nixpkgs/doc/hooks/check-phase-thread-limit-hook.section.md

checkPhaseThreadLimitHook

This hook defaults a variety of environment variables known to control thread counts to 1. Many of these otherwise default to $(nproc), which causes massive overloads on build machines if nix build jobs and build cores are already tuned to fully utilize compute capacity of a builder without additional parallelism.

Currently sets the following environment variables:

• OMP_NUM_THREADS
• OPENBLAS_NUM_THREADS
• MKL_NUM_THREADS
• BLIS_NUM_THREADS
• VECLIB_MAXIMUM_THREADS: Only affects darwin, see man 7 Accelerate
• NUMBA_NUM_THREADS
• NUMEXPR_NUM_THREADS

The NIX_CHECK_PHASE_DEFAULT_NUM_THREADS environment variable can be used to override the default thread count limit. dontLimitCheckPhaseThreads = true; can be used to disable thread limiting on an individual package.

This hook will not attempt to override already existing definitions for thread count environment variables.