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{{AfC topic|stem}}
In [[computer programming]], '''virtual threads''' are [[Thread (computing)|threads]] that are scheduled by a [[runtime library]] instead of natively by the underlying [[operating system]] (OS). Virtual threads allows for tens of millions of preemptive tasks and events without swapping on a 2021 consumer-grade computer.<ref name="javaworld2">{{cite web |last1=Rudell |first1=Harald |date=2022-03-19 |title=massivevirtualparallelism |url=https://codeberg.org/haraldrudell/massivevirtualparallelism/src/branch/main/README.md |website= |quote= |df= |accessdate=}}</ref>, compared to low thousands of operating system threads<ref>{{Cite web |last=baeldung |date=2022-01-02 |title=Maximum Number of Threads Per Process in Linux {{!}} Baeldung on Linux |url=https://www.baeldung.com/linux/max-threads-per-process |access-date=2022-03-30 |website=www.baeldung.com |language=en-US}}</ref>. Preemptive execution<ref>{{Cite web |title=Go 1.14 Release Notes - The Go Programming Language |url=https://go.dev/doc/go1.14#runtime |access-date=2022-03-30 |website=go.dev}}</ref> is important to performance since constructs that are not preemptive, such as [[Coroutine|coroutines]] or the largely single-threaded [[Node.js]], introduce delays in responding to asynchronous events such as every incoming request in a server application<ref>{{Cite web |last=Node.js |title=Don't Block the Event Loop (or the Worker Pool) |url=https://nodejs.org/en/docs/guides/dont-block-the-event-loop/ |access-date=2022-03-30 |website=Node.js |language=en}}</ref>.
Virtual threads increase possible concurrency by many orders of magnitudes while the actual [[Parallel computing|parallelism]] achieved is limited by available execution units and pipelining offered by present processors and processor cores. In 2021, a consumer grade computers typically offer a parallelism of tens of concurrent execution units<ref>{{Cite web |title=MacBook Pro 14-inch and MacBook Pro 16-inch |url=https://www.apple.com/macbook-pro-14-and-16/ |access-date=2022-03-30 |website=Apple |language=en-US}}</ref>. For increased performance through parallelism, the language runtime need to use all present hardware<ref>{{Cite web |title=Frequently Asked Questions (FAQ) - The Go Programming Language |url=https://go.dev/doc/faq#number_cpus |access-date=2022-03-30 |website=go.dev}}</ref>, not be single-threaded or feature global synchronization such as [[global interpreter lock]]▼
The many magnitudes of increase in possible preemptive items offered by virtual threads is achieved by the language runtime managing resizable thread stacks<ref>{{Cite web |title=JEP draft: Virtual Threads (Preview) |url=https://openjdk.java.net/jeps/8277131#Memory-use-and-interaction-with-garbage-collection |access-date=2022-03-30 |website=openjdk.java.net}}</ref>. Those stacks are smaller in size than those of operating system threads. The maximum number of threads possible without swapping is proportional to the amount of main memory<ref>{{Cite web |last=Rudell |first=Harald |date=2022-03-22 |title=Maximum number of virtual threads in Go |url=https://www.reddit.com/r/golang/comments/tifbow/comment/i1owqo9}}</ref>▼
In order to support virtual threads efficiently, the language runtime has to be largely rewritten to prevent blocking calls from holding up an operating system thread assigned to execute a virtual thread<ref>{{Cite web |last=Szczukocki |first=Denis |date=2020-03-18 |title=Difference Between Thread and Virtual Thread in Java {{!}} Baeldung |url=https://www.baeldung.com/java-virtual-thread-vs-thread |access-date=2022-03-30 |website=www.baeldung.com |language=en-US}}</ref> and to manage thread stacks<ref>{{Cite web |date=2018-04-12 |title=Why you can have millions of Goroutines but only thousands of Java Threads |url=https://rcoh.me/posts/why-you-can-have-a-million-go-routines-but-only-1000-java-threads/ |access-date=2022-03-30 |website=rcoh.me |language=en-us}}</ref>. An example of a retrofit of virtual threads is Java Loom<ref>{{Cite web |title=Main - Main - OpenJDK Wiki |url=https://wiki.openjdk.java.net/display/loom/Main |access-date=2022-03-30 |website=wiki.openjdk.java.net}}</ref>. An example of a new language designed for virtual threads is Go<ref>{{Cite web |last= |first= |date=2022-03-22 |title=The Go Programming Language |url=https://go.dev/ |access-date=2022-03-30 |website=go.dev}}</ref>▼
== Definition ==
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** Unlike coroutines, if a virtual thread is in an infinite loop, it does not block the program. Execution continues at a higher cpu load, even if there are more looping threads than available execution units
* '''Virtual Threads
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*
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* '''Virtual Threads can be allocated quickly'''
** Because allocating a virtual thread is akin to allocate memory structures, they can be allocated very quickly, like 600,000 per second. This is not possible for OS threads that would crash the host
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** The simpler concurrency, in which a single resource is shared in small time increments so that it appears always available, is easier to implement and program but do not offer any gains in performance
==
Java servers have featured extensive and memory consuming software constructs allowing dozens of pooled operating system threads to preemptively execute thousands of requests per second without the use of virtual threads. Key to performance here is to reduce the initial latency in thread processing and minimize the time operating system threads are blocked<ref>{{Cite web |title=Principles to Handle Thousands of Connections in Java Using Netty - DZone Performance |url=https://dzone.com/articles/thousands-of-socket-connections-in-java-practical |access-date=2022-03-30 |website=dzone.com |language=en}}</ref>
▲Virtual threads increase possible concurrency by many orders of magnitudes while the actual [[Parallel computing|parallelism]] achieved is limited by available execution units and pipelining offered by present processors and processor cores. In 2021, a consumer grade computers typically offer a parallelism of tens of concurrent execution units<ref>{{Cite web |title=MacBook Pro 14-inch and MacBook Pro 16-inch |url=https://www.apple.com/macbook-pro-14-and-16/ |access-date=2022-03-30 |website=Apple |language=en-US}}</ref>. For increased performance through parallelism, the language runtime need to use all present hardware<ref>{{Cite web |title=Frequently Asked Questions (FAQ) - The Go Programming Language |url=https://go.dev/doc/faq#number_cpus |access-date=2022-03-30 |website=go.dev}}</ref>, not be single-threaded or feature global synchronization such as [[global interpreter lock]]
▲The many magnitudes of increase in possible preemptive items offered by virtual threads is achieved by the language runtime managing resizable thread stacks<ref>{{Cite web |title=JEP draft: Virtual Threads (Preview) |url=https://openjdk.java.net/jeps/8277131#Memory-use-and-interaction-with-garbage-collection |access-date=2022-03-30 |website=openjdk.java.net}}</ref>. Those stacks are smaller in size than those of operating system threads. The maximum number of threads possible without swapping is proportional to the amount of main memory<ref>{{Cite web |last=Rudell |first=Harald |date=2022-03-22 |title=Maximum number of virtual threads in Go |url=https://www.reddit.com/r/golang/comments/tifbow/comment/i1owqo9}}</ref>
▲In order to support virtual threads efficiently, the language runtime has to be largely rewritten to prevent blocking calls from holding up an operating system thread assigned to execute a virtual thread<ref>{{Cite web |last=Szczukocki |first=Denis |date=2020-03-18 |title=Difference Between Thread and Virtual Thread in Java {{!}} Baeldung |url=https://www.baeldung.com/java-virtual-thread-vs-thread |access-date=2022-03-30 |website=www.baeldung.com |language=en-US}}</ref> and to manage thread stacks<ref>{{Cite web |date=2018-04-12 |title=Why you can have millions of Goroutines but only thousands of Java Threads |url=https://rcoh.me/posts/why-you-can-have-a-million-go-routines-but-only-1000-java-threads/ |access-date=2022-03-30 |website=rcoh.me |language=en-us}}</ref>. An example of a retrofit of virtual threads is Java Loom<ref>{{Cite web |title=Main - Main - OpenJDK Wiki |url=https://wiki.openjdk.java.net/display/loom/Main |access-date=2022-03-30 |website=wiki.openjdk.java.net}}</ref>. An example of a new language designed for virtual threads is Go<ref>{{Cite web |last= |first= |date=2022-03-22 |title=The Go Programming Language |url=https://go.dev/ |access-date=2022-03-30 |website=go.dev}}</ref>
== Implementations ==
=== Google Chrome Browser ===
Virtual threads are used to serialize singleton input/output activities. When a virtual thread is executing, it can hop on different OS thread. The Chrome browser first appeared in 2008
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