年轻一代的垃圾收集器

Question

Have sort question - is it true that all GC in JDK 7 (other than G1) always use stop-the-world for young generation collection?

thanks

Answer 1

For OpenJDK, JRockit, IBM JVM, and Sun/Oracle JDK, the young collection is always stop the world for every available collector.

The only JVM I know of which does not have a stop the world collector is Azul's Zing. (Not free)

While OpenJDK/Hotspot has CMS this is mostly concurrent. There is still stop the world portions and in some cases CMS will fall back to a Full GC which is stop-the-world.

AFAIK, It is hard to find real world examples where G1 is faster in terms of pause time than CMS, however it is improving all the time.

Do your GC logs speak to you

Answer 2

All (almost) Java garbage collectors has some sort of a Stop-the-world phase where all the Java threads (not native threads) are suspended waiting for exclusive system operations to complete. This state is sometimes referred to as a safepoint.

The modern garbage collectors are concurrently running together with the applications threads, which means that the garbage collector perform its work at the same time as the application threads are running. During the garbage collector process there are phases where exclusive access memory is needed, in that phase the application Java threads goes into the safepoint state.

One alternative to get rid of the stop-the-world garbage collections is to go for the Zing JVM with the C4 collector from Azul systems. The implementation has a low pause approach with no stop-the-world collections at all. Instead it is using a concurrent compacting approach with no stop-the-world phase.

Answer 3

No it is not true. Java 7 also supports the older Concurrent Mark Sweep (CMS) collector. CMS is a low pause collector, just like G1.

UPDATE

Apparently CMS is only for the tenured generation ... according to the blog posting that you found at http://blogs.oracle.com/jonthecollector/entry/our_collectors

So that means that your proposition is in fact true.

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One could argue that all of the low-pause collectors: - need to stop the mutator threads to do some phases of their work, and - may fall back to a Full GC using the mark/sweep collector when they can't keep up.

However, there is a qualitive difference between "mostly concurrent" collectors like G1 and CMS, and other collectors that suspend non-GC threads for the entire duration of the collection process. That is what is normally meant by a "stop the world" strategy.