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    <p>Things do speed up, but that has nothing to do with parallel versus sequential, you are not comparing apples to apples. The JVM has a JIT (just in time) compiler that will compile some byte code only after the code is used a certain number of times. So what you see in the first iterations is slower execution for code that is not yet JIT-ed as well as time for ongoing JIT compilation itself. Removing the <code>.par</code> so that it's all <strong>sequential</strong> here is what I see on my machine (10x less iteration cause I'm using an older machine):</p> <pre><code>Sequential (ms): 312 Iteration[1] Executed in 117 ms Iteration[2] Executed in 112 ms Iteration[3] Executed in 112 ms Iteration[4] Executed in 112 ms Iteration[5] Executed in 114 ms Iteration[6] Executed in 113 ms Iteration[7] Executed in 113 ms Iteration[8] Executed in 117 ms Iteration[9] Executed in 113 ms Iteration[10] Executed in 111 ms </code></pre> <p>But it's all sequential! You can see what the JVM does in terms of JIT by using the JVM <code>-XX:+PrintCompilation</code> (set in <code>JAVA_OPTS</code> or use <code>-J-XX:+PrintCompilation</code> scala option. In the first iterations you will see a large numbers of JVM print statements showing what's being JIT-ed, then it stabilizes later on.</p> <p>So to compare apples to apples, you first run without the par, then add the par and run the same program. On my dual core, when using <code>.par</code> I get:</p> <pre><code>Sequential (ms): 329 Iteration[1] Executed in 197 ms Iteration[2] Executed in 60 ms Iteration[3] Executed in 57 ms Iteration[4] Executed in 58 ms Iteration[5] Executed in 59 ms Iteration[6] Executed in 73 ms Iteration[7] Executed in 56 ms Iteration[8] Executed in 60 ms Iteration[9] Executed in 58 ms Iteration[10] Executed in 57 ms </code></pre> <p>So more or less a 2x speedup once it's stable.</p> <p>On related note, the other thing you want to be careful with is boxing and un-boxing, especially if you are comparing to just Java. The scala library high order functions like filter are doing boxing and un-boxing of primitive types and this is typically source of initial disappointment for those that convert code from Java to Scala. </p> <p>Though it does not apply in this case as the <code>for</code> is outside the timing, there is also some cost for using <code>for</code> instead of <code>while</code>, but the 2.9.1 compiler should be doing a decent job when using the <code>-optimize</code> scalac flag.</p>
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