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Subject: Re: [ublas] Status of development /Benchmarks
From: Riccardo Rossi (rrossi_at_[hidden])
Date: 2013-12-07 17:35:42

Dear Oswin,

my first point is really trivial. i do not doubt that the current
expression template mechanism is not optimal.

the only thing i propose is to mantain a small backward compatibility layer
so that code that works with the old ublas is not unnecessarily broken.

for example if you assume that the novel syntax is

C += A*B

i believe that it could be useful to define some wraper between the old and
new syntax. the easiest way (dirty) would be to define two macros

"noalias" and " prod"

so that
noalias(C) += prod(A,B)

expands to the new format... indeed using macros is not a good idea but it
gives the point

my second point is mostly about matrices of size say 30*30, whose size is
NOT known at compile time.
for such matrix size openmp is not useful, however one may perform in
parallel a lot of operations on different small matrices, parallelizing
some sort of outer iteration.
for this outer parallelization to be useful, one needs guarantees that
openmp is not used in the inner loop... otherwise problems may arise with
nested parallelism.
...i only implied this...


On Sat, Dec 7, 2013 at 10:10 PM, oswin krause <
oswin.krause_at_[hidden]> wrote:

> Hi,
> I like your ideas. But i am not sure what you mean with 1. and 2.
> The problem I see is, that uBLAS right now doesn't offer the desired
> quality and speed for single core, let alone multi core. Therefore I will
> focus on the internal infrastructure instead of motivating new algorithms.
> short description:
> 1. Rock solid implementation of the BLAS algorithms( "compute kernels")
> 2. Rewriting of the expression mechanic.
> 2.1 automatic use of the compute kernels for the operations
> 2.2 automatic optimisation of expressions
> long description
> 1. Rock solid implementation of the BLAS algorithms: matrix x vector and
> matrix x matrix in all useful combinations (sparse/sparse dense/dense
> dense/sparse ... triangular/dense...etc there are a lot), but also: matrix
> assignment. uBLAS will not be compared in #Features but in runtime compared
> to Armadillo/Eigen/Atlas/GotoBlas. Having a fast implementation of this is
> also crucial for multi-core and distributed computations. Also all higher
> level decompositions (Cholesky, LU, SVD, RankDecomp,...) and triangular
> solvers rely on fast BLAS.
> 2. Rewriting of the expression mechanic. The way uBLAS is implemented
> right now, writing
> noalias(A)+=prod(B,C)+D
> or
> vec = prod(prod<matrix>(B,C),vec2);
> is in most cases a bad decision. The second expression wouldn't even
> compile without the template parameter.
> What I would like to have is the following:
> 2.1 automatic use of the fast algorithms. That is the above expression
> should be translated to
> axpy_prod(B,C,A,false);
> noalias(A) +=D;
> right now the matrix_assign algorithms are used which do a terrible job on
> prod(B,C).
> 2.2 automatic optimization of expressions. The second expression above is
> really inefficient. First the matrix-matrix prod is evaluated and the
> result is afterwards used for a single matrix-vector prod.
> now compare to this:
> prod(B,prod<vector>(C,d));
> only 2 matrix-vector products are performed which might save 99% of the
> FLOPS of the first expression. So it would be better if this would be
> transformed automatically. Normally the user could do that himself but
> sometimes it is not possible. Consider a Conjugate Gradient Solver:
> x=CG(A,b);
> a usual pattern of A is A=XX^T+c*Identity. CG uses internally only
> matrix-vector products - and possibly not many of them. Therefore you don't
> want to compute A because it is a) slow b) might require more memory than
> is available. Without expression optimizations it would be impossible to
> write a good CG that could do that.
> Greetings,
> Oswin
> On 07.12.2013 11:38, David Bellot wrote:
> Hi,
> it has been a long discussion we all had for many months now. Should we
> rewrite ublas from scratch or simply improve it.
> Joaquim and Oswin wants a brand new ublas
> Nasos was more in favor of improving it.
> I personally find very exciting the idea of writing something new, but
> ublas is very well known now. On the other hand, Eigen and Armadillo took
> the crown of the main C++ blas library in users' hearts.
> On my todo list for ublas, there are things that will require ublas to
> be deeply changed. At this stage, we can almost talk about a new library.
> Christmas is very close now, so maybe it's a good time to start talking
> about the features we wish for ublas and see if they can be implemented
> with the current version or if a new uBLAS 2.0 is necessary.
> After all, Boost::signal did the same a few years ago. We can definitively
> do the transition.
> I begin:
> - unified representation of vectors and matrices to represent the fact
> that a vector IS a matrix. Matlab does the same
> - automated use of different algorithm to let the compiler "chooses" the
> best implementation (if possible) and switch on SSE, distributed or whateve
> we need
> - implementation of solvers and decompositions algorithms
> and this is what Nasos and I think should be integrated too:
> 1. Matrix multiplication
> 2. Algorithms infrastructure (so that we can have real useful features)
> 3. Matrix/vector views for interoperability <- I think this is ultra
> critical because now ublas is monolithic in the sense that you have to use
> it everywhere you manipulate data. This would really help into letting
> people for example have a list of vectors (they are plotting) and ublas
> working on top of that to do for example transformations
> 4. NEW DOCUMENTATION - examples and the rest
> 5. Incorporate some critical bindings (i.e. mumps bindings which is
> currently probably the most efficient smp and distributed open source
> linalg solver)
> 6. matlab binding?
> 7. distributed ublas
> Please add and ESPECIALLY, please tell me your view on the current
> infrastructure of uBLAS. It seems many people are not happy with the
> current "expression template" grammar.
> I'm open to everything
> Best,
> David
> On Thu, Dec 5, 2013 at 11:14 AM, Joaquim Duran <jduran.gm_at_[hidden]>wrote:
>> I think that al stuff pending of merge listed by David, should be merged
>> and migrate to uBlas 2.0 and while uBlas 2.0 is in development/maintenance
>> then design from scratch uBlas 3.0.
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Dr. Riccardo Rossi, Civil Engineer
Member of Kratos Team
International Center for Numerical Methods in Engineering - CIMNE
Campus Norte, Edificio C1
c/ Gran Capitán s/n
08034 Barcelona, España
Tel:        (+34) 93 401 56 96
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