Currently OpenBlas does what it wants for multithreading. We hesitated to disable it but prefered to wait and think about it: see https://trac.sagemath.org/ticket/21323.
You can still influence its use of threads setting OPENBLAS_NUM_THREADS. See the trac ticket, just note that this is not Sage specific. And as you discovered, it seems it is also influenced by OMP_NUM_THREADS... On Wednesday, October 5, 2016 at 9:28:23 AM UTC+2, tdumont wrote: > > What is the size of the matrix you use ? > Whatever you do, openmp in blas is interesting only if you compute with > large matrices. > If your computations are embedded in an @parallel and launch n > processes, be careful that your OMP_NUM_THREADS be less or equal to > ncores/n. > > My experience is (I am doing numerical computations) that there are > very few cases where using openmp in blas libraries is interesting. > Parallelism should generally be searched at a higher level. > > One of the interest of multithreaded blas is for constructors: with > Intel's mkl blas, you can obtain the maximum possible performances of > tah machines when you use DGEMM (ie product of matrices), due to the > high arithmetic intensity of matrix vector products. On my 2x8 core > sandy bridge à 2.7GHZ, I have obtained more that 300 giga flops, but > with matrices of size > 1000 ! And this is only true for DGEMM.... > > t.d. > > Le 04/10/2016 à 20:26, Jonathan Bober a écrit : > > See the following timings: If I start Sage with OMP_NUM_THREADS=1, a > > particular computation takes 1.52 cpu seconds and 1.56 wall seconds. > > > > The same computation without OMP_NUM_THREADS set takes 12.8 cpu seconds > > and 1.69 wall seconds. This is particularly devastating when I'm running > > with @parallel to use all of my cpu cores. > > > > My guess is that this is Linbox related, since these computations do > > some exact linear algebra, and Linbox can do some multithreading, which > > perhaps uses OpenMP. > > > > jb12407@lmfdb1:~$ OMP_NUM_THREADS=1 sage > > [...] > > SageMath version 7.4.beta6, Release Date: 2016-09-24 > > [...] > > Warning: this is a prerelease version, and it may be unstable. > > [...] > > sage: %time M = ModularSymbols(5113, 2, -1) > > CPU times: user 509 ms, sys: 21 ms, total: 530 ms > > Wall time: 530 ms > > sage: %time S = M.cuspidal_subspace().new_subspace() > > CPU times: user 1.42 s, sys: 97 ms, total: 1.52 s > > Wall time: 1.56 s > > > > > > jb12407@lmfdb1:~$ sage > > [...] > > SageMath version 7.4.beta6, Release Date: 2016-09-24 > > [...] > > sage: %time M = ModularSymbols(5113, 2, -1) > > CPU times: user 570 ms, sys: 18 ms, total: 588 ms > > Wall time: 591 ms > > sage: %time S = M.cuspidal_subspace().new_subspace() > > CPU times: user 3.76 s, sys: 9.01 s, total: 12.8 s > > Wall time: 1.69 s > > > > -- > > You received this message because you are subscribed to the Google > > Groups "sage-devel" group. > > To unsubscribe from this group and stop receiving emails from it, send > > an email to sage-devel+...@googlegroups.com <javascript:> > > <mailto:sage-devel+unsubscr...@googlegroups.com <javascript:>>. > > To post to this group, send email to sage-...@googlegroups.com > <javascript:> > > <mailto:sage-...@googlegroups.com <javascript:>>. > > Visit this group at https://groups.google.com/group/sage-devel. > > For more options, visit https://groups.google.com/d/optout. > > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to sage-devel@googlegroups.com. Visit this group at https://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
