JVM running Tomcat - high non-DB paging rates

Good afternoon,

We have an instance of Tomcat running on IBM I (V7R3). It runs in a subsystem with a 4Gb private memory pool and is the only job that runs in that subsystem.

After the application of the latest cumulative release (Java Group Level 15) we have, on several occasions, seen very high Non-DB paging rates in this pool and a complete degradation in performance (sub-second TTFB degrading to 20+ seconds). This usually occurs around 9.30am – possibly because there is a spike in users arriving for work..

We placed a call with IBM support regarding this and the response has been less than acceptable so I am turning to the mailing list!

We had an instance of this earlier today and at the time it happened, the GC information we collected was as follows:

Initial heap size: 512.000M
Max heap size: 1534.000M
Current heap size: 619.875M
Heap in use: 439.007M

The job’s maximum temporary storage for the whole day today is around 1Gb – and yet we saw paging faults over 1000 and pages 5000+ in the 4Gb memory pool. These are usually zero or negligible.

If we allocate more storage to the pool – i.e. increase it to 5Gb, the issue subsides and performance returns to normal levels. We can later dial down the storage again without seeing recurrence of the issue.

Does anyone know why we could potentially see such symptoms? The memory requirements of the JVM seem to me to be more than amply served by the 4GB allocated to the pool but it is clearly looking for more memory, and I am at a loss to explain why.

Thanks for any assistance in advance!
Stu

Stuart Bramley
Technology Manager

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