_ of the threads are exiting >> that are not possible when _many_ threads are exiting. > > Ah! Now I get it. This explains why the dput() lock contention was > so important. A new thread starting would block on that lock as it > tried to create its new /proc/$pid/task/ directory. > > Terminating thousands of threads but not the entire process isn't going > to hit many of the locks (eg exit_signal() and exit_mm() aren't going > to be called). So we need a more sophisticated micro benchmark that is > continually starting threads and asking dozens-to-thousands of them to > stop at the same time. Otherwise we'll try to fix lots of scalability > problems that our customer doesn't care about. Has anyone come up with a more sophisticated microbenchmark or otherwise made any progress in tracking this down farther? Eric
