# Tagged Questions

Basic Linear Algebra Subprograms - A standard API library with vector-vector, matrix-vector, and matrix-matrix operations.

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### Efficiently rotate vector in 2D (and 3D)

I need to efficiently rotate a 2D (and 3D) vector in a CUDA kernel. I was thinking about generating random unitary rotation matrices. I don't need to know the angle, it just has to be randomly ...
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### Best Open Source BLAS / LAPACK package

I was wondering what is a more optimized open source BLAS/LAPACK package with respect to modern multi-core processors (Haswell and beyond). Is there any distribution that can attain performance close ...
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### Optimized parallel routine for $X' W X$ with $W$ diagonal

$X$ is a dense matrix of real doubles, typically of size 20 million rows and 500 columns, and $W$ is a diagonal matrix of real, non-negative doubles stored as a vector. I'm working in C and have ...
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### Do BLAS routines compute their respective operations with minimum error?

Do all BLAS routines compute the respective operation with minimum error ? i.e. Is the reduction in sdot computed with least error ? I need to call these ...
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### Kronecker products and basis contractions (ie. B.A.Transpose[B]) in C?

I have implemented a basis transformation in C of the following form kron[A,A]*B*Transpose[kron[A,A]] where A and ...
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### Sparse Linear Algebra vs Dense Linear Algebra

I am interested in a reference in the literature that discusses the performance of Dense Linear Algebra (blas routines) and dense linear algebra (sparse blas routines). I am interested in knowing for ...
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### BLAS, LAPACK or ATLAS for Matrix Multiplication in C

I am trying to find the most optimized way to perform Matrix Multiplication of very large sizes in C language and under Windows 7 or Ubuntu 14.04. And searching led me to BLAS, LAPACK and ATLAS. ...
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### Sudden drops in matrix multiplication performance

I've been reading about implementing dense matrix multiplication when the matrix doesn't fit in cache. One of the graphs I've seen (slide 9 from these slides) shows sudden drops in performance using ...
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### BLAS/LAPACK subroutine to add two matrices with different offsets and leading dimensions

I currently searching for a subroutine from BLAS or LAPACK which realizes the following operation A = alpha*A + beta * B where A and B have different leading ...
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### Does PETSc ever make use of LAPACK libraries for sparse matrix math?

Does compiling PETSc with an external BLAS/LAPACK library significantly affect performance on sparse matrices, or does it only use those libraries for dense matrix math?
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### Estimating time for running serial/parallel codes

Assume I am running an iterative method, I have a rough estimate of how many iterations it will need, How do best estimate the time it will run for in serial? For instance, If I have Conjugate ...
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### Are DAXPY, DCOPY, DSCAL overkills?

I have implemented CG in FORTRAN by linking it to Intel MKL. When there are statements like: (Refer Wikipedia) p=r; x=x+alpha*p r=r-alpha*Ap; or similar ...
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### Why isn't my Matrix-Vector Multiplication Scaling?

Sorry for the long post but I wanted to include everything that I thought was relevant in the first go. What I want I am implementing a parallel version of Krylov Subspace Methods for Dense Matrices....
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### How do I use ScaLapack/PBLAS for Matrix-Vector Multiplication?

After going to all possible "Introductions" to ScaLapack, I still can't understand how to carry out a simple PDGEMV operation using it. Here is what I must do : ...
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### How much better are Fortran compilers really?

This question is an extension of two discussions that came up recently in the replies to "C++ vs Fortran for HPC". And it is a bit more of a challenge than a question... One of the most often-heard ...