RFR (XXS): 7110173: GCNotifier::pushNotification publishes stale data.

[Frederic Parain]

Hi,

The reason why the memory usage data are bundled with the notification
is to prevent data lost. If the notification doesn't provide the
memory usage data, the client has to perform a getLastGcInfo() call
to retrieve the data each time it receives a notification. But several
GC cycles might occur between the time the notification is received and
the time getLastGcInfo() is invoked. And there's no API to get memory
usage data older than the one from the last GC cycle. With the
notification including the memory usage data, the client is ensured
to get data for all GC cycles.

That said, the notification contains two memory usage datasets, one
created at the beginning of the GC cycle and one created at the end
of the cycle. There's no reason to condition the notification to
the recording of the post GC cycle data recording. It's reasonable
to send a notification for a GC which only collects the pre GC data
for instance. So I think it's ok to move the notification code out
of the recordPostGCUsage==true condition as long as the values in
the non recorded memory usage data set can clearly be identified
as being invalid. Right now they are initialized to zero, so it
looks ok to me.

Moving the notification code after the "if(countCollection)" block
looks good too.

However, I'm not an official reviewer, so you'll need more approvals
for this changeset.

Fred

On 11/16/11 03:26 AM, Mandy Chung wrote:
> 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
>> -----
>>
>>>
>>> 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
>>>

-- 
Frederic Parain - Oracle
Grenoble Engineering Center - France
Phone: +33 4 76 18 81 17
Email: Frederic.Parain at Oracle.com