While naive
garbage collection examines every living object in the heap, generational
collection exploits several empirically observed properties of most applications
to avoid extra work.
infant mortality
Efficient
collection is made possible by focusing on the fact that a majority of objects
die young.
To do this, memory is managed in generations: memory pools holding
objects of different ages. Garbage collection occurs in each generation
when it fills up;
Objects are allocated in eden, and because
of infant mortality most objects die there. When Eden fills up it causes
a minor collection, in which some surviving objects are moved to
an older generation. When older generations need to be collected there
is a major collection that is often much slower because it involves
all living objects.
The young generation consists of eden plus two survivor spaces
. Objects are initially allocated in eden. One survivor space
is empty at any time, and serves as the destination of the next copying
collection of any living objects in eden and the other survivor space.
Objects are copied between survivor spaces in this way until they age enough
to be tenured (copied to the old generation.)
One portion
called the permanent generation is special because it holds all the
reflective data of the JVM itself, such as class and method objects.
Pauses are
the times when an application appears unresponsive because garbage collection
is going on.
The parameters NewSize and
MaxNewSize bound the young
generation size below and above. Setting these equal to one another
fixes the young generation, just as setting
-Xms and -Xmx equal
fixes the total heap size. This is useful for tuning the young generation
at a finer granularity than the integral multiples allowed by
NewRatio.
If desired, the parameter SurvivorRatio
can be used to tune the size of the survivor spaces, but this is often
not as important to performance. For example,
-XX:SurvivorRatio=6 sets the ratio between each survivor space
and eden to be 1:6; in other words, each survivor space will be one eighth
of the young generation
If survivor spaces are too small, copying collection overflows directly
into the old generation. If survivor spaces are too large, they will
be uselessly empty