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Matthew Wilson

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Recursive search examples, pt3: C++

April 28, 2010

In two previous posts, I looked at implementations of a simple recursive-search task, implemented first in C# and then in C. In this post I'll examine how to write the same algorithm in C++, and continue the comparative performance analysis.

The first two implementations were in C# (using recls 100% .NET) and in C (using recls). Defining the C++ implementation has taken a while longer, as I wanted to wait until the release of recls 1.9, in order to utilise the substantial improvements in the C++ API over version 1.8. That's now available, and, so in this post I'll examine how to write the same algorithm in C++, and update the continue the comparative performance analysis started in the second part of this series.

Here's the code:

 <strong>#include <recls/recls.hpp></strong>
 #include <fastformat/fastformat.hpp>
 #include <fastformat/sinks/ostream.hpp>
 #include <stlsoft/string/case_functions.hpp>
 
 int main()
 {
   <strong>recls::search_sequence</strong> files(NULL, "*.cs", <strong>recls::RECURSIVE</strong> | <strong>recls::RECLS_F_DIRECTORY_PARTS</strong>);
   unsigned numFiles = 0;
   stlsoft::uint64_t totalSize = 0;
 
   { for(recls::search_sequence::const_iterator i = files.begin(); i != files.end(); ++i)
   {
     if("ASSEMBLYINFO.CS" == stlsoft::to_upper((*i).get_file()))
     {
       <strong>recls::directory_parts</strong> const parts((*i).get_directory_parts());
       if(!parts.empty())
       {
         if("PROPERTIES\\" == stlsoft::to_upper(*(parts.end() - 1)))
         {
           continue;
         }
       }
     }
     ++numFiles;
     totalSize += (*i).get_size();
   }}
   fastformat::fmtln(std::cout, "{0} file(s); {1} bytes", numFiles, totalSize);
   return EXIT_SUCCESS;
 }

This code illustrates the simple nature of the recls C++ API. Interestingly, it more closely approximates the C# version than the C version, showing that, in suitable cases (and with suitable libraries), C++ application code can be in the ballpack with C# for expressiveness.

For convenience, I've also used the FastFormat library for results output for portability, because 64-bit integers are not universally acceptable to the IOStreams. Consequently the program is portable across all platforms that are supported by recls (and FastFormat), which include Linux, Mac OS-X and Windows (including 32-/64-bit versions of each).

Now let's look at the performance. Further to the performance criteria examined in the second part of this series (time(s), handles, "I/O Other"), I've also updated each of the programs to yield its maximum working set size. The results for executing each of the programs under the same conditions on my host system (32-bit Windows) are shown in the following table. As before, Handles and "I/O Other" metrics were observed via Task Manager, and are therefore approximate.

Language Average Elapsed Time (ms) Average Kernel Time (ms) Average User Time (ms) Maximum Working Set (KB) Observed Handles Observed "I/O Other"s
C 9,429 4,656 4,500 1,380 ~23 ~230,000
C++ 9,753 4,812 4,531 1,380 ~20 ~230,000
C#/.NET 26,515 15,984 10,125 13,512 ~80 ~641,000

The results seem pretty clear cut. Accessing the file-system in .NET is substantially more expensive in time than the lower level accesses performed by C and C++. It also appears that it's more expensive in memory, but that's a long bow to draw from such a small test: it could equally be that the .NET runtime has a higher overhead, but further memory use increases linearly with algorithmic complexity in a roughly similar manner to C/C++. Further, more complex, tests may reveal more ...

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