# 7.2. Hostfiles

Hostfiles (sometimes called “machine files”) are a combination of two things:

1. A listing of hosts on which to launch processes.

2. Optionally, limit the number of processes which can be launched on each host.

Hostfile syntax consists of one node name on each line, optionally including a designated number of “slots”:

    # This is a comment line, and will be ignored
    node01  slots=10
    node13  slots=5

    node15
    node16
    node17  slots=3
    ...

Blank lines and lines beginning with a `#` are ignored.

A “slot” is the PRRTE term for an allocatable unit where we can launch a process. See the section on definition of the term `slot` for a longer description of slots.

In the absence of the `slot` parameter, PRRTE will assign either the number of slots to be the number of CPUs detected on the node or the resource manager-assigned value if operating in the presence of an RM.

Important

If using a resource manager, the user-specified number of slots is capped by the RM-assigned value.