Appendices

Table of Contents

• Environmental variable reference
  • Logging level details
  • Gather-scatter communication backend details
  • Coarray Fortran signaling mode details

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
• Governing equations
• Neko .nmsh mesh format
• Neko .fld field format
• Publications

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_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)

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.