OpenMathLib / OpenMathLib/OpenBLAS
Performance of dgemm
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Description
When I perform calculations of the type C = (transpose(A))B I noticed that with openBLAS I don't gain any speedup when I call cblas_dgemm with the right flags to indicate that A is transposed, compared to if I just do a manual transposition myself, i.e. D = transpose(A) which involves a memory allocation for D followed by manually filling in D(i,j)=A(j,i), and only then C = DB via cblas_dgemm. I observe the same behavior when doing A*(transpose(B)). On the other hand using MKL BLAS, the first option which doesn't do a manual transposition results in 50% faster execution. Is this because OpenBLAS makes hidden copies when transA and transB are set to CBlasTrans?
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Research direction
Start by reproducing the comparison using the cblas_dgemm entry point, with transposed operands versus manually transposed matrices. Inspect the handling of CBlasTrans and determine whether hidden copies account for the observed performance difference; done means explaining the behavior and its contrast with MKL BLAS.
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Assessment
- Tech stack
- c
- Domain
- backend, performance
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 30/100