Monday, April 27, 2009

Heap size nnnnnK exceeds notification threshold (2048K)

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Lately I'm working a lot with XDB.
At the same time, having some spare time to investigate a few outstanding "issues", i decided to search the web for information about alert log messages like the following:
Memory Notification: Library Cache Object loaded into SGA
Heap size 2208K exceeds notification threshold (2048K)
KGL object name :XDB.XDbD/PLZ01TcHgNAgAIIegtw==
I wasn't particularly worried as this message is marked as a notification, so i was pretty sure that it was just informational stuff, however i wanted to learn more about and see what others say about it.
I quickly found a useful article on Frits's blog.

The suggested workaround, even if this isn't a serious issue as per Oracle Support note, consists in increasing the threshold level by executing the following statement:
alter system set "_kgl_large_heap_warning_threshold"=8388608 scope=spfile;
followed by a db restart.

The value of 8Mb has no special meaning, i guess we can increase or decrease it as necessary, because the concept of threshold depends on how much memory is allocated for the SGA: the bigger the SGA, the higher the threshold, but i don't recommend making any arbitrary assumptions here, it's probably better to adjust the value one step at a time, just to reduce the frequency of the notification.

If we work a lot with large objects in memory and the alert log is cluttered by many of these notifications, but the performances of the database are still ok, we might want to increase this value to avoid logging useless entries. For example, if most of the notification messages refer to objects in the range between 2 and 4Mb, then it makes sense to set the threshold around 4Mb, so that we can still "watch" any objects above that level.

We are interested in peaks, not in the average size objects, but what average size means depends on our applications.

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