Re: [PATCH] proc: Avoid a thundering herd of threads freeing proc dentries

[Eric W. Biederman]

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