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Observability of the Java Virtual Machine

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The JVM is one of the most observable runtimes. It provides us lots of tools for troubleshooting a JVM application in production. 1. Thread observability Threads are how the JVM actually does work. When something is wrong in production, the symptom is almost always a thread: stopped, blocked, leaking etc. Thread dumps work on any JVM with no  instrumentation, no agents, no restarts. <Example project link with /threaddump endpoint>         // (1) Deadlock — two threads grab the same pair of locks in opposite order.         new Thread(() -> grab(LOCK_A, LOCK_B), "deadlock-A-then-B").start();         new Thread(() -> grab(LOCK_B, LOCK_A), "deadlock-B-then-A").start(); http://localhost:8080/actuator/threaddump To list the JVMS, we can use the command below. PS C:\observe-jvm> jps -lv 25296 jdk.jcmd/sun.tools.jps.Jps -Dapplication.home=C:\Program Files\Microsoft\jdk-21.0.3.9-hotspot -Xms8m -Djdk.module.main=...
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A* Search Algorithm and Priority Queues

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In this blog post, I'll examine the fifteen-puzzle problem . We have an n x n grid filled with numbers from 1 to n-1. Our aim is to reach the position called the "goal board." For example, for a 3x3 grid, our goal board is: 1  2  3 4  5  6 7  8 To reach there, we will be sliding blocks horizontally and vertically to the blank square. The solution approach to this problem utilizes the A* search algorithm . These kinds of algorithms use heuristics to arrive better positions. Let's explain the steps of the algorithm: Create a priority queue and enqueue the initial position. Dequeue the best item using the heuristics. Insert all neighbours of the dequeued item into the queue. Repeat this procedure until the goal board is dequeued.  For each position we need to calculate the heuristic function. For this problem, we can use one of the two priority fuctions: 1) Hamming priority function: The number of blocks in the wrong position. 2) Manh...
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A Pattern Recognition Problem: Finding Line Segments Effectively In Java

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Lets say we are given a set of  n  points and we need to find every line segment that connects 4 or more of the points. We have an immutable data type for Point: public class Point implements Comparable {     private final int x;        private final int y;        //...     public int compareTo(Point that) {         if (that.y == y && that.x == x) {             return 0;         }         if (y < that.y) {             return -1;         }         if (y > that.y) {             return 1;         }         if (x < that.x) {             return -1;         }         // ...
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Dynamic Connectivity Problem and the Union-Find Algorithm

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Lets say we need to connect some objects and query if some objects are connected or not. Below is an example test: @Test     public void testConnectivity() {         MyConnectionDb connectDb = new MyConnectionDb();         connectDb.connect(4, 3);         connectDb.connect(3, 8);         connectDb.connect(6, 5);         connectDb.connect(9, 4);         connectDb.connect(2, 1);         assertTrue(connectDb.isConnected(8, 9));         assertFalse(connectDb.isConnected(5, 4));         connectDb.connect(5, 0);         connectDb.connect(7, 2);         connectDb.connect(6, 1);         connectDb.connect(7, 3);         assertTrue(connectDb.isConnected(5, 3));     } So whe...
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The WeakReference class, monitoring memory leak and garbage collection in a Java application

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 Below is a Stack implementation that uses an internal resizeable array structure.  public class MyStack< T > implements Stack< T > { private static final int CAPACITY = 100 ; private Object[] array ; private int pos = 0 ; public MyStack () { this . array = new Object[ CAPACITY ] ; } @Override public void push ( T item) { if ( pos >= array . length / 2 ) { Object[] newArray = new Object[ pos * 2 ] ; System. arraycopy ( array , 0 , newArray , 0 , array . length ) ; array = newArray ; } array [ pos ++] = item ; } @Override public T pop () { if (isEmpty()) { throw new RuntimeException( "empty stack" ) ; } @SuppressWarnings ( "unchecked" ) T item = ( T ) array [ pos - 1 ] ; pos -= 1 ; return item ; } @Override @SuppressWarnings ( "unchecked" ) public T peek...
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Simplescalar Simulator - Part 2: sim-outorder.c

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sim-outorder implements a very detailed out-of-order issue superscalar processor with a two-level memory system and speculative execution support. This simulator is a performance simulator, tracking the latency of all pipeline operations. Let's have a look at SimpleScalar simulator's code related to iTLB. (Instruction Translation Lookaside Buffer) First, there are simulator options related to iTLB: /* register simulator-specific options */ void sim_reg_options(struct opt_odb_t *odb) { /* TLB options */ opt_reg_string(odb, "-tlb:itlb", "instruction TLB config, i.e., { |none}", &itlb_opt, "itlb:16:4096:4:l", /* print */TRUE, NULL); opt_reg_int(odb, "-tlb:lat", "inst/data TLB miss latency (in cycles)", &tlb_miss_lat, /* default */30, /* print */TRUE, /* format */NULL); } /* check simulator-specific option values */ void  sim_check_options(... ) { /* sim-outorder.c  line: 1098*/   /* u...
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Notes on Java Performance

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Performance monitoring, performance profiling and performance tuning are different activities. For example, monitoring is not an intrusive action but profiling is. Because it may be changing the responsiveness of the running application. User CPU utilization vs Kernel CPU utilization: The ideal situation is when Kernel CPU utilization is 0%. So ideally, all the CPU cycles must be spent on our application code. JVM just in time compiler performs dynamic optimizations. So it makes decisions while the program is running and generates native code with better performance. Java 1.0 was completely interpreted but beginning with Java 1.1.8, there was a just-in-time compiler. It was 8 times faster. After that, paralellizing the garbage collector was a huge improvement. There are hundreds of Java tuning flags. https://github.com/ScottOaks/JavaPerformanceTuning Boolean flags use this syntax: -XX:+FlagName enables the flag, and -XX:-FlagName disables the flag. 32 BIT VS 64 BIT Fr...
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