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

Peter Zijlstra
2018-07-13T16:42:57Z

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 :-) :-)