Java HotSpot Virtual Machine

I have a tomcat process running on linux which generates a lot of garbage.

Throughout the day the machine will be at around 80% idle, and then suddenly jump to 20% idle cpu. It can stay at 20% idle for 1-4 hours, then goes back to 80% idle where it can stay for hours as well. The graph looks like a step-function.

This does not correlate to the usage graph; the usage graph is smooth, typical of a B->C webapp.

In conjunction with the 20% idle, i also see about 20% spent in "system" cpu, as well as an increase in "user" cpu consumption. When at 80% idle, system% is around 1%-2%.

I log GC stats, and when the cpu is at 80% idle, i see:

75324.001: [GC 75324.001: [ParNew: 65408K->0K(65472K), 0.0102260 secs] 472166K->407009K(1535936K), 0.0104630 secs]
75327.998: [GC 75327.998: [ParNew: 65408K->0K(65472K), 0.0118190 secs] 472417K->407212K(1535936K), 0.0120560 secs]
75333.342: [GC 75333.343: [ParNew: 65408K->0K(65472K), 0.0105900 secs] 472620K->407480K(1535936K), 0.0108260 secs]

About 10 msecs spent in GC every 4 secs, but when it gets to 20% idle i see:

30336.581: [GC 30336.581: [ParNew: 65408K->0K(65472K), 0.5277730 secs] 244204K->203369K(1535936K), 0.5280800 secs]
30338.403: [GC 30338.403: [ParNew: 65408K->0K(65472K), 0.4632960 secs] 268777K->224890K(1535936K), 0.4635790 secs]
30340.256: [GC 30340.256: [ParNew: 65408K->0K(65472K), 0.5050930 secs] 290298K->248727K(1535936K), 0.5053670 secs]

So now it's taking 500 msecs every 2 secs or so.

I'm using 1.5 with the following options:

-Xms1500m -Xmx1500m -XX:+UseConcMarkSweepGC -XX:NewSize=64m

So a 1.5G heap, with a 64M young generation. My inclination is to bump up the size of the young generation, but when i see these long 500 msecs young gen collection times, i get worried.

Can anybody explain the strange step-function behavior of the process over time that would be strictly related to GC?

Note that i see exactly the same pattern using the jrockit jvm (although the absolute numbers are different). I see the same step-function graph for cpu utilization vs a smooth graph for usage.

Thanks.