Is Android garbage collector pausing other apps while running?

Question

I've found some information about the Android garbage collector that are contradictions to me.

Android Devevelopers Guide says:

Android 3.0 is the first version of the platform designed to run on either single or multicore processor architectures. A variety of changes in the Dalvik VM, Bionic library, and elsewhere add support for symmetric multiprocessing in multicore environments. These optimizations can benefit all applications, even those that are single-threaded. For example, with two active cores, a single-threaded application might still see a performance boost if the Dalvik garbage collector runs on the second core. The system will arrange for this automatically."

Ok, now the other thing

According to this link: The Dalvik Virtual Machine Architecture android uses mark and sweep aproach.

The current strategy in the Dalvik garbage collector is to keep mark bits, or the bits that indicate that a particular object is “reachable” and therefore should not be garbage collected, separate from other heap memory.

If we check how mark and sweep works on this link: Mark and Sweep Garbage Collection Algorithm , we can see this:

The main disadvantage of the mark-and-sweep approach is the fact that that normal program execution is suspended while the garbage collection algorithm runs. In particular, this can be a problem in a program that interacts with a human user or that must satisfy real-time execution constraints. For example, an interactive application that uses mark-and-sweep garbage collection becomes unresponsive periodically.

So my question now is, how does it really work? Does garbage collector pause everything while he is working, or is he capable of running completely independent on the other active processor core?

Answer 1

The Dalvik VM in the Gingerbread and beyond version is using the Mostly Concurrent partial collection garbage collector with pause times usually around 5ms. Therefore, yes, the GC is influencing the other apps by stopping them but the concurrent GC algorithm is able to minimaze these pauses.

You should look at :

• Technical details of Android Garbage Collector
• Does the DalvikVM Garbage Collector halt the whole VM?

In general, the Garbage Collection theory [Garbage Collection Wiki] explains:

• Stop-the-world garbage collectors completely halt execution of the program to run a collection cycle

• Incremental and concurrent garbage collectors are designed to reduce this disruption by interleaving their work with activity from the main program. Incremental garbage collectors perform the garbage collection cycle in discrete phases, with program execution permitted between each phase (and sometimes during some phases).

• Concurrent garbage collectors do not stop program execution at all, except perhaps briefly when the program's execution stack is scanned.