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cpu times [message #13484] Fri, 11 May 2012 18:36 Go to previous message
Gianluigi Boca is currently offline  Gianluigi Boca
Messages: 177
Registered: March 2004
first-grade participant
From: *gsi.de
dear collaborators,
we all know at least from oral tradition, that the Standard Template Library containers are somewhat slower compared to the 'traditional' C code style arrays.

But how much slower are they actually ?

I checked the difference in cpu consumption when using a conventional C array or a Standard Template Library <vector> instead, using a very simple program.

I measured the cputime consumption
of 10,000,000,000 assignment operations [avoiding
though a calculation that can be optimized heavily by the compiler].

I wrote two almost identical simple loop programs :

1) Conventional C array program :

int main ()
{
int v[10],b ;
itmp = 500000;
for(int j=0;j<10000;j++){
for(int i=0;i<itmp;i++){
b=i+j;
v[3]=j+i;
b=v[3];
}
}

2) Template <vector> program :

#include <vector>
int main ()
{
vector <int> v(10,0) ;
int b;
int itmp = 500000;

for(int j=0;j<10000;j++){
for(int i=0;i<itmp;i++){
b=i+j;
v.at(3)=i+j;
b=v.at(3);
}
}
return 0;
};


I measured the cpu consumption of the two programs.
I also measured (and subtracted) the cputime consumption of the NON RELEVANT part of the code, namely :

int main ()
{
int b;
int itmp = 500000;

for(int j=0;j<10000;j++){
for(int i=0;i<itmp;i++){
b=i+j;
}
}
return 0;
};


THE FOLLOWING IS THE CpuTime CONSUMPTION OF THE TRADITIONAL C
STYLE

v[3]=j+i;
b=v[3];

STATEMENTS : 9.352 sec

while THE FOLLOWING IS THE CpuTime CONSUMPTION OF THE Standard
Template Library

v.at(3)= i+j;
b=v.at(3);

STATEMENTS : 166 sec

In other words, a factor almost 18 worse of the Template <vector>.

As you very well know, the Template <vector> gives the advantage
of the array boundary check (only when you use the at() function
though, NOT when you use the [] form! ) but I am wondering if we can afford a factor of speed 18 slower in acces time for a code such as the PANDA code that is supposed to digest billion and billion of events in the future.

Please comment, thanks

Gianluigi
 
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