The Host List File

• The host list file is not required if you let the Resource Manager allocate which nodes your job uses (MP_HOSTFILE set to NULL or ""). The host list file is required if you want to do any of the following:

  1. Explicitly specify the actual nodes which your job should use. Node names are listed one per line. Note: actual names will depend upon your system.

     
         ! Specific node allocation for 4 tasks
         fr3n05.mhpcc.edu
         fr3n01.mhpcc.edu
         fr3n03.mhpcc.edu
         fr3n07.mhpcc.edu
         

  2. Use non-specific node allocation from more than one pool. Each line specifies a pool number preceded by the "at" symbol (@). In the example below, the first two tasks will be allocated to processors in pool 6, the third task will go to pool 9 and the fourth task will execute in pool 12. Note: the actual pool numbers will depend upon your system.

     
         ! Non-specific allocation from different pools for 4 tasks
         @6
         @6
         @9
         @12
         

  3. Override the default use of the node's CPU and / or adapter. This is not recommended in most cases. See the CPU and adapter usage discussion for details.

• By default, the host list file is called host.list and resides in your current directory. If you want to use a filename other than the default name host.list , then you will need to set the MP_HOSTFILE environment variable to that file's name. For example:

  
      C Shell:      setenv  MP_HOSTFILE  /u/jsmith/parallel/16hosts
      Korn Shell:   export  MP_HOSTFILE=/u/jsmith/parallel/16hosts
      

• The actual number of nodes used by your job depends upon the MP_PROCS variable. Some considerations:

  • Nodes are used in the order they are listed in the host list file.

  • If a listed host is unavailable, the Partition Manager returns an error and the program is not executed - even if there are more than the number of nodes needed in your host list file.

  • For jobs using IP communications, if a host is listed more than once in the host list file, then multiple tasks will be allocated to that particular host. This can not be done for US communications because the switch adapter must be dedicated.

  • For interactive jobs using IP communications, if MP_PROCS exceeds the actual number of available nodes, multiple processes may be started on a node.

• If the environment variable MP_SAVEHOSTFILE is used during a program execution , the resulting output file can be saved and used as a host list file later if desired. An example appears below. Note that it automatically includes additional information about CPU usage, adapter usage and adapter network addresses. Comment lines begin with an exclamation mark (!).

  
      ! myhosts: allocation file 4 tasks
        fr29install4.mhpcc.edu  dedicated  multiple 
      ! external_IP 164.122.53.46  MPL_IP 164.122.217.113
      ! @3
        fr29n02.mhpcc.edu  dedicated  multiple 
      ! external_IP 164.122.189.102  MPL_IP 164.122.217.114
      ! @3
        fr29n03.mhpcc.edu  dedicated  multiple 
      ! external_IP 164.122.189.103  MPL_IP 164.122.217.115
      ! @3
        fr29n04.mhpcc.edu  dedicated  multiple 
      ! external_IP 164.122.189.104  MPL_IP 164.122.217.116
      ! @3