What is GC , Heap Memory & Metaspace ?

What is GC (Garbage Collection)?

• GC stands for “Garbage Collection”.
• The JVM (Java Memory Management) has a feature of Garbage Collection,
• The GC remove the unused objects from the program in order to make heap memory free.

The GC works in three steps format: “MARK”, “SWEEP” & “Deletion with Compacting”

1. MARK: This step, mark the live objects in the heap memory.
2. SWEEP: This step, removes (sweep) the dead/unreferenced objects from the heap memory.
3. Deletion with Compacting: After removing unreferenced objects, this step compact the live/referenced objects together, which makes more memory allocation faster.

The Heap memory is divided into many generations:

1. Young Generation.

a.   Eden.

b.  Survivor (S0).

c.  Survivor (S1).

2. Old Generation (Tenured).
3. Permanent generation.

1. Young Generation.    

• The (YG) young generations are the space where the new objects reside.
• As young generations full, the Minor GC takes place.
• Minor GC is always triggered when JVM is unable to allocate space for a new Object in Eden.
• All Minor GC trigger Stop the world pauses, means stopping the application threads but mostly it is negligible.
• Referenced objects are moved to the first survivor space (S0). Unreferenced objects are deleted when the Eden space is cleared. 

2. Old Generation (Tenured).

• The (OG) Old Generation is the space where long survived objects reside.
• Major GC is used to clean the Tenured space (OG). 

3. Permanent generation (PG).

• The Permanent generation is also named as “Perm-Gen”, “Non-Heap Memory” and “Metaspace” space from JAVA 8.
• Perm gen is used to store the class and methods objects, in the native memory which is used by the application.
• During Full-GC the objects are cleared from Young - Old – Permanent.
• This Perm gen is separated from main heap memory.

What is Metaspace ?

• The metaspace is the new memory space. It has been replaced by “perm-gen” space.
• Dynamic allocation of native memory space during class & memory leak.
• GC calls, if the dead classes and classloaders reach the “MaxMetaspaceSize”.
• The memory leaks can be identified by “java.lang.OutOfMemoryError “exception along with stack trace log.

Types of OutOfMemoryError Exception:

• java.lang.OutOfMemoryError: Java heap space
• java.lang.OutOfMemoryError: GC Overhead limit exceeded
• java.lang.OutOfMemoryError: Requested array size exceeds VM limit
• java.lang.OutOfMemoryError: Metaspace
• java.lang.OutOfMemoryError: request size bytes for reason. Out of swap space?
• java.lang.OutOfMemoryError: Compressed class space
• java.lang.OutOfMemoryError: reason stack_trace_with_native_method

References :

https://www.journaldev.com/2856/java-jvm-memory
https://plumbr.io/blog/garbage-collection/minor-gc-vs-major-gc
https://docs.oracle.com/javase/8/docs/technotes/guides/
https://docs.oracle.com/javase/8/docs/technotes/guides/

Thanks :-)