Continuations: The Magic Behind Virtual Threads - Spring IO

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30 May 2024 Balkrishna Rawool Continuations The magic behind virtual threads

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2 Balkrishna Rawool IT Chapter Lead, ING Bank @BalaRawool

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Introduction to Virtual Threads

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4 Platform Threads and 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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How to create virtual threads?

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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 Continuations in Java 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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Continuations in Java

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

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

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Uses of continuations 14 Ø Virtual threads or green 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 “A”; //… //… yield “B”; //… //… yield “C”; } Generator while () { // … } Consumer

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Generator using Continuations

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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 // … }