Neko 1.99.3
A portable framework for high-order spectral element flow simulations
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Appendices

Table of Contents

The appendices contain a few extra pages that are not directly related to the usage of the code. But can be useful for users and developers alike.

Environmental variable reference

Name Description Default value
NEKO_AUTOTUNE Force Ax auto-tuning strategy ('1D','KSTEP') Unset
NEKO_LOG_FILE Log file name, uses stdout if not set. Unset
NEKO_LOG_TAB_SIZE Number of spaces added for each level of indentation in the log file. 1
NEKO_LOG_LEVEL Log verbosity level (integer > 0, default: 1) Unset
NEKO_GS_STRTGY Gather-scatter device MPI sync. strategy (0 < integer < 5 ) Unset
NEKO_GS_COMM Gather-scatter communication backend Unset
NEKO_GS_CAF_SIGNALING Coarray Fortran gather-scatter signaling mode Unset
NEKO_COMM_ID Communicator id for this process (non-negative integer) 0
NEKO_MPI_THREAD_LEVEL Requested MPI (and SHMEM) thread support level Unset
NEKO_DEPRECATION_ERROR Whether to treat deprecated features as errors (boolean) Unset

Logging level details

A number of logging levels are supported.

  • NEKO_LOG_LEVEL=0 : Quiet mode, minimal logging during execution.
  • NEKO_LOG_LEVEL=1 : Default information mode, adding step informations.
  • NEKO_LOG_LEVEL=2 : Verbose mode, logging extra details.
  • NEKO_LOG_LEVEL=5 : Deprecated features will be logged if used.
  • NEKO_LOG_LEVEL=10 : Debug mode.

Gather-scatter communication backend details

A number of gather-scatter backends are supported.

  • NEKO_GS_COMM=MPI : Host based MPI
  • NEKO_GS_COMM=MPIGPU : Device based MPI
  • NEKO_GS_COMM=NCCL : NCCL/RCCL using its point-to-point interface
  • NEKO_GS_COMM=SHMEM : NVSHMEM based on GPU builds (NVIDIA GPUs); OpenSHMEM based on CPU builds (requires a native OpenSHMEM library, e.g. Cray OpenSHMEMX, enabled at configure time with --with-openshmem)
  • NEKO_GS_COMM=CAF : Coarray Fortran (requires a coarray-capable compiler)

MPI thread level details

NEKO_MPI_THREAD_LEVEL overrides the default thread support requested from the MPI runtime (and, when built with --with-openshmem, from the SHMEM runtime as well). When unset, the level is chosen automatically: MPI_THREAD_MULTIPLE when running with more than one OpenMP thread per rank, falling back to MPI_THREAD_FUNNELED on host-only backends, and MPI_Init otherwise. Device backends require MPI_THREAD_MULTIPLE under the automatic policy. Setting the variable bypasses these heuristics; if the runtime cannot honor the requested level, initialisation aborts.

  • NEKO_MPI_THREAD_LEVEL=single : MPI_THREAD_SINGLE (calls MPI_Init).
  • NEKO_MPI_THREAD_LEVEL=funneled : MPI_THREAD_FUNNELED.
  • NEKO_MPI_THREAD_LEVEL=serialized : MPI_THREAD_SERIALIZED.
  • NEKO_MPI_THREAD_LEVEL=multiple : MPI_THREAD_MULTIPLE.

Coarray Fortran signaling mode details

When NEKO_GS_COMM=CAF, the per-pair synchronisation strategy is selected by NEKO_GS_CAF_SIGNALING. The mode is bound on the first gather-scatter initialisation and cannot change thereafter. The default (when unset) is sync.

  • NEKO_GS_CAF_SIGNALING=sync : sync images over the union of neighbour pairs, with a double-buffered receive coarray (F2008).
  • NEKO_GS_CAF_SIGNALING=atomic : Per-pair atomic counters via atomic_define/atomic_ref with a busy-wait spin (F2008).
  • NEKO_GS_CAF_SIGNALING=event : F2018 events (event post / event wait); requires a runtime that implements F2018 event semantics.