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Ublas : |
From: Karl Meerbergen (karl.meerbergen_at_[hidden])
Date: 2008-01-12 13:01:20
Sorkin Dima wrote:
>>> Can anybody tell me the reason, why Matlab get faster, the bigger the
>>> program gets?
>>>
>
> I will add my part on Matlab vs. uBLAS:
>
> 1) As it was previously said Matlab uses optimized libraries. Moreover,
> Matlab developers add homemade changes in these libs, to make them even
> faster (but I'm not sure about precision in corner cases). For example,
> I've payed attention that Matlab uses at-home-modified version of
> LAPACK's SVD for dense matrices (see the documentation for 'svd' on their
> site, when it will return back to normal operation). It will be very hard
> to beat Matlab for large matrices (Our local gurus recommended to learn
> Fortran for such tasks ... :)
>
The question is not fortran or not. Usually, the BLAS libraries are not
written in fortran but C with assembler kernels. For dense matrices,
there is no doubt that optimized is the choice to make. For sparse, I
have no data to make any statement.
> 2) Two reasons made me abandon Matlab programming and switch to C++/Boost:
> a) Code maintenance in Matlab was too hard for me. If you write object
> oriented code in Matlab, each method is in a separate file. I
> couldn't stand this scattering of code in many tiny files. Maybe
> I got it wrong.
> b) In Matlab arguments are passed to functions by value (for good
> reasons - this allows very effective debug and inspection of
> state in case something fails). This has little effect on
> performance, since _actually_ objects are not copied.
> But this also has one exception which heavily harms performance
> of many algorithms: if you want to modify small part of large
> object (object containing large matrix some entries of which
> you want to modify) - you will pay for copying of the whole
> matrix. See the below attached message for details.
>
In addition: generic programming has some advantages for reusing code
which matlab code does not have ...
Matlab is nice for small tests on a new algorithm or graphics, or if you
need to use their toolboxes in a research project.
Best,
Karl
> Regards,
> Dima.
>
> P.S.
> I last checked it 3 years ago, things could be improved since,
> you can rerun the code by yourself on your MAtlab and examine the results.
> Below attached the reply from Matlab support and my original message:
>
> --------
> Sun, 23 May 2004 13:22:31 +0300 (IDT)
> From: Batia Perry
> To: Sorkin Dmitri
>
> Shalom,
>
> Following the answer I have received from the support of Matlab
>
> ----------------------------------------------
> MATLAB does not support passing arguments by reference (i.e., pointers).
> Functions can change only their private, temporary copy of an object.
> Therefore, to change an existing object, you must create a new one, and
> then replace the old one.
>
> The MATLAB parser will pass parameters by reference because it is faster
> to pass a pointer to a chunk of data as opposed to passing the whole chunk
> of data. However, the MATLAB parser will pass parameters by value if it
> knows that the function is going to modify the chunk of data in some way.
>
> In the case of passing structures or cell arrays, only the field or cell
> data being modified by the function will be passed by value. When the
> function returns, the calling workspace's copy of the structure or cell
> array is replaced by the function's copy efficiently, such that a minimal
> amount of copying occurs.
> ------------------------------------------
>
> Batia Perry D.Sc.
> ...
> Taub Computer Center, Technion, Haifa, Israel
>
> On Wed, 19 May 2004, Sorkin Dmitri wrote:
>
>> Hi.
>> I have 2 (advanced) questions on matlab.
>>
>> 1.Am I right that matlab implements it's objects
>> using a "reference counting with copy on write" technique ?
>>
>> 2.Regarding "A = subsasgn(A,S,B)" method :
>> When calling
>> >> myObj.field = 10 ;
>> ,is there any way to make matlab to not to
>> duplicate a copy of the object "myObj" in memory ?
>>
>> I have a problem with encapsulating large data structures.
>> Each attemt to modify "large" object makes Matlab to
>> duplicate the whole obj in memory , which hurts the performance
>> (As I think).
>> See example below for more information.
>>
>> Thank you.
>> Dmitri.
>>
>> Example:
>> ----------------------------
>> @myclass/myclass.m:
>> function result = myclass(N,M);
>> d.m = cell(N,M);
>> result = class(d,'myclass');
>>
>> @myclass/subsasgn.m:
>> function A = subsasgn(A,S,B);
>> A.m{1,1} = 'hello world' ;
>>
>> @myclass/dummy_method.m:
>> function result = dummy_method(A);
>> result = A.m{1,1};
>>
>> -------
>> Running
>> >> N = 10000 ; M = 1000; ; L = 100;
>> >> Q = myclass(N,M);
>> >> for i = 1:1:L
>> >> Q.use_of_subsasgn = 10 ; % dummy use, only for example
>> >> end
>>
>> has the complexity of O(L*M*N), because of copying.
>>
>> Running
>> >> for i=1:1:L
>> >> qqq = dummy_method(Q);
>> >> end
>>
>> has the complexity of O(L) , because of referencecounting.
>>
>>
>>
>
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