Ruby Memory Model

Garden City Ruby Conference - Ruby Memory Model
What is a Memory Model?
Is Ruby really Threadsafe because it is single threaded?
How is single threading implemented in MRI?
Threadsafety in Rubinius and JRuby without GIL.

Video: http://www.confreaks.com/videos/2922-gardencityruby-ruby-memory-model

Transcript

1. Ruby Memory Model Hari Krishnan @harikrishnan83
2. Why this talk?
3. Memory model is not about
6. A quick exercise
9. Increment operation is not a single instruction • Retrieve the current value of @count. • Increment the retrieved value by 1. • Store the incremented value back in @count.
10. Thread A Thread B @count = 0 Load @count = 0 Increment Store Load Increment Store @count = 1 @count = 2 @count = 1
11. Thread A Thread B @count = 0 Load @count = 0 Increment Store Load Increment Store @count = 1
12. Where does reordering happen Core 1 Cache Core 2 Cache Compiler Memory
13. What is a memory model? A memory model describes the interactions of threads through memory and their shared use of the data
14. Atomicity
15. Visibility
16. Ordering
17. Should rubyists care about this? Ruby is single threaded
18. Are single threaded languages like Ruby thread safe?
19. Does single core imply thread safe?
20. Concurrency is about Interleavings Thread A - Load Thread A - Increment Thread B - Load Thread A - Store Thread B - Increment Thread B - Store
21. Threading in Ruby
22. How is Ruby made Single Threaded • 1.8 - Green Threads - Global Interpreter Lock • 1.9 - OS Threads - Global VM Lock
23. Does GIL make you thread safe?
24. How does GIL work? Ruby Thread 1 Ruby Thread 2 OS Thread 1 OS 2 Timer Thread Wake up! Interrupt
25. GIL Details • GIL is acquired at start of Thread block and released once done • To ensure fairness a timer thread sends an interrupt to the Thread holding GIL when other threads are waiting • The Thread holding GIL may choose to release it based on many parameters Refer to this excellent blog post by Jesse Storimer - http://www.jstorimer.com/blogs/workingwithcode/8085491- nobody-understands-the-gil
26. Now do you still think it is thread safe?
27. Never depend on GIL for Thread Safety • GIL does not have a specification • It also ties us to MRI • Rubinius and JRuby do not have GIL
28. Ruby does not have a documented Memory Model
29. Ruby right now depends on underlying Virtual Machines • MRI is not much of an interpreter. It is a VM. - No specification for Memory Model • JRuby - JVM - JSR 133 • Rubinius - LLVM for JIT -
30. What do we do?
31. Start Engineering Code for Thread Safety
33. Do not leave innocent accessors lying around
34. Methods that mutate parameters are generally dangerous
35. Gems to the rescue • atomic - https://github.com/headius/ruby- atomic • thread_safe - https://github. com/headius/thread_safe
36. Thank you! Hari Krishnan @harikrishnan83