[concurrency-interest] Scalability of ConcurrentHashMap#computeIfAbsent

Josh Humphries jhump at bluegosling.com
Thu Jul 6 11:01:54 EDT 2017


On Thu, Jul 6, 2017 at 10:47 AM, Doug Lea <dl at cs.oswego.edu> wrote:

> On 07/06/2017 10:14 AM, Benoit Daloze wrote:
>
> > I have been chasing a scalability problem that only happens with over 8
> > cores on my benchmark and it turns out
> > ConcurrentHashMap#computeIfAbsent when used as a cache with the same key
> > does not scale.
>

This is necessary since the spec
<https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentHashMap.html#computeIfAbsent-K-java.util.function.Function->
indicates
that the operation (including computation of the value) is atomic. To make
this atomic, it must use pessimistic locking, so it seems you are observing
lock contention. (Interestingly, this behavior contrasts with the default
implementation
<https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentMap.html#computeIfAbsent-K-java.util.function.Function->
in
the ConcurrentMap interface, which uses optimistic concurrency and retries
and makes no promise of atomicity.) The scalability of the map relies on
many threads operating on different keys, not all operations using the same
key.

Your work-around, to have every thread eagerly execute the function in
parallel and then "first one wins" to store the value in the map, looks
appropriate.


>
> Please try this using JDK9, which locks only on (possible) hash
> collision. (See discussion in list archives.)
>

The original question indicates the operation not scaling is for the same
key, so I think it will encounter a hash collision and the same scalability
issue even with this patch.


> If we had known that JDK9 would face so much adoption delay,
> we might have tried to arrange backports of this and other minor
> improvements and fixes...
>
> -Doug
>
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