Re: [Boost-users] Efficiacy of std::vector

From: Stefan Tarrant (stefan.tarrant_at_[hidden])
Date: 2006-08-28 14:47:37

Hi,

I profiled some example code (not yours) using AQTime. Here is a screen 
shot of the results. You can see that in the inner loop I am accessing 
the elements using the [] operator, the array and the vector operations 
perform almost identically, accessing with at() is quite a bit more 
expensive.

Stefan

Janos Vegh wrote:
> Hi,
> 
> I was interested in the efficiacy of std::vector.
> Using the test program below, I receive the following results.
> The "simple" handling is equivalent with the C array access.
> The [] and iterator access time of the vector element
> is an order of magnitude worse, than that of the simple access.
> This is not because of index limit control, it is done by at()
> and its use increases that access time by a factor of three.
> What is going on in the background here, that the implementation
> is so ineffective?
> ( I did my tests with WinXP/cygwin/g++/time,
> using different setting of the #define-s at the beginning)
> Shall I expect similar ineffectivity with the other STL objects?
> 
> Regards
> 
> Janos
> 
> 
> Empty cycle
> real    0m2.173s
> user    0m1.892s
> sys     0m0.080s
> 
> STL=1, SIMPLE=1, size=0
> real    0m4.917s
> user    0m4.646s
> sys     0m0.070s
> 
> STL=1, size=0
> real    0m33.939s
> user    0m33.658s
> sys     0m0.090s
> 
> STL=1, ITERATOR=1, size=1000
> real    0m34.059s
> user    0m33.688s
> sys     0m0.100s
> 
> STL=1, AT=1, size=1000
> real    1m22.769s
> user    1m22.368s
> sys     0m0.090s
> 
> #define STL 1
> #define SIMPLE 0
> #define ITERATOR 1
> #define AT 0
> #define EMPTY 1
> #if STL
>     #include <vector>
> #endif
>     using namespace std;
> #include <iostream>
> #define ARR_SIZE 1000
> #define CYCLES 1000000
> main (int argc, char** argv)
> {
>     long int i,j,*P;
> #if EMPTY   
>     cout << "Empty cycle";
> #else
> #if STL
>     #if ITERATOR || AT
>         std::vector<long int>::iterator pos;
>         std::vector<long int> arr(ARR_SIZE);
>     #else
>         std::vector <long int> arr;
>         arr.reserve(ARR_SIZE);
>     #endif
> 
>     cout << "STL=1";
>     #if SIMPLE
>     cout << ", SIMPLE=1";
>     #endif
>     #if ITERATOR
>     cout << ", ITERATOR=1";
>     #endif
>     #if AT
>     cout << ", AT=1";
>     #endif
>     cout << ", size=" << arr.size();
> #else
>     long int *arr;
>     arr = new long int[ARR_SIZE];
>     cout << "STL=0";
> #endif
> #endif
>     cout << endl;
>     #if SIMPLE && STL
>         P = &arr[0];
>     #endif
>     for(j=0;j<CYCLES; j++)
>     {
>         #if STL && ITERATOR && !EMPTY
>         i = 0;
>         for(pos=arr.begin(); pos < arr.end(); ++pos)
>         {
>             *pos = i;
>         }
>         #else
>         for(i=0; i<ARR_SIZE; i++)
>         {
>             #if !EMPTY
>                 #if SIMPLE && STL
>                 P[i] = i;
>                 #elif STL && AT
>                 arr.at(i) = i;
>                 #else
>                 arr[i] = i;
>                 #endif
>             #endif
>         }
>         #endif
>     }
>     exit(0);
> }
> 
> 
> ------------------------------------------------------------------------
> 
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Stefan Tarrant
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