September 4, 2008

High precision performance measurement

Sometimes you need to get a feel for how performant a specific method is. I've seen lots of developers use DateTime.Now for this. Like this:

DateTime start = DateTime.Now;
TimeSpan ts = DateTime.Now - start
Console.WriteLine("Time taken: {0}ms", ts.TotalMilliseconds);

Now, there's a couple of problems with this. First off, DateTime.Now performs way worse than DateTime.UtcNow since it has to get the UTC time and the calculate timezone and DST offsets.

The second and more concerning problem is that DateTime has a resolution of about 15ms, it can't be more precise than that. Now, in development code you could of course get around this by running your code a couple of thousand time to compensate for the low precision but what if you're code can't easily be run more than one time (perhaps it performs expensive and hard-to-recover database work).

Stopwatch to the rescue

The System.Diagnostics.Stopwatch class provides high-precision measurement of elapsed time. Using hardware frequency counters it achieves nanosecond precision and it's really easy to use.

Stopwatch sw = Stopwatch.StartNew();

Console.WriteLine("Time taken: {0}ms", sw.Elapsed.TotalMilliseconds);

The stopwatch falls back on DateTime.UtcNow if your hardware doesn't support a high frequency counter. You can check to see if Stopwatch utilizes hardware to achieve high precision by looking at the static field Stopwatch.IsHighResolution.

This post is essentially a ripoff of my answer to a question regarding time measurement on the kickass Q/A site (not open to public yet).

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