And I figured out the answer to my own problem. The suspect line in the `addprocs` behavior is here: https://github.com/JuliaLang/julia/blob/v0.4.3/base/multi.jl#L1120 This function calls `blas_set_num_threads(1)` so I can undo this behavior by just calling `Base.blas_set_num_threads(16)` after `addprocs`.
I am going to open an issue asking for this to become a keyword argument. Thanks, Michael On Thu, Jun 2, 2016 at 2:44 PM, Michael Eastwood < michael.w.eastw...@gmail.com> wrote: > Hello julia-users, > > I've noticed that if I start a worker blas is all of the sudden restricted > to only running on one thread. See for example the following log. Here I > multiply a large matrix by itself, add a remote worker, and repeat the > multiplication (on the master process). In the latter case blas only uses > one thread despite the fact it was happily using 16 threads before adding > the worker. (Note that this log only shows timings but I've verified the > number of threads being used by watching top). > > Can this behavior be circumvented? > > $ julia > _ > _ _ _(_)_ | A fresh approach to technical computing > (_) | (_) (_) | Documentation: http://docs.julialang.org > _ _ _| |_ __ _ | Type "?help" for help. > | | | | | | |/ _` | | > | | |_| | | | (_| | | Version 0.4.3 (2016-01-12 21:37 UTC) > _/ |\__'_|_|_|\__'_| | > |__/ | x86_64-unknown-linux-gnu > > julia> function multiply(A) > A'*A > end > multiply (generic function with 1 method) > > julia> A = randn(5000, 5000); > > julia> ENV["OMP_NUM_THREADS"] > "16" > > julia> @time multiply(A); # 16 threads used as expected > 1.302345 seconds (634.91 k allocations: 220.681 MB, 3.01% gc time) > > julia> addprocs([("astm10", 1)]) # add a process on a remote machine > 1-element Array{Int64,1}: > 2 > > julia> ENV["OMP_NUM_THREADS"] # unchanged > "16" > > julia> @time multiply(A); # only one thread used > 10.126372 seconds (9 allocations: 190.735 MB, 0.02% gc time) > > julia> versioninfo() > Julia Version 0.4.3 > Commit a2f713d* (2016-01-12 21:37 UTC) > Platform Info: > System: Linux (x86_64-unknown-linux-gnu) > CPU: Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz > WORD_SIZE: 64 > BLAS: libopenblas (USE64BITINT NO_AFFINITY NEHALEM) > LAPACK: libopenblas64_ > LIBM: libopenlibm > LLVM: libLLVM-3.3 > > > Thanks, > Michael > >
