memory-aware-build

Memory-aware builds

QDK Chemistry compilation can require up to 8 GB of RAM per concurrent compile job. Limit build parallelism by both the available memory and the CPU count.

1. Determine the number of logical CPUs and the available RAM.
2. Reserve 8 GB of available RAM for each compile job.
3. Set the job count to the smaller of:
   • the logical CPU count;
   • floor(available RAM in GB / 8).
4. Always use at least one job. If available memory cannot be determined, use one job.
5. Export the result as CMAKE_BUILD_PARALLEL_LEVEL before invoking a build, including builds started indirectly by pip.

For example, a machine with 32 GB of available RAM and 16 logical CPUs should compile with at most four jobs:

export CMAKE_BUILD_PARALLEL_LEVEL=4
cmake --build cpp/build

For a Python package build, apply the same limit:

CMAKE_BUILD_PARALLEL_LEVEL=4 python -m pip install ./python

If a build is killed or reports an out-of-memory error, reduce CMAKE_BUILD_PARALLEL_LEVEL further, preferably by half, and retry. Never increase parallelism beyond the 8 GB-per-job memory limit merely to use all available CPU cores.