Where Are the Sharp Edges?

In general GC ergonomics works best for an application that has reached a steady state behavior in terms of its allocation pattern. Or at least it is not changing its allocation pattern quickly. GC ergonomics measures the pause times and throughput of the application and changes the size of the heap based on those measurements.

The measurements of pause time and throughput are kept in terms of a weighted average where (as one would expect) the most recent measurements are weighted more heavily. By using a weighted average GC ergonomics is not going to turn on a dime in response to a change in behavior by the the application, but it is also not going to go flying off in a wrong direction because of normal variations in behavior.

If past behavior is not a good indicator of future performance, then GC ergonomics can lag behind in its decision making. If a change is just an occasional bump in the road, GC ergonomics will catch up. If behavior is all over the map, well, what can I say.

The easiest way to get into trouble with GC ergonomics is to specify a pause time goal that is not reachable. Typically what happens is that GC ergonomics reduces the pause times by reducing the size of the heap. As the heap is shrunk the frequency of collections goes up and throughput goes down. GC ergonomics is willing to drive throughput to nearly zero (by doing collections nearly all the time) in order to reach the pause time goal. I tell people to run without a pause time goal initially and see how large the collection pauses get. That gives a baseline for experimenting with a pause time goal. Then have a little fun and try some pause times.

Another thing you should be aware of is that GC ergonomics is going to run at every collection. If your applications has settled into a stable steady state, GC ergonomics is still looking to see if anything is changing so it can adjust. It does cost you some cycles, but I don't think it's significant. Let me put it this way. I've never seen GC ergonomics code show up in a significant way on performance profiles. This is probably less a sharp edge than a mild poke in the ribs. If you think that your application is really not going to be changing its behavior after it has settled in and want those last few cycles, run with GC ergonomics until your application has reached its steady state and look to see how the heap is sized. You'll have to pay attention to how the generations are sized also. Then select those sizes on the command line and turn GC ergonomics off. At least for most of you that should be plenty good. If performances is not quit as high and you don't already know about survivor spaces, you may have to learn about them. The document "Tuning Garbage Collection with the 5.0 Java Virtual Machine" should help. It can be found under the URL below (same one as in "Magic"). If performances is actually better, rejoice and let me know how we can be doing better.

http://java.sun.com/docs/hotspot

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