[concurrency-interest] The very best CAS loop

Peter Levart peter.levart at gmail.com
Tue Sep 27 10:39:45 EDT 2016


Hi Andrew,


On 09/27/2016 02:56 PM, Andrew Haley wrote:
> On 27/09/16 11:44, Peter Levart wrote:
>
>> What I was hoping to measure with the benchmark is some difference
>> between using weak CAS and strong CAS in a getAndUpdate retry loop
>> that always invokes the update function (even on spurious failures
>> of weak CAS).
> That's going to be very difficult.  In practice truly spurious
> failures are rare enough that you won't see them unless you have false
> sharing.

And that's exactly what my benchmark tries to provoke. It uses CAS to 
concurrently update two distinct locations from two distinct threads (no 
data contention) with a common cache-line. I was trying to measure the 
worst case impact of re-invoking the update function on spurious failure 
of weak CAS vs. using strong CAS that would never fail in my benchmark 
(as it is strong), as a function of the update function execution time. 
But it appears there is no difference on Oracle's arm32 no matter how 
long the update function executes.

> (In which case, of course, strong CAS slows down too.)  You
> benefit from using weak CAS at times of high contention by not
> spinning to retry a SC which is going to fail anyway.
>
>> I can understand that this is not possible on x86_64 as both are
>> mapped to the same strong CAS intrinsic. But I also haven't observed
>> any meaningful difference on arm32. So is it possible that Oracle's
>> JDK 9 b137 for arm32 also maps both weak and strong CAS to the same
>> implementation which is a strong CAS simulation?
> Certainly.  The only way to be enlightened is to look at the generated
> source.
>
>> Is there any platform where weak CAS is actually a weak CAS
>> currently?
> AArch64.

Ah. I don't have any device for that (yet) ;-)

Thanks, Peter

> Andrew.
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