-verbose:gc logs garbage collector runs and how long they're taking.
It is generally use as first tool to investigate if GC is a bottleneck for a given application.
-XX:+PrintGCDetails includes the data from -verbose:gc but also adds information about the size of the new generation and more accurate timings.
-XX:-PrintGCTimeStamps Print timestamps at garbage collection.

If you want to understand G1 GC, forget everything you know about the young generation and the old generation. As you can see in the picture, one object is allocated to each grid, and then a GC is executed.

Then, once one area is full, the objects are allocated to another area, and then a GC is executed. The steps where the data moves from the three spaces of the young generation to the old generation cannot be found in this GC type. This type was created to replace the CMS GC, which has causes a lot of issues and complaints in the long term. The biggest advantage of the G1 GC is its performance.

It is faster than any other GC types that we have discussed so far. But in JDK 6, this is called an early access and can be used only for a test. It is officially included in JDK 7.

In my personal opinion, we need to go through a long test period (at least 1 year) before NHN can use JDK7 in actual services, so you probably should wait a while. Also, I heard a few times that a JVM crash occurred after applying the G1 in JDK 6. Please wait until it is more stable.

I will talk about the GC tuning in the next issue, but I would like to ask you one thing in advance. If the size and the type of all objects created in the application are identical, all the GC options for WAS used in our company can be the same.

But the size and the lifespan of the objects created by WAS vary depending on the service, and the type of equipment varies as well. In other words, just because a certain service uses the GC option "A," it does not mean that the same option will bring the best results for a different service

It is necessary to find the best values for the WAS threads, WAS instances for each equipment and each GC option by constant tuning and monitoring. This did not come from my personal experience, but from the discussion of the engineers making Oracle JVM for JavaOne 2010. In this issue, we have only glanced at the GC for Java. Please look forward to our next issue, where I will talk about how to monitor the Java GC status and tune GC