java Project loom: Why are virtual threads not the default?

Each thread has a separate flow of execution, and multiple threads are used to execute different parts of a task simultaneously. Usually, it is the operating system’s job to schedule and manage threads depending on the performance of the CPU. In the case of IO-work (REST calls, database calls, queue, stream calls etc.) this will absolutely yield benefits, and at the same time illustrates why they won’t help at all with CPU-intensive work (or make matters worse).

java project loom

„Java is used very heavily on the back end in business applications, which is where we focus on helping businesses. … If we want to maintain and help people build new stuff, it’s important that the language keeps up with that.” JEP 428, Structured Concurrency (Incubator), proposes to simplify multithreaded programming by introducing a library to treat multiple tasks running in different threads as a single unit of work. This can streamline error handling and cancellation, improve reliability, and enhance observability. A point to be noted is this suspension or resuming occurs in the application runtime instead of the OS. As a result, it prevents the expensive context switch between kernel threads. To cater to these issues, the asynchronous non-blocking I/O were used.

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Continuation is a programming construct that was put into the JVM, at the very heart of the JVM. There are actually similar concepts in different languages. Continuation, the software construct is the thing that allows multiple java project loom virtual threads to seamlessly run on very few carrier threads, the ones that are actually operated by your Linux system. Much digital ink has already been spilled on this subject, and the consensus appears to be “probably not”.

Then on line 16, something really exciting and interesting happens. The function bar voluntarily says it would like to suspend itself. The code says that it no longer wishes to run for some bizarre reason, it no longer wishes to use the CPU, the carrier thread. What happens now is that we jump directly back to line four, as if it was an exception of some kind. Then we move on, and in line five, we run the continuation once again.

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If you know the operating system’s utility called top, which is a built in one, it has a switch -H. With the H switch, it actually shows individual threads rather than processes. After all, why does this top utility that was supposed to be showing which processes are consuming your CPU, why does it have a switch to show you the actual threads? Virtual threads represent a lighter-weight approach to multi-threaded applications than the traditional Java model, which uses one thread of execution per application request. Few new methods are introduced in the Java Thread class.

java project loom

There’s nothing really exciting here, except from the fact that the foo function is wrapped in a continuation. Wrapping up a function in a continuation doesn’t really run that function, it just wraps a Lambda expression, nothing specific to see here. However, if I now run the continuation, so if I call run on that object, I will go into foo function, and it will continue running. It runs the first line, and then goes to bar method, it goes to bar function, it continues running.

Project loom: what makes the performance better when using virtual threads?

If you suspend such a virtual thread, you do have to keep that memory that holds all these stack lines somewhere. The cost of the virtual thread will actually approach the cost of the platform thread. Because after all, you do have to store the stack trace somewhere. Most of the time it’s going to be less expensive, you will use less memory, but it doesn’t mean that you can create millions of very complex threads that are doing a lot of work.

java project loom

Cases 2 & 3 are so close in performance, they are within the margin of error. With Loom, we write synchronous code, and let someone else decide what to do when blocked. And debugging is indeed painful, and if one of the intermediary stages results with an exception, the control-flow goes hay-wire, resulting in further code to handle it. Fourth level architecture to have better understanding. For details and insights, be sure to read the articles and watch the presentations and interviews by Ron Pressler, Alan Bateman, or other members of the Project Loom team.

Structured concurrency

Deepu is a polyglot developer, Java Champion, and OSS aficionado. He mainly works with Java, JS, Rust, and Golang. He co-leads JHipster and created the JDL Studio and KDash. He’s a Senior Developer Advocate for DevOps at Okta. He is also an international speaker and published author. For these situations, we would have to carefully write workarounds and failsafe, putting all the burden on the developer.

java project loom

Those should be covered completely via project Loom. And of course, there would have to be some actual I/O or other thread parking for Loom to bring benefits. „Apps might see a big performance boost without having to change the way their code is written,” he said. „That’s very appreciated by our customers who are building software for not just a year or two, but for five to 10 years — not having to rewrite their apps all the time is important to them.” Fiber class would wrap the tasks in an internal user-mode continuation.

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In this article, we will be looking into Project Loom and how this concurrent model works. We will be discussing the prominent parts of the model such as the virtual threads, Scheduler, Fiber class and Continuations. Loom and Java in general are prominently devoted to building web applications. Obviously, Java is used in many other areas, and the ideas introduced by Loom may well be useful in these applications.

  • Whenever the caller resumes the continuation after it is suspended, the control is returned to the exact point where it was suspended.
  • Project Loom has made it into the JDK through JEP 425.
  • Web servers like Jetty have long been using NIO connectors, where you have just a few threads able to keep open hundreds of thousand or even a million connections.
  • You have coroutines or goroutines, in languages like Kotlin and Go.

There’s also a different algorithm or a different initiative coming as part of Project Loom called structured concurrency. Essentially, it allows us to create an ExecutorService that waits for all tasks that were submitted to it in a try with resources block. This is just a minor addition to the API, and it may change. Assumptions leading to the asynchronous Servlet API are subject to be invalidated with the introduction of Virtual Threads. The async Servlet API was introduced to release server threads so the server could continue serving requests while a worker thread continues working on the request.

Project Loom: Revolution in Java Concurrency or Obscure Implementation Detail?

More importantly, every thread you create in your Java Virtual Machine consumes more or less around 1 megabyte of memory, and it’s outside of heap. No matter how much heap you allocate, you have to factor out the extra memory consumed by your threads. This is actually a significant cost, every time you create a thread, that’s why we have thread pools. That’s why we were taught not to create too many threads on your JVM, because the context switching and memory consumption will kill us. It turns out that user threads are actually kernel threads these days.