linux-kernel - Re: Question about cacheline bounching with percpu-rwsem and rcu-sync

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Date:   Sat, 8 Jun 2019 20:24:36 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>, rcu <rcu@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question about cacheline bounching with percpu-rwsem and rcu-sync

On Fri, May 31, 2019 at 10:43 AM Joel Fernandes <joel@...lfernandes.org> wrote:
[snip]
> >
> > Either way, it would be good for you to just try it.  Create a kernel
> > module or similar than hammers on percpu_down_read() and percpu_up_read(),
> > and empirically check the scalability on a largish system.  Then compare
> > this to down_read() and up_read()
>
> Will do! thanks.

I created a test for this and the results are quite amazing just
stressed read lock/unlock for rwsem vs percpu-rwsem.
The test is conducted on a dual socket Intel x86_64 machine with 14
cores each socket.

Test runs 10,000,000 loops of rwsem vs percpu-rwsem:
https://github.com/joelagnel/linux-kernel/commit/8fe968116bd887592301179a53b7b3200db84424

Graphs/Results here:
https://docs.google.com/spreadsheets/d/1cbVLNK8tzTZNTr-EDGDC0T0cnFCdFK3wg2Foj5-Ll9s/edit?usp=sharing

The completion time of the test goes up somewhat exponentially with
the number of threads, for the rwsem case, where as for percpu-rwsem
it is the same. I could add this data to some of the documentation as
well.

Thanks!

 - Joel