I'm including Frederic and serviceability-dev since this is part of the
instrumentation done for serviceability.
On 11/15/2011 4:59 PM, David Holmes wrote:
Hi John,
On 16/11/2011 3:33 AM, John Cuthbertson wrote:
Thanks for the review. I did and I did that consciously. As you say -
with the current code the listener would only be notified if the
_recordPostGCUsage field of the TraceMemoryManagerStats object is true
(which it is by default) and none of the TraceMemoryManagerStats
instances created by the collectors change this. But there's nothing to
stop a collector creating a TraceMemoryManagerStats object with
_recordPostGCUsage false and_recordGCEndTime true. In this case wouldn't
we still want to notify the listener? I may be wrong (in which case I'll
add the extra guard) but I believe that recording (some) data and
notification should be two independent operations.
It depends on what data the listener is expecting to receive. If you
push the notification when there hasn't been an update does that make
sense? I don't know what the spec is for this.
The GC notifier and pushNotification call was added as this changeset:
http://hg.openjdk.java.net/jdk8/jdk8/hotspot/rev/78542e2b5e35
The spec for this is:
http://download.oracle.com/javase/7/docs/jre/api/management/extension/com/sun/management/GarbageCollectionNotificationInfo.html
Anyway I just wanted to flag the change in potential behaviour. If
no-one from GC has any issue with this then it's fine by me.
I think David is right that it should check recordPostGCUsage==true to
push a notification. The notification is sent with the last GC
statistics. If my memory serves correctly, if recordPostGCUsage is
false, it doesn't update the last GC usage but sending a notification
when recordPostGCUsage is false seems to be incorrect. Also, the
recording was specifically added for CMS since CMS has separate phases
that it needs to record different things.
Frederic would be the best person to comment on this. Frederic - it
seems to me that the pushNotification can be moved to the end of gc_end
function but within the if (countCollection) statement. Sorry I don't
have time to check the details out. It'd be good if you can help.
Thanks
Mandy
David
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Thanks,
JohnC
On 11/14/11 22:35, David Holmes wrote:
Hi John,
On 15/11/2011 4:46 AM, John Cuthbertson wrote:
Can I have a couple of volnteers to review the fix for this CR? The
webrev can be found at:
http://cr.openjdk.java.net/~johnc/7110173/webrev.0/.
In the original code the pushNotification is conditional on
recordPostGCUsage, but you've removed that guard by moving the code.
David
-----
The issue here was that the routine GCNotifier::pushNotification(),
which uses GC data held in GCMemoryManager::_last_gc_stat, was being
called before the values in GCMemoryManager::_last_gc_stat were being
populated for the current GC. As a result the JVM could pass
uninitialized or stale data to a listener. The fix is to move the call
to GCNotifier::pushNotification() after the code that populates
GCMemoryManager::_last_gc_stat. I also modified the GCStatInfo
constructor to fully initialize instances of that class.
Testing: The supplied test case on Windows, a crafted test case on
Solaris, and the nsk GC tests.
Thanks,
JohnC
