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

From: Brian Budge (brian.budge_at_[hidden])
Date: 2006-08-28 12:49:43

This is most likely that you are not getting your operator[] inlined.
The function call overhead for such a small operation is going to be
very large.  Optimizations are your friends.

On 8/26/06, Janos Vegh <vegh.janos_at_[hidden]> wrote:
>
>
>
> On 8/26/06, me22 <me22.ca_at_[hidden]> wrote:
> >
> > main has to return int; Your code isn't legal C++.  Also, it seems
> > like you end up using memory in the vector after only reserving it,
> > not resizing it.
>
>
> Yes, a forgotten  int, my mistake.
>  After reserving the  memory, I guess I can  write the memory  locations.
> I think neither of the above affects the subject.
>
> > Your empty cycle time makes it obvious you're not even doing the most
> > rudimentary of optimizations.
>
>
> My goal was to measure the time of the empty processing,
> in order to estimate the time of the net calculation,
> rather than to find out that for no operation I need no time.
>
> > Here's what I get in g++ (g++ (GCC) 4.1.1 (Gentoo 4.1.1)) after making
> > a few changes while using -O3 -DNDEBUG as arguments to the compiler
> > (though -DNDEBUG didn't make any signifigant difference):
> >
> > Empty cycle
> > real    0m0.003s
> > user    0m0.000s
> > sys     0m0.000s
> >
> > STL=1, SIMPLE=1, size=1000
> > real    0m1.357s
> > user    0m1.168s
> > sys     0m0.008s
> >
> > STL=1, size=1000
> > real    0m1.337s
> > user    0m1.172s
> > sys     0m0.016s
> >
> > STL=1, ITERATOR=1, size=1000
> > real    0m1.315s
> > user    0m1.172s
> > sys     0m0.012s
> >
> > STL=1, AT=1, size=1000
> > real    0m1.357s
> > user    0m1.176s
> > sys     0m0.004s
> >
> > I really don't see an issue here.
> >
> >
> I neither, except that it looks like that in the case of AT checking the
> lower and upper
> limits of the index needs no extra time(??).
>
> Otherwise, on my system
> $ g++ -oeffic2.exe effic2.cpp
> $ time ./effic2.exe
> STL=1, SIMPLE=1, ITERATOR=0, AT=0, size=1000
> real    0m4.775s
> user    0m4.476s
> sys     0m0.080s
>
> $ g++ -o3 -oeffic2.exe effic2.cpp
> $ time ./effic2.exe
> STL=1, SIMPLE=1, ITERATOR=0, AT=0, size=1000
> real    0m4.749s
> user    0m4.496s
> sys     0m0.060s
>
> and
>
> $ g++ -o3 - oeffic2.exe effic2.cpp
> $ time ./effic2.exe
> STL=1, SIMPLE=0, ITERATOR=0, AT=0, size=1000
> real    0m33.413s
> user    0m32.076s
> sys     0m0.100s
>
> $ g++ -oeffic2.exe effic2.cpp
> $ time ./effic2.exe
> STL=1, SIMPLE=0, ITERATOR=0, AT=0, size=1000
> real    0m34.554s
> user    0m33.668s
> sys     0m0.060s
>
> and
>
> $ g++ -oeffic2.exe effic2.cpp
> $ time ./effic2.exe
> STL=1, SIMPLE=0, ITERATOR=0, AT=1, size=1000
> real    1m26.753s
> user    1m24.301s
> sys     0m0.080s
>
> $ g++ -o3 -oeffic2.exe effic2.cpp
> $ time ./effic2.exe
> STL=1, SIMPLE=0, ITERATOR=0, AT=1, size=1000
> real    1m29.311s
> user    1m27.035s
> sys     0m0.050s
>
> After that you were so kind to run my  test on your Linux,
> maybe I need to ask MingW people about this  problem.
>
> Anyhow, thanks for replying and sorry for bothering the list with my problem
>
> Janos
>
>
>
>
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