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Date:   Fri, 13 Jul 2018 18:42:39 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        andrea.parri@amarulasolutions.com,
        Will Deacon <will.deacon@arm.com>,
        Akira Yokosawa <akiyks@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Nick Piggin <npiggin@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and
 remove it for ordinary release/acquire
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Hi Michael,

On Fri, Jul 13, 2018 at 11:15:26PM +1000, Michael Ellerman wrote:
> I reran some numbers today with some slightly updated tests.
> 
> It varies quite a bit across machines and CPU revisions.
> 
> On one I get:
> 
> Lock/Unlock    Time             Time %    Total Cycles     Cycles  Cycles Delta
> lwsync/lwsync   79,290,859,955  100.0 %   290,160,065,087  145     -
> lwsync/sync    104,903,703,237  132.3 %   383,966,199,430  192     47
> 
> Another:
> 
> Lock/Unlock    Time             Time %    Total Cycles     Cycles  Cycles Delta
> lwsync/lwsync  71,662,395,722   100.0 %   252,403,777,715  126     -
> lwsync/sync    84,932,987,977   118.5 %   299,141,951,285  150     23
> 
> 
> So 18-32% slower, or 23-47 cycles.

Very good info. Note that another option is to put the SYNC in lock() it
doesn't really matter which of the two primitives gets it. I don't
suppose it really matters for timing either way around.

> Next week I can do some macro benchmarks, to see if it's actually
> detectable at all.
> 
> The other question is how they behave on a heavily loaded system.
> 
> 
> My personal preference would be to switch to sync, we don't want to be
> the only arch finding (or not finding!) exotic ordering bugs.
> 
> But we'd also rather not make our slow locks any slower than they have
> to be.

I completely understand, but I'll get you beer (lots) if you do manage
to make SYNC happen :-) :-)