Continuations The Magic Behind Virtual Threads

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Alignment indicator 10 Jan 2024 Balkrishna Rawool Continuations The magic behind virtual threads

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2 Balkrishna Rawool IT Chapter Lead, Business Lending @BalaRawool

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Alignment indicator Virtual Threads

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4 Virtual Threads JVM OS Platform Threads Platform Threads: Ø Thin wrapper over OS threads Ø One-to-one mapping with OS threads Ø Thread pools Virtual Threads Virtual Threads: Ø Lightweight user threads Ø Highly scalable Ø No need for pooling

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5 Virtual Thread Scheduler Platform/ Carrier Thread Virtual Threads

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Alignment indicator How to create virtual threads?

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Alignment indicator Continuations

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Continuation 8 Ø A continuation can be viewed as Ø a representation of the current state of the program

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Continuation 9 Ø A continuation can be viewed as Ø a representation of the current state of the program or Ø ‘the rest of the computation’ Ø It allows us to pause execution of a program (or part thereof) and resume it later. Ø WARNING: Continuation API in Java is not supposed to be used directly by application developers. For now, it is only used internally to implement virtual threads.

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Alignment indicator Continuations in Java

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11 Continuations in Java are One Shot Multi-Prompt Delimited Continuations

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Continuations in Java 12 Continuation.yield() yield() d() c() b() run() a() main() Stack Heap d() c() b() run() a() main() Stack Heap yield()

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Continuations in Java 13 Continuation.run() c() b() run() a() main() Stack Heap d() c() b() run() a() main() Stack Heap yield() yield() d()

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Uses of continuations 14 Ø Green threads or virtual threads Ø Coroutines Ø Exception handling Ø Generators

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Generator 15 Ø A generator allows us to write a function that ‘yields’ values. Ø These values are lazily retrieved. Ø They can then be iterated over by using a loop. { //… //… yield 1; //… //… yield 2; //… //… yield 3; //… //… yield 4; } Generator while () { // … } Consumer

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Alignment indicator Generator using Continuations

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Alignment indicator A Simple VirtualThread using Continuations

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Simple VirtualThread 18 Ø Goals Ø A VirtualThread instance should not use CPU (or platform thread) when waiting/ blocking. Ø A VirtualThread can get mounted on several platform threads during its lifetime. Ø A platform thread can support several VirtualThread-s during its lifetime.

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Simple VirtualThread 19 VirtualThread VirtualThreadScheduler Virtual threads Platform threads // Consumer { // … // Blocking call // … }