Re: [PATCH 0/3] use rwlock in order to reduce ep_poll_callback() contention

From: Davidlohr Bueso
Date: Thu Dec 13 2018 - 13:13:27 EST

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:


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.


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.