Multithreading

[Snuffy4]

Hi, i have tried to use it in a few Threads, but it crashed. Do you have any solutions?

[rodrigo-speller]

Can you post any sample code?

[Snuffy4]

Can you post any sample code?

using System;
using System.IO;
using System.Collections.Generic;
using System.Reflection;
using JNet.Runtime;
using System.Threading;

namespace testJVMBridge {
    public class Program {
        public static JNetVirtualMachine vm = null;
        public static void Main(string[] args) {
            vm = JNetVirtualMachine.Initialize(new() {
                JavaRuntimePath = "/Library/Java/JavaVirtualMachines/jdk-18.0.2.1.jdk/Contents/Home/",
                Classpath = new List<string>() { "testfile.jar" },
            });

            int thread_count = 10;

            for (int i = 0; i < thread_count; i++) {
                Thread myThread = new Thread(Print);
                myThread.Start();
            }

        }

        static void Print() {
            var rt = vm.AttachCurrentThread();
            var mainClass = rt.FindClass("com/testClass");
            var initMethod = rt.GetMethodID(mainClass, "<init>", "()V");
            var initObj = rt.NewObject(mainClass, initMethod);

            var initMainMethod = rt.GetMethodID(mainClass, "mainFunc", "()V");
            rt.CallVoidMethod(initObj, initMainMethod);
            vm.DetachCurrentThread();

        }
    }
}

For example, if i specify 1 thread, it takes about ten seconds. But if i specify 10 threads, it takes more than 30 seconds.

[rodrigo-speller]

Managing the JNetRuntime instances by AttachCurrentThread and DettachCurrentThread can be complicated in your project. To attach and detach multiple threads, you should implement a host to handle the runtime operations to avoid race concerns.

JNet.Hosting is a library that implements a host to manage the JNetRuntime instances. It is a wrapper library around the JNet.Runtime library. I recommend you to use JNet.Hosting to orchestrate the java calls to handle rece conditions.

using JNet.Hosting;

static class Program
{
    static void Main(string[] args)
    {
        var debug = true;

        JNetHost.Initialize(new() {
            EnableDiagnostics = debug
        });

        JNetHost.Run(runtime => {

            // The "runtime" parameter is a JNetRutime object, that exposes the JNI function for usage.
            // See more: https://docs.oracle.com/en/java/javase/15/docs/specs/jni/functions.html

            // java.lang.System class
            var clz_System = runtime.FindClass("java/lang/System");
            // System.out field
            var f_out = runtime.GetStaticFieldID(clz_System, "out", "Ljava/io/PrintStream;");

            // java.io.PrintStream class
            var clz_PrintStream = runtime.FindClass("java/io/PrintStream");
            // PrintStream.println method
            var m_println_A = runtime.GetMethodID(clz_PrintStream, "println", "(Ljava/lang/String;)V");   

            // gets the System.out value
            var j_out = runtime.GetStaticObjectField(clz_System, f_out);

            // creates the "Hello, World!" string
            var j_str = runtime.NewString("Hello, World!");

            // call: System.out.println("Hello, World!")
            runtime.CallVoidMethod(j_out, m_println_A, j_str);

        });

        JNetHost.Destroy();
    }
}

Calling JNetHost.Run is thread safe, so it can be called inside your threads.

Checkout the sample code to understand how JNetHost class can be initialized and used handle these operations.

[Snuffy4]

And how can I run this in two threads?

[rodrigo-speller]

Calling JNetHost.Run is thread safe, so it can be called inside your threads.

[Snuffy4]

I need to run threads as needed without deleting the Java VM, but for some reason after the thread finishes, they do not free up RAM

[Snuffy4]

I also trying your example using

using JNet.Hosting;
using JNet.Runtime;

static class Program {
    static void Main(string[] args) {
        var debug = true;

        JNetHost.Initialize(new() {
            EnableDiagnostics = debug,
            JavaRuntimePath = "/Library/Java/JavaVirtualMachines/jdk-18.0.2.1.jdk/Contents/Home/",
        });

        JNetHost.Run(runtime => {

            // The "runtime" parameter is a JNetRutime object, that exposes the JNI function for usage.
            // See more: https://docs.oracle.com/en/java/javase/15/docs/specs/jni/functions.html

            // java.lang.System class
            var clz_System = runtime.FindClass("java/lang/System");
            // System.out field
            var f_out = runtime.GetStaticFieldID(clz_System, "out", "Ljava/io/PrintStream;");

            // java.io.PrintStream class
            var clz_PrintStream = runtime.FindClass("java/io/PrintStream");
            // PrintStream.println method
            var m_println_A = runtime.GetMethodID(clz_PrintStream, "println", "(Ljava/lang/String;)V");

            // gets the System.out value
            var j_out = runtime.GetStaticObjectField(clz_System, f_out);

            // creates the "Hello, World!" string
            var j_str = runtime.NewString("Hello, World!");

            // call: System.out.println("Hello, World!")
            runtime.CallVoidMethod(j_out, m_println_A, j_str);

        });

        Console.ReadLine();
        //JNetHost.Destroy();
    }
}

https://user-images.githubusercontent.com/44814286/231010411-34fdddae-b0e1-40d2-bd10-b5433c354548.png

During execution, about 25 MB is allocated, but after the execution of the thread, during the standby mode, the memory is not released...

[rodrigo-speller]

I believe that this memory is the JNI libraries overhead. I simulate 100 threads and the memory comsumtion was very stable.

I will investigate this when I have more time.

using System;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using JNet.Hosting;
using JNet.Runtime;

static class Program
{
    static void Main(string[] args)
    {
        PrintMemoryUsage("Program starts");

        var debug = true;

        PrintMemoryUsage("Before JNetHost.Initialize");

        JNetHost.Initialize(new() {
            EnableDiagnostics = debug
        });

        PrintMemoryUsage("Before JNetHost.Run");

        var threads = Enumerable.Range(0, 100).Select(x => {
            var thread = new Thread(() => {
                JNetHost.Run(runtime => {
                    PrintMemoryUsage("Inside JNetHost.Run (starts)");

                    // The "runtime" parameter is a JNetRutime object, that exposes the JNI function for usage.
                    // See more: https://docs.oracle.com/en/java/javase/15/docs/specs/jni/functions.html

                    // java.lang.System class
                    var clz_System = runtime.FindClass("java/lang/System");
                    // System.out field
                    var f_out = runtime.GetStaticFieldID(clz_System, "out", "Ljava/io/PrintStream;");

                    // java.io.PrintStream class
                    var clz_PrintStream = runtime.FindClass("java/io/PrintStream");
                    // PrintStream.println method
                    var m_println_A = runtime.GetMethodID(clz_PrintStream, "println", "(Ljava/lang/String;)V");   

                    // gets the System.out value
                    var j_out = runtime.GetStaticObjectField(clz_System, f_out);

                    // creates the "Hello, World!" string
                    var j_str = runtime.NewString("Hello, World!");

                    // call: System.out.println("Hello, World!")
                    runtime.CallVoidMethod(j_out, m_println_A, j_str);
                    runtime.ThrowExceptionOccurred();
                    PrintMemoryUsage("Inside JNetHost.Run (completes)");
                });

            });

            thread.Start();

            return thread;
        }).ToArray();

        foreach (var thread in threads) {
            thread.Join();
        }

        PrintMemoryUsage("Before JNetHost.Destroy");
        JNetHost.Destroy();

        PrintMemoryUsage("Program completes");
    }

    static void PrintMemoryUsage(string when) {
        Process currentProcess = Process.GetCurrentProcess();
        long mProcess = currentProcess.WorkingSet64;
        var mGc = GC.GetTotalMemory(false);

        Console.WriteLine($"Memory: {when,32}: {mProcess / (1024f * 1024f),6:0.00} {mGc / (1024f * 1024f),6:0.00}");
    }
}

Thank you for your analisys.

[Snuffy4]

Ok, I want to use this to run a heavy function via JNI in multiple threads. One thread consumes about 200 MB, so after some time the program runs out of RAM and the kernel terminates the program. I will be very grateful when you manage to study the problem. Thanks!

[rodrigo-speller]

@Snuffy4, this problem, that the kernel terminates the program, is happening before or after you use JNetHost.Run?

I'm questioning this, because I found (and fix) a bug that terminates the program on JNetHost.Run.

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