Java 20 Doubling down on the Loom Project by Oskar

There’s a list of APIs that do not play well with Project Loom, so it’s easy to shoot yourself in the foot. Involved in open-source, DZone’s Most Valuable Blogger, used to be very active on StackOverflow. Author, trainer, conference speaker, technical reviewer, runner. Claims that code not tested automatically is not a feature but just a rumor. For example, if you want to serialize one task after another, we would use an executor service backed by a single thread.

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. Project Loom allows the use of pluggable schedulers with fiber class. In asynchronous mode, ForkJoinPool is used as the default scheduler.

Why are some Java calls blocking?

We will call that feature unwind-and-invoke, or UAI. It is not the goal of this project to add an automatic tail-call optimization to the JVM. It is the goal of this project to add a public delimited continuation construct to the Java platform. Here once again, to properly handle backpressure, we need a run-time coordinator.

  • 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.
  • The approach can be scaled up to much larger systems.
  • Because when your virtual thread runs, it’s a normal Java thread.
  • This was actually an experiment done by the team behind Jetty.
  • This may actually give you some overview like how heavyweight Java threads actually are.
  • Project Loom team has done a great job on this front, and Fiber can take the Runnable interface.
  • After making the improvement, after the same number of requests only 6m14s of simulated time (and 240ms of wall clock time!) had passed.

First, let’s see how many platform threads vs. virtual threads we can create on a machine. My machine is Intel Core i H with 8 cores, 16 threads, and 64GB RAM running Fedora 36. So in a thread-per-request model, the throughput will be limited by the number of OS threads available, which depends on the number of physical cores/threads available on the hardware.

gRPC vs REST: Comparing Approaches For Making APIs

If they are serializable, we might as well make them cloneable, as the ability to clone continuations actually adds expressivity . It is, however, a very serious challenge to make continuation cloning useful enough for such uses, as Java code stores a lot of information off-stack, and to be useful, cloning would need to be “deep” in some customizable way. In any event, a fiber that blocks its underlying kernel thread will trigger some system event that can be monitored with JFR/MBeans.

Reasons for Using Java Project Loom

The goal is to allow most Java code to run inside fibers unmodified, or with minimal modifications. It is not a requirement of this project to allow native code called from Java code to run in fibers, although this may be possible in some circumstances. It is also not the goal of this project to ensure that every piece of code would enjoy performance benefits when run in fibers; in fact, some code that is less appropriate for lightweight threads may suffer in performance when run in fibers.

Structured Concurrency and Java: Project Loom’s Solution

The wiki says Project Loom supports “easy-to-use, high-throughput lightweight concurrency and new programming models on the Java platform.” Instead of allocating one OS thread per Java thread , Project Loom provides additional schedulers that schedule the multiple lightweight threads on the same OS thread. This approach provides better usage and much less context switching. And yes, it’s this type of I/O work where Project Loom will potentially shine.

Reasons for Using Java Project Loom

A virtual thread is not a wrapper around an OS thread, but a Java entity. This document explains the motivations for the project and the approaches taken, and summarizes our work so far. Like all OpenJDK projects, it will be delivered in stages, with different components arriving in GA at different times, likely taking advantage of the Preview mechanism, first. Like any ambitious new project, Loom is not without its challenges. Dealing with sophisticated interleaving of threads is always going to be a complex challenge, and we’ll have to wait to see exactly what library support and design patterns emerge to deal with these situations.

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Fast creation time, as fast as creating string object. Such a code base would be better, clearer, more obvious to comprehend if explicit limiting/throttling mechanisms were utilized. If you browse through Executors class Javadoc, you will see a variety of options.

Reasons for Using Java Project Loom

But we also wish to correct past design mistakes and begin afresh. Both choices have a considerable financial cost, either in hardware or in development and maintenance effort. Because threads are costly to create, we pool them. The cost of creating a new thread is so high that to reuse them we happily pay the price of leaking thread-locals and a complex cancellation protocol. A virtual thread is a Thread — in code, at runtime, in the debugger and in the profiler.

Side note: how Loom fibers compare to Kotlin coroutines/Scala fibers

We can achieve the same functionality with structured concurrency using the code below. Imagine that updateInventory() fails and throws an exception. Then, the handleOrder() method throws an exception when calling inventory.get(). Since it runs on its own thread, it can complete successfully. But now we have an issue with a mismatch in inventory and order.

I maintain some skepticism, as the research typically shows a poorly scaled system, which is transformed into a lock avoidance model, then shown to be better. I have yet to see one which unleashes some experienced developers to analyze the synchronization behavior of the system, transform it for scalability, then measure the result. But, even if that were a win experienced developers are a rare and expensive java project loom commodity; the heart of scalability is really financial. The good times we got without messing with thread priority are almost kept but .. Obviously, those times are really hardware dependant, but those will be used as a reference to compare to the other running scenarios. Supposing the filter object will be loaded and available globally to the program, , the process function will be like the one below.

Structured Concurrency

There is plenty of good information in the 2020 blog post ‘State of Loom’ although details have changed in the last two years. It’s typical to test the consistency protocols of distributed systems via randomized failure testing. Two approaches which sit at different ends of the spectrum are Jepsen and the simulation mechanism pioneered by FoundationDB. The former allows the system under test to be implemented in any way, but is only viable as a last line of defense. The latter can be used to guide a much more aggressive implementation strategy, but requires the system to be implemented in a very specific style.

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