# Maximum BLAS threads number

**URL:** <https://discourse.julialang.org/t/maximum-blas-threads-number/25320>\
**Category:** General Usage\
**Tags:** question\
**Created:** [June 14, 2019, 8:06pm UTC](https://discourse.julialang.org/t/maximum-blas-threads-number/25320 "2019-06-14T20:06:48Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![shipengcheng1230](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/shipengcheng1230/32/4089_2.png) [@shipengcheng1230](https://discourse.julialang.org/u/shipengcheng1230)\
**Post date:** [June 14, 2019, 8:06pm UTC](https://discourse.julialang.org/t/maximum-blas-threads-number/25320/1 "2019-06-14T20:06:48Z")

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Hi, consider the following example:

```julia
julia> versioninfo()
Julia Version 1.1.1
Commit 55e36cc308 (2019-05-16 04:10 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Xeon(R) Gold 6136 CPU @ 3.00GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.1 (ORCJIT, skylake)
Environment:
  JULIA_NUM_THREADS = 24

julia> using LinearAlgebra

julia> BLAS.set_num_threads(24)

julia> ccall((:openblas_get_num_threads64_, Base.libblas_name), Cint, ())
16

julia> BLAS.set_num_threads(12)

julia> ccall((:openblas_get_num_threads64_, Base.libblas_name), Cint, ())
12

```

It won’t let me get more than 16 threads for BLAS. The computer has 2 CPUs, each of which has 12 cores (24 HT). How does the limitation `16` come from?

Thanks!

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<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [June 14, 2019, 8:25pm UTC](https://discourse.julialang.org/t/maximum-blas-threads-number/25320/2 "2019-06-14T20:25:00Z")

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[https://github.com/JuliaLinearAlgebra/OpenBLASBuilder/blob/5a6eca317de505abc3678e47f08f4355646f511e/build\_tarballs.jl#L42-L47](https://github.com/JuliaLinearAlgebra/OpenBLASBuilder/blob/5a6eca317de505abc3678e47f08f4355646f511e/build_tarballs.jl#L42-L47).

IIRC, OpenBLAS allocates memory on initialization for the largest number of threads it can start so it isn’t possible to put a super high number. I guess 16 was good at the time but with high core counts becoming more mainstream, perhaps it would make sense to increase it.

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<div class="post-metadata">

**Author:** ![shipengcheng1230](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/shipengcheng1230/32/4089_2.png) [@shipengcheng1230](https://discourse.julialang.org/u/shipengcheng1230)\
**Post date:** [June 15, 2019, 1:03am UTC](https://discourse.julialang.org/t/maximum-blas-threads-number/25320/3 "2019-06-15T01:03:53Z")

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Thanks for the link!

Is it just as simple as changing the number in `flags+=(NUM_THREADS=16)` or is there some nontrivial modifications to make more use of threads for now?