Go Stack: Should it be Moving Like That?

Transcript

1. Go Stack: Should it be Moving Like That? Yarden Laifenfeld
2. 2 Who Am I? • Software Engineer at Rookout • Go, Java, Ruby • C#, Python, JavaScript, C++ • Amateur sewist
3. What is Stack? • Section of memory ◦ One stack per thread • Works like a stack (LIFO) • Stores function parameters, local variables, return addresses 3
4. What is Stack? 4 … func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... } Stack
5. What is Stack? 5 … 1 a Local variables Stack func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... }
6. What is Stack? 6 … 1 a Local variables Stack 1 b Function parameters 2 c func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... }
7. What is Stack? 7 … 1 a Local variables Stack 1 b Function parameters 2 c 0x12345 Return address func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... }
8. What is Stack? 8 … 1 a Local variables Stack 1 b Function parameters 2 c 0x12345 Return address func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... } 0 d Local variables
9. What is Stack? 9 … 1 a Local variables Stack 1 b Function parameters 2 c 0x12345 Return address 0 d Local variables func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... }
10. What is Stack? 10 … 1 a Local variables Stack 1 b Function parameters 2 c 0x12345 Return address func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... }
11. What is Stack? • Grows down - last thing we pushed onto the stack will be at the lowest address • RSP - “variable” that holds the last address of the stack 11
12. What is Stack? 12 … 1 0x1238 Local variables Stack 1 0x1230 Function parameters 2 0x1228 0x12345 func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... } Return address 0x1220
13. What is Stack? 13 … 1 0x1238 Local variables Stack 1 0x1230 Function parameters 2 0x1228 0x12345 func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... } Return address 0x1220 rsp
14. What is Stack? 14 … 1 0x1238 Local variables Stack 1 0x1230 Function parameters 2 0x1228 rsp Bigger rsp → smaller stack
15. What’s So Special About Go? 15 • Threads ≠ Goroutines • Threads - 1-10MB • Goroutines - 4K
16. What If I Need morestack? 16
17. When Is morestack Called? 17 • The compiler adds a prologue and epilogue to every function • Prologue checks if more stack is needed • Epilogue calls `morestack`
18. When is morestack Called? 18
19. needsMorestack() 19 • Not a real function • A condition that compares: ◦ Available stack ◦ Stack in use • How does that look like?
20. g struct 20 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... rsp: 0x1090
21. g struct 21 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x rsp: 0x1078 g.stack.lo g.stack.hi
22. g struct 22 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1010 x g.stack.lo g.stack.hi
23. g struct 23 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x
24. g struct 24 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x
25. 25 `morestack` needs 0x10 stack!
26. `morestack` needs 0x10 stack 26 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
27. `morestack` needs 0x10 stack 27 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
28. `morestack` needs 0x10 stack 28 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
29. `morestack` needs 0x10 stack 29 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
30. `morestack` needs 0x10 stack 30 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
31. `morestack` needs 0x10 stack 31 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
32. `morestack` needs 0x10 stack 32 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
33. `morestack` needs 0x10 stack 33 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
34. `morestack` needs 0x10 stack 34 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
35. `morestack` needs 0x10 stack 35 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
36. `morestack` needs 0x10 stack 36 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0x1000 x g.stack.lo g.stack.hi x x 0xff8 0xff0
37. `morestack` needs 0x10 stack 37 Stack 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... x x x x x x x x x x x x x x x rsp: 0xff0 x g.stack.lo g.stack.hi x x x x 0xff8 0xff0
38. `morestack` needs 0x10 stack 38 38 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1090 0x1088 0x1080 0x1010 ... rsp: 0x1090 g.stack.lo+0x10
39. `morestack` needs 0x10 stack 39 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1010 x x x x x x x x x x x x x x x x g.stack.lo+0x10
40. `morestack` needs 0x10 stack 40 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1010 x x x x x x x x x x x x x x x x g.stack.lo+0x10
41. `morestack` needs 0x10 stack 41 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1010 x x x x x x x x x x x x x x x x g.stack.lo+0x10
42. `morestack` needs 0x10 stack 42 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1000 x x x x x x x x x x x x x x x x g.stack.lo+0x10 x x
43. 43 g.stackguard0 = g.stack.lo + 0x10
44. Stackguard 44 • Used to check if we have enough stack • Always between stack.lo and stack.hi • Will leave enough memory for calls to functions like `morestack`
46. Stack Usage 46
47. Stack Usage 47
48. Stack Usage 48 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1020 x x x x x x x x x x x x x x g.stackguard0
49. Stack Usage 49 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1020 x x x x x x x x x x x x x x g.stackguard0
50. Stack Usage 50 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1020 x x x x x x x x x x x x x x g.stackguard0
51. Stack Usage 51 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0xf20 x x x x x x x x x x x x x x g.stackguard0 x x x x x x x x x x x x x x x x
52. What is Stack? 52 … func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... } Stack
53. What is Stack? 53 … 1 a Local variables Stack func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... }
54. What is Stack? 54 … 1 a Local variables Stack 1 b Function parameters 2 c func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... }
55. What is Stack? 55 … 1 a Local variables Stack 1 b Function parameters 2 c func runFirst() { a := 1 runSecond(a, 2) return } func runSecond(b, c int) { var d int ... }
56. What is Stack? 56 … 1 a Local variables Stack 1 b Function parameters 2 c func runFirst() { a := 1 if shouldRunSecond { runSecond(a, 2) } return }
57. Stack Usage 57 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1020 x x x x x x x x x x x x x x g.stackguard0
58. Stack Usage 58 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1020 x x x x x x x x x x x x x x g.stackguard0
59. Stack Usage 59 Stack g.stack.lo g.stack.hi 0x1000 0x1008 0x1088 0x1080 0x1010 ... rsp: 0x1020 x x x x x x x x x x x x x x g.stackguard0
60. Stack Usage 60
61. 61 What Goes Up Must Come Back Down
62. Stack Shrink • `shrinkstack` • Called by GC • Shrink stack if less than a quarter is being used 62
63. Thank You! @yardenlaif 63