Performance issue: comparing to String.Format

Question

A while back a post by Jon Skeet planted the idea in my head of building a CompiledFormatter class, for using in a loop instead of String.Format().

Answer 1

The framework provides explicit overrides to the format methods that take fixed-sized parameter lists instead of the params object[] approach to remove the overhead of allocating and collecting all of the temporary object arrays. You might want to consider that for your code as well. Also, providing strongly-typed overloads for common value types would reduce boxing overhead.

Answer 2

I gotta believe that spending as much time optimizing data IO would earn exponentially bigger returns!

This is surely a kissin' cousin to YAGNI for this. Avoid Premature Optimization. APO.

Answer 3

It seems to me that in order to get actual performance improvement, you'd need to factor out any format analysis done by your customFormatter and formattable arguments into a function that returns some data structure that tells a later formatting call what to do. Then you pull those data structures in your constructor and store them for later use. Presumably this would involve extending ICustomFormatter and IFormattable. Seems kinda unlikely.

Answer 4

Have you accounted for the time to do the JIT compile as well? After all, the framework will be ngen'd which could account for the differences?

Answer 5

Don't stop now!

Your custom formatter might only be slightly more efficient than the built-in API, but you can add more features to your own implementation that would make it more useful.

I did a similar thing in Java, and here are some of the features I added (besides just pre-compiled format strings):

1) The format() method accepts either a varargs array or a Map (in .NET, it'd be a dictionary). So my format strings can look like this:

StringFormatter f = StringFormatter.parse(
   "the quick brown {animal} jumped over the {attitude} dog"
);

Then, if I already have my objects in a map (which is pretty common), I can call the format method like this:

String s = f.format(myMap);

2) I have a special syntax for performing regular expression replacements on strings during the formatting process:

// After calling obj.toString(), all space characters in the formatted
// object string are converted to underscores.
StringFormatter f = StringFormatter.parse(
   "blah blah blah {0:/\\s+/_/} blah blah blah"
);

3) I have a special syntax that allows the formatted to check the argument for null-ness, applying a different formatter depending on whether the object is null or non-null.

StringFormatter f = StringFormatter.parse(
   "blah blah blah {0:?'NULL'|'NOT NULL'} blah blah blah"
);

There are a zillion other things you can do. One of the tasks on my todo list is to add a new syntax where you can automatically format Lists, Sets, and other Collections by specifying a formatter to apply to each element as well as a string to insert between all elements. Something like this...

// Wraps each elements in single-quote charts, separating
// adjacent elements with a comma.
StringFormatter f = StringFormatter.parse(
   "blah blah blah {0:@['$'][,]} blah blah blah"
);

But the syntax is a little awkward and I'm not in love with it yet.

Anyhow, the point is that your existing class might not be much more efficient than the framework API, but if you extend it to satisfy all of your personal string-formatting needs, you might end up with a very convenient library in the end. Personally, I use my own version of this library for dynamically constructing all SQL strings, error messages, and localization strings. It's enormously useful.

Answer 6

Here's the final result:

I changed the format string in a benchmark trial to something that should favor my code a little more:

The quick brown {0} jumped over the lazy {1}.

As I expected, this fares much better compared to the original; 2 million iterations in 5.3 seconds for this code vs 6.1 seconds for String.Format. This is an undeniable improvement. You might even be tempted to start using this as a no-brainer replacement for many String.Format situations. After all, you'll do no worse and you might even get a small performance boost: as much 14%, and that's nothing to sneeze at.

Except that it is. Keep in mind, we're still talking less than half a second difference for 2 million attempts, under a situation specifically designed to favor this code. Not even busy ASP.Net pages are likely to create that much load, unless you're lucky enough to work on a top 100 web site.

Most of all, this omits one important alternative: you can create a new StringBuilder each time and manually handle your own formatting using raw Append() calls. With that technique my benchmark finished in only 3.9 seconds. That's a much greater improvement.

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In summary, if performance doesn't matter as much, you should stick with the clarity and simplicity of the built-in option. But when in a situation where profiling shows this really is driving your performance, there is a better alternative available via StringBuilder.Append().