Know Your Runtime

My talk at devfest.cz held in Prague, 2013. In this talk I try to explain why clean code isn't slow and I explain what your runtime system does to make your clean code fast.

Transcript

1. Know Your Runtime marcel.hlopko@gmail.com
2. Who am I?
3. . .
4. Is clean code slow?
5. Is %s slow? ▶ allocating many new objects ▶ calling virtual methods ▶ having many small methods ▶ throwing exceptions ▶ creating threads
6. Beware JVM specific, but principles apply everywhere
7. Problem Huge object (1000 fields)
8. Naive Implementation HugeObj hugeObj = new HugeObj(); hugeObj.doSomething();
9. Java Performance Tuning You should look at the object and consider whether it is possible to reset the fields and then reuse the object, rather than throw it away and create another.
10. Caching Implementation HugeObj hugeObj = cache.get(); hugeObj.reset(); hugeObj.doSomething();
11. Benchmark Cache is not faster than naive implementation!
12. . .
13. Garbage Collection ▶ Mark & Sweep ▶ Reference Counting ▶ Baker Algorithm ▶ Generational GC
14. Reference Counting each object remembers how many references point to it
15. Reference Counting 1 2 1 1 1 2
16. Reference Counting 1 2 1 1 1 2
17. Reference Counting 1 1 1 1 1 2
18. Reference Counting 1 1 1 1 1 2
19. Reference Counting 1 1 1 2
20. Reference Counting 1 1 1 1
21. Reference Counting ✓ incremental × cycles × fragmentation
22. Mark & Sweep 1. stop the world 2. walk the heap 3. find garbage 4. collect it
23. Mark & Sweep
24. Mark & Sweep
25. Mark & Sweep
26. Mark & Sweep ✓ cycles × stops the world × fragmentation
27. Baker's algorithm Two semispaces, copy survivors over
28. Baker's algorithm
29. Baker's algorithm
30. Baker's algorithm
31. Baker's algorithm ✓ garbage is for free!! ✓ one pass over heap ✓ no fragmentation × stops the world × memory demanding
32. Generational GC Most objects die young ▶ small eden with baker ▶ big oldgen with incremental mark & sweep
33. Generational GC Eden OldGen
34. Caching vs. naive ▶ Baker in eden - fast allocation, trash for free ▶ M&S in oldgen - need to remember and update pointers
35. Don’t be afraid of creating many objects (be afraid of remembering them)
36. Question Method lookup is slower for deep class hierarchies.
37. Method Lookup Object AbstractCollection AbstractList ArrayList public void log(Collection o) { print(o.toString()); }
38. Inline Caches Object AbstractCollection AbstractList ArrayList public void log(Collection o) { print(o.toString()); } ArrayList Object>>toString()
39. Inline Caches Method lookup is very very fast.
40. Question One big method is faster than multiple small methods.
41. Just-In-Time Interpreted code simple native code optimized native code when called few times when hot when wrong when wrong
42. JIT Inlines ▶ final methods ▶ static methods ▶ unique methods ▶ virtual methods *if it wants to
43. JIT Inlines If your small methods are called often enough, they will be inlined and there will be no overhead.
44. Question Exceptions are slower than if check
45. if check public void hello(String name) { if (name == null) { return; } sayHelloTo(name); }
46. throw/catch public void hello(String name) { try { sayHelloTo(name); } catch (NullPointerException e) { return; } }
47. Exceptions are slower But sometimes in practice the amortized cost of throw is smaller.
48. Question Threads are expensive to create. We should always use thread pools.
49. OS Threads vs. Green Threads OS Thread OS Thread JVM Scheduler Green Thread Green Thread Green Thread Green Thread
50. OS Threads vs. Green Threads Green threads are cheap(ish) to create and to context switch.
51. Disclaimer For every benchmark you’ll make somebody smart will make two benchmarks with opposite results.
52. . .
53. The Message Write Clean Code™first profile later
54. Thank you!
55. . .
56. Marcel Hlopko ▶ developing awesome Rails apps by day and hacking and breaking virtual machines by night ▶ PhD candidate in Language Interoperability @ fit.cvut.cz
57. STX:LIBJAVA Java implementation for Smalltalk/X VM. Ask me I’ll tell you more.
58. Questions?
59. Acknowledgement ▶ http://www.mi9.com/wallpaper/dream-spring-2012-what- scared-the-kitty-make-a-guess_96714/ ▶ http://wumocomicstrip.com/2012/04/16
60. hlopko.com marcel.hlopko@gmail.com