[jmm-dev] weakCompareAndSet memory semantics

[David Holmes]

On 22/04/2016 10:03 PM, Andrew Haley wrote:
> On 04/22/2016 12:32 PM, David Holmes wrote:
>> I consider it a different scenario because it requires high contention.
>> If you have low contention (CAS ideal case) the spurious failures are
>> unlikely to make your data stale so jumping back out to the outer loop
>> to re-fetch doesn't gain anything - potentially the opposite!
>
> Potentially?  Maybe that would be true if spurious store exclusive
> failures were common, but they aren't.

If they aren't common you neither gain nor lose - so no motivation to 
use weakCAS.

>> Obviously this all depends on the exact circumstances - but that is why
>> I question these blanket statements about weakCAS performing better on
>> ll/sc systems. Given we're dealing with WORA Java code this may push
>> people towards always using weakCAS just in case they run on a ll/sc system.
>
> I just did 10^9 uncontended ldx/stx on my system here.  Guess how many
> spurious failures there were?  Zero.  This is a tool for people who
> know what they're doing.

I don't get your point. You could also write code that gets many 
spurious failures.

We are saying "use weakCAS on ll/sc because it will be better if there 
are spurious failures".

But now you are saying you won't get spurious failures anyway.

So why even bother with weakCAS?

David

> Andrew.
>