Re: [boost] [local] Help for the Alternatives section

Subject: Re: [boost] [local] Help for the Alternatives section
From: lcaminiti (lorcaminiti_at_[hidden])
Date: 2011-03-29 12:37:29

lcaminiti wrote:
> 
> benchmark numbers. I will run correct tests more along the lines of what
> you suggested.
> 

Hello all,

I am looking into why Boost.Local has a long run-time and I run into
something strange that I cannot explain... I am still looking into this but
I wanted to ask if anyone has some insight.

Consider the following code which uses Boost.ScopeExit type deduction and a
local functor class (these are bits that Boost.Local uses internally):

#include <boost/local/function.hpp>
#include 
#include 
#include 

int main() {
    double sum = 0.0;
    int factor = 10;

    typedef void (*tag)(int factor); 
    typedef BOOST_TYPEOF(boost::scope_exit::aux::wrap(
            boost::scope_exit::aux::deref(factor, (tag)0))) wrapped_t;
    typedef wrapped_t::type capture_t;
    struct params_t {
        typedef capture_t param_t;
        boost::scope_exit::aux::member<param_t, tag> param;
    } params = {
          { boost::scope_exit::aux::deref(factor, (tag)0) } 
    }; 

    // *******
    std::cout << params.param.value << std::endl;   // (1)
    std::vector v(1000000);                 // (2)
    // *******

    class local_add { 
    public:
        explicit local_add(double& _sum, int _factor):
                sum_(_sum), factor_(_factor) {
        } 
        void operator()(double num) {
            return body(factor_, sum_, num);
        } 
    private: 
        double& sum_;
        int factor_;
        
        void body(const int factor, double& sum, double num)
                const {
            sum += factor * num;
        }
    } add(sum, factor);

    std::fill(v.begin(), v.end(), 10);
    for (size_t i = 0; i < 10000; ++i) {
        for (size_t j = 0; j < v.size(); ++j) {
            add(v[j]);
        }
    }

    std::cout << sum << std::endl;
    return 0;
}

This compiles and runs as:

$ time g++ -O3 -Wall -Wextra -I../../.. l09.cpp -ol09

real    0m5.969s
user    0m0.994s
sys     0m1.435s

$ time ./l09
10
1e+12

real    0m16.297s
user    0m15.827s
sys     0m0.109s

But if I simply swap lines (1) and (2):

$ diff -u l09.cpp l08.cpp
--- l09.cpp     2011-03-29 12:21:16.034974600 -0400
+++ l08.cpp     2011-03-29 12:21:08.160478600 -0400
@@ -20,8 +20,8 @@
     }; 
 
 
-    std::cout << params.param.value << std::endl;
     std::vector v(1000000);
+    std::cout << params.param.value << std::endl;
 
 
     class local_add { 

Then it runs _much_ slower:

$ time g++ -O3 -Wall -Wextra -I../../.. l08.cpp -ol08

real    0m5.875s
user    0m0.821s
sys     0m1.231s

$ time ./l08
10
1e+12

real    0m44.469s
user    0m43.874s
sys     0m0.015s

Why swapping lines (1) and (2) changes the run-time? I don't understand it
at all... I must be missing something...

Thank you in advance.

--
Lorenzo
http://boost.2283326.n4.nabble.com/file/n3415414/l08.exe l08.exe 
http://boost.2283326.n4.nabble.com/file/n3415414/l09.exe l09.exe 
--
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