[boost] [Xpressive] vs libpcre performance

Subject: [boost] [Xpressive] vs libpcre performance
From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2008-09-13 14:12:01

Hi,

A German journal recently published a small programming contest with a 
very simple text processing problem. The program needs to renumber and 
sort footnotes in a long text. I wrote a solution in C++ using a number 
of Boost libraries, among them Xpressive, but also Interprocess for 
memory-mapping the input file.

Then I looked at the submissions page and found one C++ solution already 
there, an admirably short (but rather inflexible) program using libpcre.
http://www.linux-magazin.de/static/listings/magazin/2008/10/leser/cpp/footnotes.cpp

Comparing the performance, my program was sightly but consistently 
slower than this small program, even when I felt that my usage of 
memory-mapping and then scanning the whole file instead of going line by 
line ought to give me a speed advantage.

So to test the performance I took the existing submission and replaced 
libpcre with Xpressive (see attached file). I believe the solutions to 
be functionally equivalent. However, the original takes 6 seconds to 
process a 55MB file, whereas my variation takes ~15 seconds. That's on 
the second run of each program, meaning that the entire file is in the 
OS cache. This seems awfully slow.

Has anyone done a proper performance comparison between Xpressive and 
libpcre? Boost.Regex's performance pages lists PCRE, but in version 4.1, 
where 7.8 is the most recent release.

My system:
Athlon 64 2000MHz (64-bit mode)
1GB RAM
Linux 2.6.23, GCC 4.1.2
Boost trunk as of 2008-09-11
libpcre 7.7

Sebastian Redl