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Network and interconnect

All LUMI compute nodes use the HPE Cray Slingshot-11 200 Gbps network interconnect (NIC). The LUMI-C nodes (CPU) are equipped with a single endpoint while the LUMI-G nodes (GPU nodes) have 4 endpoints - one for each AMD MI250x GPU modules. Each endpoint provides up to 50 GB/s of bidirectional bandwidth. The HPE Cray Slingshot NIC features high-performance RDMA and hardware acceleration for MPI and SHMEM based software.


LUMI uses the dragonfly network topology. In this topology, a group of endpoints, for example the compute nodes, are connected to a switch. Multiple switches connected in an all-to-all fashion to create a group. This group is sometimes referred to as an electrical group as these switches can be connected by short copper cables. The electrical groups are then connected to each other in an all-to-all fashion. Optical cables are used for group-to-group communication as distances are much larger. The figure below presents a graphical representation of a dragonfly topology.

Example dragonfly topology
Example dragonfly topology.

In summary, the LUMI network dragonfly can be summarized as these three ranks:

  • Rank 1: multiple compute nodes connected to a switch
  • Rank 2: multiple switches all-to-all connected with copper cables forming an electrical group
  • Rank 3: multiple electrical groups all-to-all connected with optical cables

The dragonfly topology allows for communication to be performed in a maximum of 3 switch hops: one hop inside the source group, one hop between the group and one hop inside the destination group.

Communication switch hops
Switch hops from source to destination

Inter- and intra-partition bandwidth

The table below provides an overview of the total available bandwidth for inter- and intra-partition communication.

LUMI-G 276 TB/s 38.4 TB/s 2.4 TB/s 7.2 TB/s 9.6 TB/s
LUMI-C 22.4 TB/s 0.8 TB/s 3.2 TB/s 3.2 TB/s
LUMI-M 0.1 TB/s 0.4 TB/s
LUMI-F 0.4 TB/s

The different partitions presented in the table correspond to

  • LUMI-G: the AMD MI250x GPU nodes
  • LUMI-C: the CPU nodes
  • LUMI-M: the login nodes, LUMI-D, the largemem nodes and the management racks
  • LUMI-P: the four Lustre filesystems using mechanical disk
  • LUMI-F: the Lustre filesystem using Flash-based storage


LUMI-C, the CPU partition, has 8 electrical groups of 256 nodes. The groups are composed of 16 switches connected in all-to-all. There are 16 nodes connected to each of the switches.

LUMI-C network overview
LUMI-C network overview


LUMI-G, the GPU partition, has 24 electrical groups of 124 nodes, except the last group which contains 126 nodes. The groups are composed of 32 switches connected in all-to-all. There are 16 endpoints connected to each of the switches. The LUMI-G compute nodes have 4 endpoints per nodes, each endpoint connected to different switches.

LUMI-G network overview
LUMI-G network overview