Gregg Wonderly wrote:
Peter Firmstone wrote:
Gregg Wonderly wrote:
I have been looking into some seemingly slow responses in several
clients running simultaneously, and I see in some stack traces that
there are synchronization points in DynamicPolicyProvider.implies()
that seem to be heavily contended. We probably need to revisit this
class and rewrite it to use copy on write mutation so that reads
(the majority of activity) are completely uncontended.
Any thoughts or experience with this issue?
This sounds like a job for
java.util.concurrent.ReentrantReadWriteLock! Da dat, da dat, da dat,
da da! Requires Java 5, works well, the javadoc is clear too. Can
you submit this as an issue on Jira?
We don't actually want to lock, we just want to use a copy on write
update strategy that does lock but set volatile references to the new
contents.
In other words you want an AtomicReference, the objects being
de-referenced must be accessed by getting the referent for every read,
it also must not be published (an implicit reference allowed to escape)
after a read. When the AtomicReference is updated it is guaranteed to
be done atomically, however if the referent has escaped, any escaped
(implicit) references will still refer to the old object. This isn't as
easy as it sounds.
Use the compareAndSet() method, in case another write occurs, if the
referent isn't the one expected (it just got updated), you can retry it.
I haven't had time to look into the details so can't comment on whether
this is appropriate or not. You might want to try this and the
ReentrantReadWriteLock and compare performance before deciding. The
contention write lock's cause might be negligible, for code, much easier
to protect, read and understand later on.
Cheers,
Peter.
Gregg Wonderly
