Ruby Memory ModelHari Krishnan

@harikrishnan83

Why this talk?

Memory model is not about

A quick exercise

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.

Thread A Thread B

@count = 0Load @count = 0

Increment

Store

Load

Increment

Store

@count = 1

@count = 2

@count = 1

Thread A Thread B

@count = 0Load @count = 0

Increment

Store

Load

Increment

Store@count = 1

Where does reordering happen

Core 1

Cache

Core 2

Cache

Compiler

Memory

What is a memory model?A memory model describes the interactions of threads through memory

and their shared use of the data

Atomicity

Visibility

Ordering

Should rubyists care about this?

Ruby is single threaded

Are single threaded languages like Ruby

thread safe?

Does single core imply thread safe?

Concurrency is about InterleavingsThread A - Load

Thread A - Increment

Thread B - Load

Thread A - Store

Thread B - Increment

Thread B - Store

Threading in Ruby

How is Ruby made Single Threaded

● 1.8 - Green Threads - Global Interpreter Lock

● 1.9 - OS Threads - Global VM Lock

Does GIL make you thread safe?

How does GIL work?

Ruby Thread 1 Ruby Thread 2

OS Thread 1 OS 2

Timer ThreadWake up!Interrupt

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

Now do you still think it is thread safe?

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

Ruby does not have a documented Memory

Model

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 -

What do we do?

Start Engineering Code for Thread Safety

Do not leave innocent accessors lying around

Methods that mutate parameters are generally

dangerous

Gems to the rescue

● atomic - https://github.com/headius/ruby-atomic

● thread_safe - https://github.com/headius/thread_safe

Thank you!Hari Krishnan

@harikrishnan83