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POWhy's Java in this case faster (and slower) than C?
 

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  1. POWhy's Java in this case faster (and slower) than C?
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    <p>Some fellow just started learning C by reading K&amp;R and came up with its fahrenheit-to-celcius conversion loop printed down on the first pages:</p> <pre><code>#include &lt;stdio.h&gt; main () { int fahr; for (fahr = 0; fahr&lt;= 200000000; fahr = fahr + 20) printf("%d\t%6.2f\n", fahr, (5.0 / 9.0) * (fahr-32)); } </code></pre> <p>He was told Java to be slow. So, told him that Java's very competitive these days but that C will in this simple case probably be faster. Wanted to proof him and basically added "System.out." in front of printf().</p> <p>It was more than 10x slower. Way too much. I was baffled. Thought about String object creation, GC, -server, yada, yada, yada.</p> <p>I was even more baffled when I figured out that nearly 100% of the time was actually spent in printf() (<code>PrintSteam.write()</code>, output piped to /dev/null).</p> <p>After some fiddling I came up with this (doesn't do %f's rounding for now):</p> <pre><code>public static void main(String... args) throws Exception { int fahr=0; PrintWriter out = new PrintWriter(Channels.newWriter(Channels.newChannel(System.out), "US-ASCII") ); int max = 2000000000; for (fahr = 0; fahr&lt;= max; fahr = fahr + 20) // out.printf("%d\t%6.2f\n", fahr, (5.0 / 9.0) * (fahr-32)); out.println( fahr + "\t" + f(((5.0 / 9.0) * (fahr-32)) )); out.close(); } private static final String f(double d) { return (int)d + "." + (int)((d - (int)d)*100); } } </code></pre> <p>So, this uses NIO. And it <strong>outperforms</strong> gcc -O2 on two machines tested.</p> <p>Questions:</p> <ul> <li>why is the literal transscript from C to Java (i.e. <code>PrintStream</code>) <strong>so</strong> slow?</li> <li>(why is commented <code>out.printf()</code> so slow [maybe performance degrades over time]?)</li> <li>and finally: why is my solution faster than C (incl. JVM startup time)?</li> </ul>
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    1. COprint some data to the console is REALLY no scientific benchmark. You run into the I/O-Bottleneck instead of testing the speed of Java vs C.
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    2. COhave you compared with C version without formatting string?
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    3. COI'd guess that caching and number of write operations has a big impact here; for question 3, at least. You're only writing *once* (AFAIKS), as opposed to the C version writing a lot of times.
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