There are several questions, like here or here, asking why extra threads decrease performance. I get those.
I have a somewhat opposite question: how can just one extra thread increase performance by a factor of e.g. 10 or even 20?
Example code:
public class MainThreaded {
public static void main(String[]
String filePath = "my\\path\\";
String[] inFiles = {filePath+"file1.txt",filePath+"file2.txt"};
String[] outFiles = {filePath + "file1Out.txt", filePath + "file2Out.txt"};
long startTime = System.currentTimeMillis();
for (int i = 0; i < inFiles.length; i++) {
Adder adder = new Adder(inFiles[i], outFiles[i]);
// adder.doAdd();
Thread thread = new Thread(adder);
thread.start();
}
long endTime = System.currentTimeMillis();
long totalTime = endTime - startTime;
System.out.println("That took " + totalTime+ " milliseconds");
}
}
public class MainNoThread {
public static void main(String[] args) {
String filePath = "my\\path\\";
String[] inFiles = {filePath+"file1.txt",filePath+"file2.txt"};
String[] outFiles = {filePath + "file1Out.txt", filePath + "file2Out.txt"};
long startTime = System.currentTimeMillis();
for (int i = 0; i < inFiles.length; i++) {
Adder adder = new Adder(inFiles[i], outFiles[i]);
try {
adder.doAdd();
} catch (IOException e) {
e.printStackTrace();
}
}
long endTime = System.currentTimeMillis();
long totalTime = endTime - startTime;
System.out.println("That took " + totalTime+ " milliseconds");
}
}
public class Adder implements Runnable {
private String inFile, outFile;
public Adder(String inFile, String outFile){
this.inFile = inFile;
this.outFile = outFile;
}
public void doAdd() throws IOException{
int lines = 0;
try(BufferedReader reader = Files.newBufferedReader(Paths.get(inFile))){
while (reader.readLine() != null) {
lines++;
}
try(BufferedWriter writer = Files.newBufferedWriter(Paths.get(outFile))){
writer.write("Total: " + lines);
}
}
}
@Override
public void run() {
try {
doAdd();
} catch (IOException e) {
e.printStackTrace();
}
}
}
- 1st app without threads takes ca. 40 milliseconds
- 2nd version, that creates just one extra
new Thread()takes 2 milliseconds.
So my question: what is the reason behind such drastic improvement?
