Re: [boost] C++ Performance

From: Steven Burns (royalstream_at_[hidden])
Date: 2007-12-03 18:09:22

Objects are getting copied over and over again, that is a big part of the 
problem.
That is not the right way to do C++ if you want to get top performing code.

With that said, you say you are a newbie so IMHO your options are:

(A) Learn some more about C++ and use techniques (such as expresion 
templates) to aviod all the copying around.
(B) Stick to basic C++ but change the way your code works (more on this 
later)
(C) Stick to MATLAB

As far as option (B), try creating your objects once and then passing them 
by reference to the actual operations.
So, instead of having an itrans() function that returns a new matrix, make 
it write the result to an output parameter and return nothing.
Not very pretty, I know, but if you want pretty go for option (A) or (C).

I've used option (A) for something similar.
I have some Matrix/Vector classes that support intuitive syntax (X = Y * W) 
yet they avoid the memory copies using expression templates. I also wrapped 
Intel's MKL Library. As far as performance goes, Intel MKL library gives me 
an edge when the matrixes are not tiny. For tiny matrixes my own hand 
written code performs faster (because it avoids calling their library which 
has some little overhead).

But honestly, there's nothing wrong with using output parameters and keeping 
things simple.

Regards,

Steven

"nisha kannookadan" <nishak44_at_[hidden]> wrote in message 
news:BAY122-W16E24F4B48411F2554E94FD97F0_at_phx.gbl...
>
> Wow, Im very impressed, I already got so much answers. Thanks a lot.
> I didnt realize, that there are different lists, sorry.
>
> The Code is quite huge. But I post here, a part, which takes quite a lot 
> of time: These are two methods for wavelet transformations:
>
>
> matrix Wavelet::ttrans(matrix At, int level)
> {
> matrix  cfe1, cfe2, cfo, cfe, c, d;
> int N,s2;
>
> N = (At.size1()+1)/2;
> s2 = At.size2();
> zero_matrix zer(N,s2);
>
> for (int ii = 1; ii <= level; ii++)
> {
> cfo.resize(N,s2,false);
> cfe.resize(N-1,s2,false);
> c.resize(N-1,s2,false);
> d.resize(N,s2,false);
> cfo.assign(subslice(At, 0,2,N, 0,1,s2));
> cfe.assign(subslice(At, 1,2,N-1, 0,1,s2));
>
> c.assign(cfe + (subrange(cfo, 0,N-1, 0,s2)+subrange(cfo, 1,N, 0,s2))*0.5);
>
> zer.resize(N,s2,true);
> cfe1.resize(N,s2,false);
> cfe2.resize(N,s2,false);
> cfe1.assign(zer);
> cfe2.assign(zer);
>
> (subrange(cfe1, 0,N-1, 0,s2)).assign(cfe);
> (subrange(cfe2, 1,N, 0,s2)).assign(cfe);
> d.assign(cfo-(cfe1+cfe2)*0.5);
>
>     (subrange(At, 0,N-1, 0,At.size2())).assign(c);
>     (subrange(At, N-1,2*N-1, 0,At.size2())).assign(d);
>
>     N = N/2;
> }
>
> cfe1.clear();
> cfe2.clear();
> cfo.clear();
> cfe.clear();
> c.clear();
> d.clear();
>
> return At;
>
> }
>
> matrix Wavelet::itrans(matrix At,int level)
> {
> matrix inv_M,tmp;
> vector f1,f2;
> int N,m;
>
> N = (At.size1()+1)/2;
> scalar_vector e(2*N-1,1);
> scalar_vector f(2*N-2,0.5);
> m = At.size2();
>
> for (int ii=1; ii<=level; ii++)
> {
>
>    e.resize(2*N-1,true);
>    f.resize(2*N-2,true);
>    f1 = f;
>    f2 = f;
>    for(int jj = 0; jj < 2*N-3; jj = jj+2)
>    {
>    f2(jj) = -0.5;
>    f1(jj+1) = -0.5;
>    }
>
>    inv_M.resize(2*N-1,2*N-1,false);
>    inv_M = MVHF::tri_inverse(e,f1,f2);
>    tmp.resize(2*N-1,m);
>    tmp.assign(prod(inv_M,subrange(At, 0,2*N-1, 0,m)));
>
>    (subrange(At, 0,N-1, 0,At.size2())).assign(subslice(tmp, 1,2,N-1, 
> 0,1,tmp.size2()));
>    (subrange(At, N-1,2*N-1, 0,At.size2())).assign(subslice(tmp, 0,2,N, 
> 0,1,tmp.size2()));
>
>    N = N/2;
> }
>
> inv_M.clear();
> tmp.clear();
> f1.clear();
> f2.clear();
>
> return At;
> }
>
> I did it like that, bcuz I realized that using 'assign' does it make a bit 
> faster,though a lot of resize is needed. I use Eclipse, so is it possible 
> to adapt the Debugging stuff there? Or does it need to be in a 
> makefile...yeah, I know, I dont know the basic stuff :). I read about the 
> library binding, but I actually dont need things like svd, lu..all I need 
> is prod,solve,element_div and the methods above. So not to freaky things, 
> so I probabaly dont need the bindings, right?
>
> Anyways, I thank you all very much..I never got answers so fast in a 
> forum..I appreciate it.
>
>
>
>
>
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