“Just leave the garbage outside and we will take care of it for you”. This is the panacea promised by garbage collection mechanisms built into most software stacks available today. So, we don’t need to think about it anymore, right? Wrong! When misused, garbage collectors can fail miserably. When this happens they slow down your application and lead to unacceptable pauses. In this talk we will go over different garbage collectors approaches in different software runtimes and what are the conditions which enable them to function well. Presented on Reversim summit 2019 https://summit2019.reversim.com/session/5c754052d0e22f001706cbd8

2. This is Me (Cicra 2015)

5. Today

6. Today

7. Memory
HeapStackStatic

8. Participants
Allocator - Allocate memory
Collector - the GC threads

9. Mutator

10. Participants
Allocator - Allocate memory
Mutator - the application threads
Collector - the GC threads

11. malloc / free
• new / malloc () to create object
• delete() / free() release an object

12. I don’t miss it
• Dangling pointers / Double free
• Memory leaks
• Fragmentation / Degradation
• Locality

13. Automatic Reference Counting (ARC)

14. Automatic Reference Counting (ARC)
1 1
Heap
2
1
1
1 1
Non Heap

15. Automatic Reference Counting - Cycles
1 1
Heap
2
1
1
1 1

16. Reference Counting Concurrency
Concurrency - Every change to reference requires a memory barrier
Very bad for highly concurrent systems
Mostly relevant for single threaded

17. The (Tracing)
Garbage Collector

19. GC Overhead
The amount of resources the GC consumes
○ Memory
○ CPU

20. Latency
What is the possibility that the GC will affect your
transaction time and for how long

21. Terminology

22. To Heap or not to Heap
HeapStatic/
Stack

23. Root Set
Heap

24. Live Set (Reachable objects)
HeapStack

25. Garbage (Unreachable Objects)
HeapStack

26. Garbage (Unreachable Objects)
HeapStack

27. Free List
HeapStack

28. Collectors Attributes
Single Threaded
Stop the World Concurrent
Parallel
VS
VS
Monolithic IncrementalVS
Conservative PreciseVS

29. STW Collectors - Serial (single threaded) - Monolith

30. STW Collectors - Incremental

31. STW Collectors - Parallel

32. Concurrent Collectors - Serial

33. Concurrent Collectors -Parallel

34. Let’s Talk about Mostly

35. When do we want to collect Garbage?

37. Conservative PreciseVS
Gets help from the compiler regarding
whether a word is a reference

38. Regional
HeapNon
Heap

39. Building
Blocks
&
Algorithms

40. Root Scanning & Safe Points
Heap

41. Mark & Sweep : Marking (a.k.a. tracing)
Heap
Work: O(#Objects in LiveSet)
Stack

42. Mark & Sweep : Sweep
Sweep through the heap and collect all unmarked objects
Maintain a list of free memory portions
Work: O(#Objects and
fragments in Heap)Heap
Unreachable
Reachable
Free

43. Mark & Sweep: Fragmentation

44. Mark Compact
Moving objects to make bigger free contiguous spaces
Heap Heap
Work: O(#Objects and
fragments in Heap)

45. Thread Local Bump allocation

46. Copy collector
Move all live objects to another space
From Space To Space
Work: O(Size of live set)
Memory: x2

47. Generational collectors
Weak Generational Hypothesis - Most object die young
The surviving ones tend to live for a long time

48. New/ Old Generation
OldNon
Heap
FromTo

49. Generational collectors
Higher throughput
Short new generation collection

50. Not Moving Moving
Non generational Generational
Low latency Latency can be high*
High Overhead (up to 25%) Lower overhead (5% considered normal)
Precise Precise
Memory overhead *2 Memory overhead configurable
* with the exception of Azul C4 zGC and Shenandoah

51. Additional Reading

52. Q&A
Q: Are you Hiring ?
A: Yes we do !