visualvm monitors memory usage

in my tomcat startup, i set -xms , -xmx to 1GB. inside visualvm when i monitor memory

1. when i do sampling of memory, it doesnt show accurate memory usage, until, if i press 'snapshot', and then the java classes that utilizing memory showing correct patterns and instances count. why like this?

2. also, as i set tomcat memory to 1gb, memory keep on increasing ing zip-zat and go up aroudn 960mb. until i press "perform gc" and the memory back to 200mb..

3. do you think needed to create scheduler to force hit perform.gc()开发者_JAVA技巧 on jvm ,everday at midnight?

any comments from my description

What I believe you see here is the concurrent Garbage Collector is doing young generation cleanups, but the heap is constantly churning so it can't get a hold of all the references for an old generation collection. When you hit the perform GC button you're doing a "stop-the-world" collection which blocks all the threads, freezing the heap, and allowing a much larger collection to occur. My main concerns here would be - does a large collection ever occur (after x hours)? Are you seeing OutOfMemory errors with a smaller heap?

1) Do you have the auto refresh button turned on? It may also be that profiler does a small GC (Eden space) before taking a snapshot.

2) This is perfectly normal. Garbage collection takes time and resources (processor cycles). If you've set your heap to a large size, it will wait until it reaches some percentage of that until triggering a garbage collection itself. I think the default free space to live object ratio is between 40% and 70% I know tomcat specifically changed the way it does garbage collection around version 5, tomcat v4 had performance issues because it spent a lot of time running the garbage collector. You may want to do some research here and see if tomcat has custom garbage collection options.

3) No. A nearly full heap is exactly what you're striving for. It may make sense to make the heap smaller so that a full garbage collection doesn't take such a long time. It's trade-off between lots of garbage collections (pauses) and lengthy garbage collections (long pauses). Each app is different, so I generally start with the defaults and tweak as needed. There are lots of options for garbage collection (and alternate collectors) if you're interested.

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