Profile and optimize your Java code Gabriel Laden CS 146 – Dr. Sin-Min Lee Spring 2004

Introduction O(n) notation is one thing Performance of your coded algorithm is another thing!!! This demo will use Homework #3 “ICBaseCycle” problem

Where to start testing? To find run-time between two sequential lines in your code insert the following: System.currentTimeMillis();

The 80/20 rule Your code spends 80% of its time in 20% of the code, or so they say… How to find this hotspot in your code without just guessing : java –Xprof class [args]

-Xprof is a built-in profiler for your code It tells you what % time is spent in running and other Java administrative tasks Caution: eliminate your user I/O and other things like that which are not constant and will throw off your testing results

Other interesting java –X options -Xloggc: log GC status to a file with time stamps Note: see use later in presentation -Xmx set maximum Java heap size Note: 64M is default, good use is for problems of moderately larger size, (go buy more memory at Fry’s) -Xss set java thread stack size Note: good use in recursive problems

javap - The Java Class File Disassembler javap [options] class The javap command disassembles a class file. Its output depends on the options used. If no options are used, javap prints out the package, protected, and public fields and methods of the classes passed to it. -c option prints out disassembled code, i.e., the instructions that comprise the Java bytecodes

Most Important Step!!! Find your inner-most loop that is in method with largest run-time % Re-write this loop to be most efficient This is where O(n) notation theory can be applied to your algorithm

Java handles your garbage So don’t forget to analyze this!!! java -Xloggc: class The cost of gc also implies the cost of the allocation The number of lines written to filegc will be evidence of the garbage collector activity Look in your code for unneccessary allocations Re-order processing to keep heap size to a minimum

Try a few different things Is your sort algorithm really the best one? Is your data structure really the best one? Save your working program to a backup!!! Compare run-times with different sorts Do the least # operations to get your output

Summary v1original code v2inner loop re-write / algorithm design v3minimize allocations v4 improved version (…repeat (1-4) if necessary) v5 final version

// version 1 private boolean sumElements(int[] permutation){ //generate sumation of terms p int walk, sum; int index = 0; int[] partSums = new int[p * (p-1) + 1]; // i = length of the elements of sum for(int i=1; i<p; i++){ //j = starting index for(int j=0; j<p; j++){ sum = 0; walk = j; //k = loop for summation for(int k=0; k<i; k++){ if(walk == p){ walk = 0; } sum += permutation[walk++]; } partSums[index++] = sum; } }// (con’t next slide…)

// version 1 (con’t) //add last element sum of all elements partSums[index] = n; //qsort rewritten from QSortAlgorithm.java in j2sdk demo directory QSort.qsort(partSums, 0, partSums.length-1); //step through array to verify success int consecutive = 1; for(int i=1; i<partSums.length; i++){ if(partSums[i] == consecutive + 1){ consecutive++; } else if(partSums[i] > consecutive + 1){ return false; } return true; }

// version 5 private boolean sumElements(int[] permutation, boolean[] partIndexes){ //generate sumation of terms p int value, walk; int pMinusOne = p - 1; Arrays.fill(partIndexes, false); for(int i=0; i<p; i++){ walk = i + 1; partIndexes[(value = permutation[i])] = true; for(int j=1; j<pMinusOne; j++, walk++){ partIndexes[(value += permutation[walk])] = true; } //skip over index 0, not used //skip over index 1 & 2, always true for(int i=3; i<partIndexes.length; i++){ if(partIndexes[i] == false){ return false; } return true; }

C:\cs146_profiling>java hw3.v1.ICBaseCycle 8 57 Here is a set of valid arrays # of solutions: 6 Total compute-time: seconds C:\cs146_profiling>java hw3.v5.ICBaseCycle 8 57 Here is a set of valid arrays # of solutions: 6 Total compute-time: seconds

Conclusion Easy to start learning how to test the performance on our homework problems Later you will be running large scale applications with multi-tier enterprise applications with networking and database, etc. it will be a lot harder to start learning how to optimize then!!!

References Effective Java - Joshua Bloch Java Platform Performance – Steve Wilson Core Java v.2 – Cay Horstmann