Wednesday, June 05, 2013

Poor man’s XSLT profiling in detail

A couple of years ago I wrote a post giving a vague idea of how to profile XSL transforms without shelling out for Visual Studio Team System. I went back to it recently and realised it didn’t really provide enough information on how to actually perform the profiling. So here’s another go at it.

The first thing we need to do is compile the XSLT into a .NET assembly. For this we need to use the XSLTC.EXE tool that ships with the Windows SDK. You can find this somewhere like this C:\Program Files (x86)\Microsoft SDKs\Windows\v8.0A\bin\NETFX 4.0 Tools. The command-line required will be something like this xsltc.exe /settings:dtd "/out:c:\temp\style.dll" /class:MyXslt "C:\temp\TEST.xsl"

So now we have an assembly for our XSLT file. Next we need a little wrapper program to load it up and call the transform. I used a simple command line app, with code as follows

    static void Main(string[] args)
      XslCompiledTransform transform = new XslCompiledTransform();
      XmlTextReader reader = new XmlTextReader(@"C:\temp\input.xml");
      XmlTextWriter writer = new XmlTextWriter(@"C:\temp\out.html", Encoding.UTF8);
      transform.Transform(reader, writer);

Now we are ready to profile the XSLT. In the past I’ve always recommended AQTime, but my old version of it doesn’t seem to work on Windows 8. I didn’t fancy paying for an upgrade, so looked around for alternatives and found the Eqatec profiler. The free version seems to work pretty well, certainly good enough for my limited needs.

So after running my little test app through the profiler, I had a much better idea of where things were slow in my XSLT. As is often the case, it was due to some XPath using “//element” to find elements. Often you can get away with it, but if the XML document is large or that piece of code is getting hit a lot then it can bite you. 

There is another slight complication with this approach to XSLT profiling. Some of the compiled templates may have a name of “compiler:generated”, meaning it’s pretty tricky to figure out how they relate to the original XSLT. From my experience, it seems that these templates are generated by the compiler itself when it decides to split larger templates into smaller compiled templates. I found ILSpy was pretty useful here to match the compiled code back to the original XSLT.

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