Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

[Peter Zijlstra <peterz@infradead.org>]

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 :-) :-)