Re: JavaMemoryModel: JMM and caches.

From: Sylvia Else (
Date: Mon Oct 27 2003 - 14:58:38 EST


The issue I'm trying to resolve is that any code that keeps running into a
memory barrier is going to compromise the gains it should be getting from
memory caches. This is a price that must be paid if the code is really in a
continuous interaction with other threads. In my configuration example,
though, the barrier occurs simply because of the extremely rare situations
where the cache may be out of date. In addition, a short period of using
stale configuration data would be of no consequence.

I've discarded the ThreadLocal subcache approach because accessing a
ThreadLocal is even slower than synchronization (on an Pentium). This may
be an artefact of the Pentium's memory model, which eliminates cache
flushes in this case, but at the moment I have no other types of system to
try it on.

In this type of problem, data races seem to be inherent (exactly when did
the configuration change anyway?), but potentially manageable. The existing
mechanisms for doing the management are unnecessarily expensive to use.


>Doug Lea wrote
> > What I'm looking for is a mechanism that introduces a happens-before
> > relationship between some defined action in a thread, and some other
> > defined action that will occur at a more-or-less arbitrary future time,
> and
> > in a different thread.
>I might be misinterpreting your intent here, but I think you may be
>making this out to be harder than it is. If you need
>ordering/visibility without locking, make sure that reader threads read
>a volatile field that is written by writer threads. Some variant of
>this is used (sometimes in conjunction with locking or atomic updates
>to coordinate writers) in most "concurrently readable data structures",
>for example in JSR-166 ConcurrentHashMap and ConcurrentLinkedQueue.

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