Go and the garbage collection
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Slides of a talk about go and the garbage collection done by Joris Bonnefoy at OVH Lyon for Golang Lyon Meetup
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Go and the garbage collection
- 1. Go & The garbage collection
- 2. “ Joris Bonnefoy DevOps @ OVH @devatoria 2
- 3. Introduction Reminder of basic algorithmic structures
- 4. » Objects have a pointer to the next element of the list » The last element have a null pointer Linked list 4
- 5. » Objects have pointers to the next and the previous element of the list » The first element have a null previous pointer » The last element have a null next pointer Doubly linked list 5
- 6. » Objects have pointers to the next and the previous element of the list » The first element have a previous pointer to the last element of the list » The last element have a next pointer to the first element of the list Circular doubly linked list 6
- 7. A simple problem How to free a dereferenced object?
- 8. How to free a dereferenced object? 8
- 9. How to free a dereferenced object? 9
- 10. How to free a dereferenced object? 10
- 11. A solution The garbage collection
- 12. The garbage collection » John McCarthy - 1959 - Lisp » Opposite of manual memory management » Used to free unused allocated memory » Can handle more types of resources (network sockets, database handlers, …) using finalization and region-based allocation 12
- 13. The garbage collection » No dangling pointers » No memory leaks » No double free » Can reduce performances » Stop-the-world issues » Can be unpredictable 13
- 14. Two types of garbage collector
- 16. How reference counting is implemented? » Each object has a count of other objects referencing it » An object is considered as garbage when its counter reach 0 16
- 17. How reference counting is implemented? 17
- 18. How reference counting is implemented? 18
- 19. Disadvantages of reference counting » Sensitive to cycle referencing 19
- 20. Disadvantages of reference counting » Increased space overhead » Must be atomic » Not real-time 20
- 22. Concept of tracing garbage collection » Determine which objects are reachable from a certain “root” object » Unreachable objects are considered as garbage » Can use a lot of algorithms 22
- 23. Two ways of reachability » The object is reachable from a certain root » Local variables » Parameters » Global variables » The object is reachable via another reachable object 23
- 24. Tracing garbage collection The naïve mark-and-sweep algorithm
- 25. The naïve mark-and-sweep algorithm » Two stages algorithm » First, run through the root set tree and mark reachable objects » Second, run through the whole memory, sweep unmarked objects and clear mark of marked objects 25
- 26. The naïve mark-and-sweep algorithm disadvantages » Must run through the memory twice » Must freeze the execution of the program to avoid mutation of the working set 26
- 27. Go and the garbage collection The tri-color marking
- 28. The tri-color marking » Proposed by Dijkstra in 1978 » Tri-color » Concurrent » Mark-and-sweep 28
- 29. The tri-color marking concept » Heap is a graph of connected objects » Uses 3 colored sets » White: contains objects to collect » Gray: contains objects reachable from the root but not yet scanned » Black: contains reachable and scanned objects 29
- 30. The tri-color marking steps » At the beginning, everyone is in the white set » Root objects are moved in the gray set » A root object is scanned » Its references are colored gray, while the root object itself is colored black » Its references are scanned too and moved to the black set » Once the gray set is empty, the remaining objects in the white set are freed and sets are recolored 30
- 31. The tri-color marking by example 31
- 32. The tri-color marking by example 32
- 33. The tri-color marking by example 33
- 34. The tri-color marking by example 34
- 35. The tri-color marking by example 35
- 36. The tri-color marking by example 36
- 37. The tri-color marking by example 37
- 39. “The tri-color marking invariant No black object may contain a pointer to a white object 39
- 40. How to always satisfy the invariant? » Stop the world until GC is complete » Use a write barrier » Function executed by the mutator to tell the GC that a pointer has been changed » Enforce an object to be gray when it is created during the garbage collector cycle 40
- 41. How to always satisfy the invariant? 41
- 42. Go back to lower versions of Go The garbage collector review by Richard Hudson
- 43. Go 1.4 and lower » Stop-the-world from the beginning of the cycle to the end » High GC latency » Not concurrent 43
- 44. Go 1.5 (and actual) garbage collector » Made by Richard Hudson and his team » Concurrent » Low latency » Decrease program performances but increase throughput 44
- 45. Go 1.5 new garbage collector 45
- 46. Benchmarking 46
- 47. Go 1.6 and further » Improvements on when to start a GC cycle, used metrics, etc. » Predictive 47
- 48. What’s new in Go 1.8? The garbage collector review
- 49. Actual garbage collector pain point 49
- 50. New type of write barrier » Hybrid write barrier » Dijkstra (1978) » Yuasa (1990) » New algorithm invariant (weak invariant) » No need to rescan the whole memory » Reduces GC cycle » 100 microseconds for the worst case » 50 microseconds for 95 percentile 50
- 51. “ Any white object pointed to by a black object is reachable from a grey object via a chain of white pointers (it is grey-protected). 51
- 52. Go 1.8 hybrid write barrier 52
- 54. Rust » Based on ownership and borrowing » Every value has a scope » Passing or returning a value means transferring its ownership » When a scope ends, owned values are destroyed » Everything is checked at compile time » Everything is allocated on the stack by default » Using stack frames » Stack frames are freed on function return » Can be allocated on the heap using boxes 54
- 55. Rust - Ownership 55
- 56. Rust - Ownership 56
- 57. Rust - Ownership 57
- 58. Rust - Borrowing 58
- 59. “ Thank you for listening 59
- 60. References » Pusher https://blog.pusher.com/golangs-real-time-gc-in-theory-and-practice/ » Golang new GC proposal https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md » Go GC 1.5 blog post https://blog.golang.org/go15gc » GopherCon 2015 GC Latency talk https://www.youtube.com/watch?v=aiv1JOfMjm0 » Dijkstra 1978 GC paper http://dl.acm.org/citation.cfm?id=359655 » Rust lang https://blog.rust-lang.org/2015/04/10/Fearless-Concurrency.html 60