Garbage Collection
Garbage Collection
■ The Java platform allows creation of as many objects as you want and you
don't have to worry about destroying them. The Java runtime environment
deletes objects when it determines that they are no longer being used. This
process is called garbage collection.
When
■ An object is eligible for garbage collection when there are no more
references to that object.
■ References that are held in a variable are usually dropped when the
variable goes out of scope. Or, you can explicitly drop an object
reference by setting the variable to the special value null.
■ Remember that a program can have multiple references to the same
object; all references to an object must be dropped before the object
is eligible for garbage collection.
Garbage Collector
■ The garbage collector does its job automatically, although, in some
situations, you may want to run the garbage collection explicitly by calling
the gc method in the System class.
System.gc();
Finalization
■ Before an object gets garbage-collected, the garbage collector gives the
object an opportunity to clean up after itself through a call to the object's
‘finalize’ method. This process is known as finalization
■ The finalize method is a member of the Object class, which is the top of
the Java platform's class hierarchy and a superclass of all classes.
■ A class can override the finalize method to perform any finalization
necessary for objects of that type. If you override finalize, your
implementation of the method should call “super.finalize” as the last thing
it does.
Historical background
■ when Java was originally developed, the JDK shipped with a
mark-and-sweep garbage collector.
■ A mark-and-sweep garbage collector proceeds in two
phases:
Mark: identifies garbage objects
Sweep: reclaims the memory for the garbage objects
■ Garbage objects are identified by traversing references
from the current application stack frames; unreachable
objects are assumed to be garbage.
Problems…
■ Mark and sweep is a "stop-the-world" garbage collection
technique; that is, all application threads stop until garbage
collection completes, or until a higher-priority thread
interrupts the garbage collector.
■ The other problem is that many types of applications can't
tolerate its stop-the-world nature. That is especially true of
applications
that require near real-time behavior or
those that service large numbers of transaction-
oriented clients.
Heap Split
■ Because of these problems, Sun Microsystems' Java HotSpot VM
split the heap into 3 sections
Permanent space: used for JVM class and method objects
Old object space: used for objects that have been around for
a while
New (young) object space: used for newly created objects
New(young) object space
■ The new object space is further subdivided into three parts:
Eden, where all newly created objects go, and
survivor space 1 and
survivor space 2, where objects go before they become old.
The J2SE 1.3 garbage collection techniques are:
■ Copy-compaction: used for new object space.
■ Mark-compact: used in old object space. Similar to mark and
sweep, mark-compact marks all unreachable objects; in the
second phase, the unreachable objects compact. This technique
avoids fragmentation problems and works well when the garbage
collector runs infrequently.
■ Incremental garbage collection (optional): Incremental GC
creates a new middle section in the heap, which divides into
multiple trains.
Garbage is reclaimed from each train one at a time.
This provides fewer, more frequent pauses for garbage
collection, but it can decrease overall application performance.
Issue !!!
■ All of these techniques are stop-the-world
techniques.
Working…
■ The garbage collector typically runs in a low-priority thread,
attempting to reclaim memory when the application is idle.
■ If there is little or no idle time, the garbage collector may
not get a chance to run.
■ Garbage collection can also be triggered if the heap's sub-
regions are nearly full.
In this case, the garbage collection thread's priority
increases.
If the new generation is full, a minor collection is
triggered; if the old generation is full, a major collection
is triggered.
Minor Collection
■ The steps in a minor collection are:
Copy objects from Eden to survivor space (1 or 2).
Copy from survivor space 1 to survivor space 2, or vice versa.
After a certain number of copies (controllable from the
command line), an object becomes tenured, that is, a
candidate for old object space.
Tenured objects move from survivor space 1 or 2 to old object
space.
Garbage collection in J2SE 1.4.1
■ The new algorithms are based on the observation that many
machines are used for low-pause or high-throughput applications
that have large amounts of memory and multiple processors. The
algorithms are optimized to take advantage of these extra
resources
The complete set of GC algorithms (*=new
in 1.4.1)
Parallel
■ The parallel (multithreaded) algorithms are optimized for
machines with multiple CPUs.
■ In J2SE 1.4.1, they are only used in the young generation.
■ Using multiple threads allows garbage collection to proceed
in parallel with the application thread, so the application
can proceed without perceptible pauses.
■ The parallel scavenging collector is optimized for very large
(gigabyte) heaps.
Concurrent
■ The concurrent collector works with the old generation. It divides
garbage collection into six phases:
Initial mark stop-the-world
Mark
Pre-cleaning
Remark stop-the-world
Sweep
Reset
■ The multi-step garbage collection allows the stop-the-world phase
to be as short as possible, which means that application pauses
for garbage collection should be minimized.
How to select a GC algorithm
■ For most Java applications, the default algorithms work fine.
■ If your application performance is reasonable, you don't need to use
another GC algorithm. As the saying goes, "If it ain't broke, don't fix it."
■ If you have a single-processor client machine and are having problems
with pause times in your application, try the incremental garbage collector.
■ If you have a single-processor server machine with lots of memory and
experience trouble with application pause times, try the concurrent
garbage collector.
■ If you have a multiprocessor machine, especially with four or more
processors, try one of the parallel garbage collection algorithms.
Sources…
■ Sun Java Tutorial
■ http://www.javaworld.com/javaworld/jw-03-2003/jw-0307-j2segc.html
