Re: [Boost-users] Performance and linking (boost_regex)

From: Vladimir Prus (vladimir_at_[hidden])
Date: 2008-07-09 13:28:14

SD wrote:

> Hi,
> 
> i've encountered a significant performance difference with boost_regex
> between linking first to it and linking last to it. I guess it is not a
> issue specific to boost_regex, but maybe someone can clarify that.
> 
> I have created the following test files and tested with gcc-4.1 and
> gcc-4.3 on 64-bit linux and gcc-4.1.2 on 32-bit linux. The results for
> 64bit are further down, but the results for 32-bit are very similar.
> 
> ****
> 
> # set your boost_extension (i.e. -mt)
> # export BOOST_EXT=-mt
> 
> cat > test.hpp <<EOF
> // test.hpp
> #ifndef TEST
> #define TEST
> 
> #include <string>
> 
> int check(const std::string & test);
> 
> #endif // TEST
> EOF
> 
> cat > test.cpp <<EOF
> // test.cpp
> #include "test.hpp"
> 
> #include <boost/regex.hpp>
> 
> int check(const std::string & test)
> {
>     int result = 0;
>     static boost::regex
> match("a*b*c*d*e*f*g*h*i*j*k*a*b*c*d*e*f*g*h*i*a*b*c*d*e*f*g*h*i*j*");
>     for (int i = 0; i < 1000; ++i) {
>         result += boost::regex_match(test, match);
>     }
>     return result;
> }
> EOF
> 
> cat > main.cpp <<EOF
> // main.cpp
> #include "test.hpp"
> 
> #include <iostream>
> 
> int main()
> {
>     std::cout << check("abcdaaaaaaaaaabcdefgaaaaaaaaaaabbbbbbbbbddeef")
> << std::endl;
> }
> EOF
> 
> 
> # Create libtest.so
> g++ -shared -fPIC test.cpp -o libtest.so
> # link boost_regex before libtest.so
> g++  main.cpp -lboost_regex${BOOST_EXT} -ltest -L${PWD} -o main_fast
> # link boost_regex after libtest.so
> g++  main.cpp -ltest -lboost_regex${BOOST_EXT} -L${PWD} -o main_slow
> 
> time env LD_LIBRARY_PATH=${PWD} ./main_slow
> # real 0m0.753s, user 0m0.746s, sys 0m0.004s
> time env LD_LIBRARY_PATH=${PWD} ./main_fast
> # real 0m0.104s, user 0m0.103s, sys 0m0.002s
> 
> As the results show the order which libs are being linked to the program
> has significant performance impacts on execution time: main_fast is
> about 7 times faster than main_slow.
....
> I would appreciate if someone could explain these surprising results.

Before we go on guessing what could cause this -- do you get the same timings
on successive runs of any given executable?

- Volodya