2. CMS to G1 - Java GC Vaibhav Choudhary (@vaibhav_c) Java Platforms Team
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4. Agenda of the day … Basics of Garbage Collector1
Basics of Garbage Collector
Concurrent Mark and Sweep (CMS)
CMS implementation and challenges
Garbage First (G1)
G1 implementation and comparison to CMS 2 3 4 5

5. Definition Mark Sweep Copy Compaction
find and mark all accessible objects
Sweep
scans through the heap and reclaims all the unmarked objects
Copy
copies all live objects from one part of the heap to another empty part
Compaction
moving all the live objects into contiguous memory locations

6. Java Heap Structure Minor GC Major GC Eden Generation S0 S1
Old Generation
Meta
Space
new Object()
After minor GC
After threshold limit (n)
Weak Generational Hypothesis :
- Most of the object die young.
- There are very few old to young reference.

7. CMS Simplified (In Young generation)…
Eden S0 S1
Eden S0 S1
Eden S0 S1
Old generation
Old generation
Old generation
When eden space will be full. Minor GC will be triggered.
Usages of S0 and S1.

8. CMS Simplified (In Young generation)…
Eden S0 S1
Eden S0 S1
Eden S0 S1 S1
Old generation
Old generation
Old generation
Empty Eden and one of the Survivors after Minor GC completion.
Stop the world process.
Some objects can be promoted to Old generation.

9. CMS Simplified (In Old generation)…
May be the first snapshot of Old generation
Old generation will mostly look like this
No compaction in CMS
Mostly concurrent

10. CMS Simplified (In Old generation)…
CMS phases :-
Initial marking phase.
Concurrent Marking phase.
Concurrent Pre-cleaning Phase.
Remarking phase.
Concurrent Sweep Phase.
Concurrent Reset Phase.
STW
STW

11. CMS Simplified (In Old generation)…
Initial Marking Phase :-
Scan the object directly reachable
from root or from Eden Generation.
2. All the mutator thread need to be
stopped (STW).
3. STW - We don’t want to mess up with
this information.
4. Generally small pause in nature. G C R O T
Eden Generation

12. CMS Simplified (In Old generation)…
Concurrent Marking Phase :-
Scan almost all live objects from the object
marked from phase 1.
2. This will be done in concurrent fashion.
3. And this will lead to a remarking phase.
4. It will take time but it’s concurrent.
Eden Generation

13. CMS Simplified (In Old generation)…
Remarking Phase :-
Scan the newly created objects.
2. We need to do STW.
3. Generally, small pause in nature.
Eden Generation
We end up creating new Objects, because
last phase is concurrent in nature.

14. CMS Simplified (In Old generation)…
Remarking Phase :-
Scan the newly created objects.
2. We need to do STW.
3. Generally, small pause in nature.
Eden Generation
We end up creating new Objects, because
last phase is concurrent in nature.

15. CMS Simplified (In Old generation)…
Concurrent Sweep Phase :-
Start sweeping all the non-marked Object.
2. Concurrent in nature.
3. Red boxes can be claimed by GC.
Eden Generation
We end up creating new Objects, because
last phase is concurrent in nature.

16. Final thoughts for CMS
It’s almost concurrent other than some small STW’s.
Memory Fragmentation - so can’t fit large object.
Less predictable.
Can Lead to promotion failure - Young to old promotion is not happening because object is too big or old space is almost full.
Can Lead to Concurrent mode failure - Old generation not finished the collection work and Old generation is full. STW and then run a different GC algorithm, probably Mark-Sweep-Compact.
If application is working fine with CMS, let it work !

17. Welcome to Garbage First (G1)
Default GC algorithm for JDK9.
G1 Goals
Low latency
Predictable (Can’t be 100 percent)
Easy to use (Less parameter settings)
Concurrent, Parallel and better Compacting.
If you are not on JDK9, use -XX:+UseG1GC.
Careful with your greedy throughput desires.

18. Garbage First (G1) - Memory layout
Memory is divided into small regions
More than 2000 regions
More flexible boundaries
Use -XX:+G1HeapRegionSize
Different regions :
Young
Survivor
Old
Humongous Y S O H H H O Y O Y O O

19. G1 - Young region View O1 O4
None of the object is reachable from GC root
in first block. Looks like the complete region
can be collected. (No you can’t) O2 O3
GC Root O1
GC root is pointing to some object. Can’t collect
this region. O2 O3

20. G1 - Young region View O1 O4 O1 O4 We have reference from Old region
to this block. Can’t be collected. O2 O3 O2 O3
GC Root
Remembered Set (Remembers the
references from old region) O1 O2 O3

21. G1 RS and Dirty Card Table Concept
APPLICATION TO UPDATE RSET
A card table is a type of remembered set.
Hotspot uses byte array as a Card Table.
Each byte is referred as a Card which corresponds to range of address in Heap.
Dirtying a card means changing the value of byte.
Processing a card means seeing a old to young pointer.
-XX:+ConcRefinementThread
MANY CRT
ONE CRT
NO CRT

22. G1 - Ease to use
java -Xmx50m -XX:+UseG1GC -XX:+MaxGCPauseMillis=200 - jar Java2DDemo.jar
MaxGCPauseMillis=200 - A soft goal. G1 will try to respect as much as possible, but can’t guarantee you.
Important to know
-XX:+InitiatingHeapOccupancyPercentage=45

23. G1 - Young GC Phases Stop the world event.
Builds collection Set (cSet) for the regions which are subject of collection.
In Young GC, cSet will contain :-
Eden Region
Survivor Region

24. G1 - Young GC Phases Phase 1 Phase 2 Phase 3 G1 [Young] Root Scanning
Find out the GC Roots like old time.
Phase 2
G1 [Young] Update RSet
Update RSet from dirty card queue.
Phase 3
G1 [Young] Process RSet
Detects old to young generation pointers.

25. G1 - Young GC Phases Phase 4 Phase 5 G1 [Young] Object copying
Traverse object graph
Copy to either New Eden Region or Survivor Region
Phase 5
G1 [Young] Reference Processing
Finding the type of reference while copying (Phase-4)

26. G1 - Young GC Phases
Dynamically setting up no. of Eden/Survivor Region
MaxPauseMillis - Can increase or decrease the no. of region to respect the pause time.
After fulfilling the latency goals, it will try to fulfil the throughput goal.
So, if possible, don’t set Xmn
Shrinking/Expansion can happen from percent.

27. G1 - Young GC View O Y O Y S O Y Y S O H H H O Y O Y O O

28. G1 - Young GC View O O O Y Y S O H H H O O O O O

29. G1 - Old GC Phases Kicks out when the heap occupancy is 45%.
-XX:+InitialHeapOccupancyPercentage=<n> (Can be changed)
G1 uses Concurrent Marking in Old region
Uses STAB [Snapshot at the beginning]
Tri-color Marking
Floating Garbage

30. Tri-color Marking
GC Root

31. Concurrency Problem - Floating Garbage
GC Root X
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32. G1 - Old GC Phases Performs a Young GC
Piggybacks root detection.
Concurrent Old region marking starts
Also, profile per region liveliness.
Which region, I can claim the best garbage.
Remarking Phase
Process STAB
Process Reference Queue
Cleanup Phase
No tree traversal in the region - So, its full of Garbage.

33. G1 - Mixed GC Phases Partial Regions
With young generation collection, G1 adds some area of Old region as well to perform GC.
-XX:MixedGCCountTarget
Which old region to give ? - One which can give max benefit
-XX:G1MixedGCLiveThresholdPercentage
Not interested in seeing the regions which are full of live objects.
-XX:G1HeapWastePercentage

34. G1 - Mixed GC View O Y O Y S O Y Y S O H H H O Y O Y O O

35. G1 - Mixed GC View O O O Y Y S O H H H O O O O O

36. G1 - Best Practices Avoid setting up too much of parameter.
You will end up G1 behaving abruptly.
Avoid full GC. Look at the adaptive policy.
Avoid allocation Failure
Very similar to Concurrent Mark Failure in CMS.
It will call STW and a full GC
Issue with heap or with the code
Careful with Humongous Regions. Change region size, if it is more.

37. Golden References Charlie Hunt - G1 Evaluation
Simone Bordet - G1 Details and Tuning
G1 - Devoxx talks
Oracle Documentation on G1
Poonam Bajaj Blogs
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