linux-kernel - Re: [PATCH 0/3] use rwlock in order to reduce ep_poll

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Date:   Thu, 13 Dec 2018 10:13:23 -0800
From:   Davidlohr Bueso <dbueso@...e.de>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     Jason Baron <jbaron@...mai.com>, Al Viro <viro@...iv.linux.org.uk>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] use rwlock in order to reduce ep_poll_callback()
 contention

On 2018-12-12 03:03, Roman Penyaev wrote:
> The last patch targets the contention problem in ep_poll_callback(), 
> which
> can be very well reproduced by generating events (write to pipe or 
> eventfd)
> from many threads, while consumer thread does polling.
> 
> The following are some microbenchmark results based on the test [1] 
> which
> starts threads which generate N events each.  The test ends when all 
> events
> are successfully fetched by the poller thread:
> 
>  spinlock
>  ========
> 
>  threads  events/ms  run-time ms
>        8       6402        12495
>       16       7045        22709
>       32       7395        43268
> 
>  rwlock + xchg
>  =============
> 
>  threads  events/ms  run-time ms
>        8      10038         7969
>       16      12178        13138
>       32      13223        24199
> 
> 
> According to the results bandwidth of delivered events is significantly
> increased, thus execution time is reduced.
> 
> This series is based on linux-next/akpm and differs from RFC in that
> additional cleanup patches and explicit comments have been added.
> 
> [1] https://github.com/rouming/test-tools/blob/master/stress-epoll.c

Care to "port" this to 'perf bench epoll', in linux-next? I've been 
trying to unify into perf bench the whole epoll performance testcases 
kernel developers can use when making changes and it would be useful.

I ran these patches on the 'wait' workload which is a epoll_wait(2) 
stresser. On a 40-core IvyBridge it shows good performance improvements 
for increasing number of file descriptors each of the 40 threads deals 
with:

64   fds: +20%
512  fds: +30%
1024 fds: +50%

(Yes these are pretty raw measurements ops/sec). Unlike your benchmark, 
though, there is only single writer thread, and therefore is less ideal 
to measure optimizations when IO becomes available. Hence it would be 
nice to also have this.

Thanks,
Davidlohr