I understand the reasons why OpenBLAS shouldn't be multithreading everything, and why it shouldn't necessarily use all available cpu cores when it does do multihreading, but the point is: it currently uses all or one, and sometimes it decides to use multithreading even when using 2 threads doesn't really seem to give me a benefit. So I guess there are two points to consider. One is a "public service announcement" that if things don't change, then in Sage 7.4 users might want to strongly consider setting OpenBLAS to be single threaded. The other is that we may want to reconsider the OpenBLAS defaults in Sage.
One possibility might be to expose the openblas_set_num_threads function at the top level, and keep the default at 1. Another possibility is to build OpenBLAS single threaded by default and force someone compiling Sage to pass some option for multithreaded OpenBLAS; that way, at least, only "advanced" users will run into and have to deal with the sub-par multithreading behavior. On Wed, Oct 5, 2016 at 10:34 AM, Clement Pernet <clement.per...@gmail.com> wrote: > To follow up on Jean-Pierre summary of the situation: > > The current version of fflas-ffpack in sage (v2.2.2) uses the BLAS > provided as is. Running it with a multithreaded BLAS may result in a slower > code than with a single threaded BLAS. This is very likely due to memory > transfer and coherence problems. > > More generally, we strongly suggest to use a single threaded BLAS and let > fflas-ffpack deal with the parallelization. This is common practice for > example with parallel versions of LAPACK. > > Therefore, after the discussion https://trac.sagemath.org/ticket/21323 we > have decided to let fflas-ffpack the possibility to force the number of > threads that OpenBLAS can use at runtime. In this context we will force it > to 1. > This is available upsteam and I plan to update sage's fflas-ffpack > whenever we release v2.3.0. > > Clément > > > Le 05/10/2016 à 11:24, Jean-Pierre Flori a écrit : > >> 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 >> > <mailto:sage-devel+unsubscr...@googlegroups.com>. >> > To post to this group, send email to sage-...@googlegroups.com >> > <mailto:sage-...@googlegroups.com>. >> > Visit this group at https://groups.google.com/group/sage-devel >> <https://groups.google.com/group/sage-devel>. >> > For more options, visit https://groups.google.com/d/optout < >> 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 <mailto:sage-devel+unsubscribe >> @googlegroups.com>. >> To post to this group, send email to sage-devel@googlegroups.com <mailto: >> 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. >> > > -- > 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. > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. 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