什么条件会阻止JVM运行FULL Garbage Collection？

Question

What conditions would prevent the JVM from running a FULL Garbage Collection when the CPU is at 5% to 8% load?

I am seeing a constant shallow GC cycle, but not able to tune the JVM to want to run FULL GC.

Where can I go to find the conditions that the JVM says "I am too busy to run".

Answer 1

When I was studying for my SCJP certification a lot of emphasis was made on

"You can not do anything to force the GC to run at any given time, you can just give hints to it"

The whole idea of having an automatic GC is precisely not having to worry about how or when it runs to clean up free memory for you. So, there is no way to actually change when or how GC does actually run... you would have to re-implement one JVM to do what you want.

There are just so many factors involved in this, there may be other, more elegant solutions for this.

Answer 2

It depends entirely on the garbage collector algorithm that you're using in your particular JDK. About all you can guarantee about garbage collection is that if the JVM throws an OutOfMemoryError , the garbage collector made its best effort to collect every unreachable/weakly reachable object. Even System.gc() doesn't guarantee anything, a no-op is a completely legal implementation.

Hence in that light I don't know if your question has any weight. If you truly believe that you need to tweak the garbage collector, it would help if you posted the problems you're seeing, and the profiling data that leads to believe that poor GC performance is the problem.

Outside of this, the garbage collector should be treated like a black box. The logic behind its implementation is surprisingly complex, and there's a very good chance it knows better than you what it ought to be doing at any given time. 99 times out of 100, trying to force the garbage collector to behave in a particular way will lower performance, not increase it.

Answer 3

这不是忙于运行，但它确实不需要额外的内存。