ClassValue perf?

[Peter Levart]

On 05/30/2016 07:47 PM, Aleksey Shipilev wrote:
> On 05/30/2016 06:59 PM, Peter Levart wrote:
>> I also employed get-acquire/put-release memory ordering semantics
>> instead of SC (volatile) in hope that it might improve a bit the
>> performance on platforms such as PowerPC or ARM, but this can be changed
>> back to SC if anyone gets scared of it :-)
> Revert, you're playing with fire here. Your _default_ modus operandi
> should be "scared" when dealing with concurrency. The correctness with
> acq/rel should be proven separately, and not by empirical testing. There
> is a reason why putOrdered is not used everywhere.
>
> Thanks,
> -Aleksey

Hi Aleksey,

You are right.  Users kind of expect from a general construct like 
ConcurrentMap to behave in a SC manner. Although my use of 
get-acquire/put-release in a Map-like construct could be correct by 
itself, it might be surprising for users to observe things like:

final static LinearProbeHashtable<Integer, Integer> t = new 
LinearProbeHashtable<>();

Thread A:
t.put(1, 10);

Thread B:
t.put(2, t.get(1));

Thread C:
Integer v2 = t.get(2);
Integer v1 = t.get(1);

with a possible outcome of (v1, v2) being (null, 10);

Will revert that to volatile semantics...

Regards, Peter

>
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