Pinning Julia Threads

The most important functions are pinthreads and threadinfo. The former allows you to pin threads. The latter visualizes the current thread-processor mapping and the system topology. Please check out the comprehensive documentation of these functions for detailed information.

Typical usage


Pinning your threads is as simple as putting the following at the top of your Julia code:

using ThreadPinning

This will successively pin all Julia threads to CPU-cores in logical order, avoiding hyperthreads if possible. Of course, you can replace :cores by all the options supported by pinthreads. Conceptually, there are three different formats to specify your desired thread-processor mapping:

  1. explicit lists of CPU IDs (e.g. 0:3 or [0,12,4]),
  2. predefined symbols (e.g. :cores or :sockets),
  3. logical specification of domains via helper functions (e.g. node and socket).

For example, instead of pinthreads(:cores) above, you could write pinthreads(1:2:10) or pinthreads(socket(1,1:3), numa(2,2:5)). Again, see pinthreads for more information.


To check and visualize the current pinning you can use threadinfo to get something like this.


As you can see, this image is taken on a dual-socket system (indicated by the two | .... | sections) where each CPU has 20 CPU-cores and Julia has been started with 40 threads. Hyperthreading is enabled - the greyed out numbers indicate hyperthreads/SMT-threads - with two CPU-threads per core.

Note that threadinfo has a few keyword arguments that let you change or tune the output. The most important ones are probably groupby and color. The former allows you to switch from socket to, say, NUMA/memory domain visualization (groupby=:numa). The latter allows you to switch to non-colored output (see below).

julia> using ThreadPinning

julia> threadinfo(; color=false)

| 0,1,_,3,4,_,_,7,8,_,10,_,_,_,_,_,
  _,_,54,_,_,57,58,_ |
| _,21,22,23,_,_,_,_,28,29,30,_,32,33,_,35,
  72,73,74,_,_,_,_,_ |

# = Julia thread, # = HT, # = Julia thread on HT, | = Socket seperator

Julia threads: 40
├ Occupied CPU-threads: 40
└ Mapping (Thread => CPUID): 1 => 63, 2 => 64, 3 => 17, 4 => 68, 5 => 4, ...

julia> pinthreads(:cores)

julia> threadinfo(; color=false)

| 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,
  _,_,_,_,_,_,_,_ |
| 20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,
  _,_,_,_,_,_,_,_ |

# = Julia thread, # = HT, # = Julia thread on HT, | = Socket seperator

Julia threads: 40
├ Occupied CPU-threads: 40
└ Mapping (Thread => CPUID): 1 => 0, 2 => 1, 3 => 2, 4 => 3, 5 => 4, ...

julia> pinthreads(:cputhreads)

julia> threadinfo(; color=false)

| 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,
  52,53,54,55,56,57,58,59 |
| _,_,_,_,_,_,_,_,_,_,_,_,_,_,_,_,
  _,_,_,_,_,_,_,_ |

# = Julia thread, # = HT, # = Julia thread on HT, | = Socket seperator

Julia threads: 40
├ Occupied CPU-threads: 40
└ Mapping (Thread => CPUID): 1 => 0, 2 => 40, 3 => 1, 4 => 41, 5 => 2, ...

Pinning via environment variables

Sometimes it can be useful to specify a desired thread-processor mapping before starting Julia, that is, without using pinthreads explicitly in your Julia code. A Julia-built-in example for this is JULIA_EXCLUSIVE=1. Similarly, ThreadPinning.jl offers pinning threads via environment variables. In this case, Julia Threads will get pinned during the initialization of the ThreadPinning package, i.e. when running using ThreadPinning. Note, though, that explicit pinthreads statements take precedence over these environment variables.

Concretely, the supported environment variables are

  • JULIA_PIN: Can be set to any symbol supported by pinthreads (e.g. JULIA_PIN=cores). Capitalization doesn't matter.
  • JULIA_LIKWID_PIN: Can be set to any string supported by pinthreads_likwidpin (e.g. JULIA_LIKWID_PIN=S0:1-3@S1:4,5,6). Capitalization does matter.

Pinning via preferences

To specify a certain pinning pattern more permanently, e.g., on a per-project basis, you can use preferences. ThreadPinning.jl provides the relevant functionality in the ThreadPinning.Prefs module. Note that environment variables and explicit pinthreads statements take precedence over these preferences.

Default pinning (for packages)

If you're developing a package you may want to provide a reasonable default pinning. If you would naively use pinthreads for this, you would enforce a certain pinning irrespective of what the user might have specified manually. This is because pinthreads has the highest precedence. To lower the latter you can set force=false in your pinthreads call, e.g. pinthreads(:cores; force=false). This way, a user can overwrite your default pinning (:cores in this example) by using environment variables, preferences, or calling pinthreads manually before running your package code.


We provide functions unpinthread(threadid) and unpinthreads() to unpin a specific or all Julia threads, respectively. This is realized by setting the thread affinity mask to all ones. While technically not really unpinning threads, you might also want to consider using pinthreads(:random) for "fake unpinning" in benchmarks as it does randomize the thread placing but keeps it fixed to reduce measurement fluctuations.

likwid-pin-compatible input

Separate from pinthreads, used and described above, we offer pinthreads_likwidpin which, ideally, should handle all inputs that are supported by the -c option of likwid-pin (e.g. S0:1-3@S1:2,4,5 or E:N:4:2:4). If you encounter an input that doesn't work as expected, please file an issue.