Ruby Memory Model - Speaker Deck

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Ruby Memory Model Hari Krishnan @harikrishnan83

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Why this talk?

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Memory model is not about

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A quick exercise

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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.

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Thread A Thread B @count = 0 Load @count = 0 Increment Store Load Increment Store @count = 1 @count = 2 @count = 1

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Thread A Thread B @count = 0 Load @count = 0 Increment Store Load Increment Store @count = 1

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Where does reordering happen Core 1 Cache Core 2 Cache Compiler Memory

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What is a memory model? A memory model describes the interactions of threads through memory and their shared use of the data

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Atomicity

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Visibility

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Ordering

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Should rubyists care about this? Ruby is single threaded

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Are single threaded languages like Ruby thread safe?

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Does single core imply thread safe?

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Concurrency is about Interleavings Thread A - Load Thread A - Increment Thread B - Load Thread A - Store Thread B - Increment Thread B - Store

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Threading in Ruby

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How is Ruby made Single Threaded ● 1.8 - Green Threads - Global Interpreter Lock ● 1.9 - OS Threads - Global VM Lock

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Does GIL make you thread safe?

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How does GIL work? Ruby Thread 1 Ruby Thread 2 OS Thread 1 OS 2 Timer Thread Wake up! Interrupt

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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

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Now do you still think it is thread safe?

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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

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Ruby does not have a documented Memory Model

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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 -

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What do we do?

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Start Engineering Code for Thread Safety

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Do not leave innocent accessors lying around

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Methods that mutate parameters are generally dangerous

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Gems to the rescue ● atomic - https://github.com/headius/ruby- atomic ● thread_safe - https://github. com/headius/thread_safe

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Thank you! Hari Krishnan @harikrishnan83