How do objects age on G1 (Garbage First) garbage collector?

Question

What is the criteria to put a young object in an old region making it an old object or keep it in survivor regions?

The point 4 of Young GC of the official tutorial state:

"Live objects are evacuated (ie, copied or moved) to one or more survivor regions. If the aging threshold is met, some of the objects are promoted to old generation regions."

But I can't find what that criteria is.

EDIT:

Amit Bhati pointed me to the MaxTenuringThreshold parameter. I don't understand much from the official doc about it but I think I started to understand how it works.

Answer 1

With your help I think I found the answer here :

-XX:InitialTenuringThreshold=7 Sets the initial tenuring threshold for use in adaptive GC sizing in the parallel young collector. The tenuring threshold is the number of times an object survives a young collection before being promoted to the old, or tenured, generation. -XX:MaxTenuringThreshold=n Sets the maximum tenuring threshold for use in adaptive GC sizing. The current largest value is 15. The default value is 15 for the parallel collector and is 4 for CMS.

It is under the Debugging Options title :)

Under Garbage First (G1) Garbage Collection Options you can find this:

-XX:MaxTenuringThreshold=n Maximum value for tenuring threshold. The default value is 15.

It is not very descriptive if you have not read InitialTenuringThreshold description on the other section. It seems InitialTenuringThreshold is not a valid G1 option but I think the algorithm is the described there.

Answer 2

The following doc is good at explaining how to alter (reduce) the rate at which items are promoted from the survivor spaces to the Old Gen in the G1 collector.

http://java-is-the-new-c.blogspot.co.uk/2013/07/tuning-and-benchmarking-java-7s-garbage.html (the section entitled Tuning The Young Generation)

As the above answers say, the MaxTenuringThreshold is the key setting, but this is only an upper limit, and would be ignored if your YoungGen wasn't big enough to allow this to be honoured. In which case you'd need to increase either the overall YoungGen via NewRatio or just the SurvivorSpace via SurvivorRatio

Answer 3

From the Javadocs :

The heap space is divided into the old and the new generation. The new generation includes the new object space (eden), and two survivor spaces. The JVM allocates new objects in the eden space, and moves longer lived objects from the new generation to the old generation.

The young generation uses a fast copying garbage collector which employs two semi-spaces (survivor spaces) in the eden, copying surviving objects from one survivor space to the second. Objects that survive multiple young space collections are tenured, meaning they are copied to the tenured generation. The tenured generation is larger and fills up less quickly. So, it is garbage collected less frequently; and each collection takes longer than a young space only collection. Collecting the tenured space is also referred to as doing a full generation collection.

The frequent young space collections are quick (a few milliseconds), while the full generation collection takes a longer (tens of milliseconds to a few seconds, depending upon the heap size).

Other GC algorithms, such as the Concurrent Mark Sweep (CMS) algorithm, are incremental. They divide the full GC into several incremental pieces. This provides a high probability of small pauses. This process comes with an overhead and is not required for enterprise web applications.

Also check this article: Java Garbage Collectors – Moving to Java7 Garbage-First (G1) Collector

The young generation comprises of one Eden and two Survivor spaces. The live objects in Eden are copied to the initially empty survivor space, labeled S1 in the figure, except for ones that are too large to fit comfortably in the S1 space. Such objects are directly copied to the old generation. The live objects in the occupied survivor space (labeled S0) that are still relatively young are also copied to the other survivor space, while objects that are relatively old are copied to the old generation. If the S1 space becomes full, the live objects from Eden or S0 that have not been copied to it are tenured, regardless of their age. Any objects remaining in Eden or the S0 space after live objects have been copied are not live and need not be examined. Figure below illustrates the heap after young generation collection:

The young generation collection leads to stop the world pause. After collection, eden and one survivor space are empty. Now let's see how CMS handles old generation collection. It essentially consists of two major steps – marking all live objects and sweeping them.