These slides may not be self explanatory. Updated for the benefit of attendees of Colombo Java Meetup keynote.

1. Taming The
Java
garbage
ColleCTor
Daya Atapattu

3. Why Learn Garbage Collection?
To select a GC algorithm
To performance debug applications
To become a better programmer

4. Summary
How Java garbage collectors work
How to monitor and tune GC
How to write GC friendly Java code

5. Dynamic Memory Allocation
Fortran77: No Dynamic Memory
C:
Dynamic Memory managed by
programmer. malloc() / free()
Java:
Dynamic Memory managed
by JVM, i.e. Java GC

7. Weak Generational Hypothesis
Most objects die young.
Only few references from old objects
to young objects.

8. All Apples in One Basket?

9. GC Regions

10. Young Generation Collection

11. Old Gen and Compaction

12. Stop The World (STW)

13. Concurrent Mark-Sweep (CMS)
Old gen is collected concurrently
1. STW, Mark roots
2. Mark concurrently with application
3. STW, mark again to catch up
4. Sweep

14. CMS

15. CMS Pros and Cons
Reduces pause
Does not collect all garbage.
Leaves the heap fragmented so allocation
is more expensive.
CMS failures.
Needs more heap.
Steals threads and CPU from application.

16. G1 Collector

17. GC Tuning
Use GC logging options with JVM
-Xloggc:gc.log
-XX:+PrintGCDetails
-XX:+PrintGCTimeStamp

18. GC Tuning
6.255: [GC [PSYoungGen: 132532K-16412K(278912K)]
141408K->25289K(453696K), 0.013 secs]
[Times: user=0.08 sys=0.01, real=0.01 secs]
6.269: [Full GC (System)
[PSYoungGen: 16412K->0K(278912K)]
[PSOldGen: 8876K->24956K(174784K)]
25289K->24956K(453696K)
[PSPermGen: 60341K->60341K(120960K)], 0.153 secs]
[Times: user=0.16 sys=0.00, real=0.16 secs]

19. GC Monitoring
Use tools to visualize GC activity
Some such tools:
JConsole
JStat
VisualVM (with GC extension)

20. Some Important Parameters
GC - The garbage collector in use.
Size of the Java heap.
Size of the young and old generation spaces.
Duration of minor garbage collections.
Frequency of minor garbage collections.
Duration of full garbage collections.
Frequency of full garbage collections.
What initiates full GC; old gen or perm space.

21. Attributes of Java Performance
Pause Time: Minimal pause time due to
GC
Throughput: Overall performance of user
process ignoring pauses due to GC
Footprint: Amount of memory required for
GC to run efficiently

22. GC Friendly Coding
GCs love small, immutable, short-lived objects
Object pooling is usually bad
Large objects are expensive to allocate,
expensive to initialize and may fragment CMS
Nulling references is usually bad
Avoid finalize()
Avoid calling System.gc()
Avoid memory Leaks
Some sources: Objects in the wrong scope
Instances of inner classes

23. Thank You!
&
Questions?