The document discusses garbage collection in Java, focusing on the allocation and management of objects across different generations. It explains the processes of minor and major garbage collection, including how objects are moved between generations and the various algorithms used, such as serial, parallel, and concurrent mark-sweep (CMS) garbage collection. The author provides insights into the mechanics of Java's garbage collection and concludes by inviting questions.

1. Garbage
Collection in
Java
Ondřej Kratochvíl

2. ▷ Young
Allocation of new objects
Often collected
Minor garbage collection when full - Stop the World event
▷ Old
Stores long surviving objects (also Stop the World)
Major garbage collection
JVM Generations

3. ▷ Permanent (Tenured)
Metadata for JVM
class/method descriptors
Data stored at runtime based on classes in use
Can be collected - when space is needed and JVM does no
longer need some old classes
included in Full garbage collection
JVM Generations

4. ▷ New object is allocated in New generation in eden space
▷ When eden space fills up, minor GC is triggered
Referenced objects are moved to first survivor space, age++
▷ At the next minor GC, referenced objects are moved to
SECOND survivor space
▷ After each minor GC, the spaces switch
▷ If referenced object’s age > threshold, it gets promoted to
Old generation
GC Process - Young generation

5. ▷ Serial GC
Mark-Sweep-Compact algorithm in the old generation
For small memory/small amount of CPU cores
▷ Parallel GC (Throughput GC)
Multithreaded for young generation
▷ Parallel Old GC
Parallel for both generations
▷ CMS GC (Concurrent mark-sweep)
Low pause time
Objects are swept while other are still being marked
GC types

6. Díky za pozornost!
Otázky?
Ondřej Kratochvíl
ondrej.kratochvil@keyup.eu