# 8.8. Rankfiles

Another way to specify arbitrary mappings is with a rankfile, which gives you detailed control over process binding as well.

Rankfiles are text files that specify detailed information about how individual processes should be mapped to nodes, and to which processor(s) they should be bound. Each line of a rankfile specifies the location of one process. The general form of each line in the rankfile is:

    rank <N>=<hostname> slot=<slot list>

For example:

    $ cat myrankfile
    rank 0=aa slot=10-12
    rank 1=bb slot=0,1,4
    rank 2=cc slot=1-2
    $ prun --host aa,bb,cc,dd --map-by rankfile:FILE=myrankfile ./a.out

Means that:

- Rank 0 runs on node aa, bound to logical cores 10-12.

- Rank 1 runs on node bb, bound to logical cores 0, 1, and 4.

- Rank 2 runs on node cc, bound to logical cores 1 and 2.

Similarly:

    $ cat myrankfile
    rank 0=aa slot=1:0-2
    rank 1=bb slot=0:0,1,4
    rank 2=cc slot=1-2
    $ prun --host aa,bb,cc,dd --map-by rankfile:FILE=myrankfile ./a.out

Means that:

- Rank 0 runs on node aa, bound to logical package 1, cores 10-12 (the 0th through 2nd cores on that package).

- Rank 1 runs on node bb, bound to logical package 0, cores 0, 1, and 4.

- Rank 2 runs on node cc, bound to logical cores 1 and 2.

The hostnames listed above are “absolute,” meaning that actual resolvable hostnames are specified. However, hostnames can also be specified as “relative,” meaning that they are specified in relation to an externally-specified list of hostnames (e.g., by `prun`’s `--host` argument, a hostfile, or a job scheduler).

The “relative” specification is of the form “`+n<X>`”, where `X` is an integer specifying the Xth hostname in the set of all available hostnames, indexed from 0. For example:

    $ cat myrankfile
    rank 0=+n0 slot=10-12
    rank 1=+n1 slot=0,1,4
    rank 2=+n2 slot=1-2
    $ prun --host aa,bb,cc,dd --map-by rankfile:FILE=myrankfile ./a.out

All package/core slot locations are be specified as logical indexes. You can use tools such as HWLOC’s `lstopo` to find the logical indexes of packages and cores.