Peter Firmstone wrote:
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.
If you have multiple references containing object state, I'd suggest
using an immutable wrapper object (no setters) containing implicit
references to the objects, where it is read or replaced using a single
AtomicReference. The problem you have then is whether the state your
wrapping has visibility elsewhere or not, it is likely that these will
all need to be created by using defensive copies in your constructor,
each time you wish to update state.
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it also must not be published (an implicit reference allowed to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
