[concurrency-interest] Should I avoid compareAndSet with value-based classes?

Alex Otenko oleksandr.otenko at gmail.com
Fri Jul 7 05:23:58 EDT 2017


No, not only updates, but reads, too.

If you assume non-atomic updates, the problem becomes just like with volatile long - has to use synchronized block for all accesses.

Alex

> On 7 Jul 2017, at 10:20, Andrew Haley <aph at redhat.com> wrote:
> 
> On 06/07/17 20:12, Gil Tene wrote:
> 
>> The answer to this is generally "But… concurrency!". Which is why
>> this is an ok place to chat about it probably. With no identity, how
>> do you achieve atomic updates? Michael's MutableClock example
>> (scroll down) of wanting to (potentially concurrently) add duration
>> amounts to some notion of a point in time, while avoiding the
>> potential for "losing" an add racy situations, requires some sort of
>> synchronization. Since Instant is not mutable, and we are
>> necessarily trying to atomically replace one Instant value with
>> another, we have a problem with the notion that these values have no
>> identity (and therefore no way to do a proper CAS on).
> 
> In practice it doesn't matter, because you don't need to CAS a
> reference to an Instant: all you need to do is to wrap all of the
> updates to that reference in synchronized blocks.  Readers don't need
> anything more than for that reference to be volatile.  The
> immutability of Instant and the sequential consistency of volatile
> provide all the guarantees that anyone needs or are possible.
> 
> I doubt that a synchronized block is going to perform much more badly
> than spinning in a CAS loop, especially in the presence of high
> contention.  In the many readers/few writers situation that Michael
> describes this is the optimal solution.
> 
> -- 
> Andrew Haley
> Java Platform Lead Engineer
> Red Hat UK Ltd. <https://www.redhat.com>
> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
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