Using the Hydra Process Manager

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This wiki page only provides information on the external usage of Hydra. If you are looking for the internal workings of Hydra, you can find it here.


Hydra is a process management system for starting parallel jobs. Hydra is designed to natively work with multiple daemons such as ssh, rsh, pbs, slurm and sge. However, in the current release, only ssh, rsh and fork are supported, with a preliminary version of slurm available.

Quick Start

Starting MPICH2-1.1, hydra is compiled into MPICH2 releases by default as a alternate process manager. You can use it as mpiexec.hydra.

Once built, the Hydra executables are in mpich2/bin, or the bin subdirectory of the install directory if you have done an install. You should put this (bin) directory in your PATH in your .cshrc or .bashrc for usage convenience:

Put in .cshrc:  setenv PATH /home/you/mpich2/bin:$PATH

Put in .bashrc: export PATH=/home/you/mpich2/bin:$PATH

To compile your application use mpicc:

shell$ mpicc app.c -o app

Create a file with the names of the machines that you want to run your job on. This file may or may not include the local machine.

shell$ cat hosts

To run your application on these nodes, use mpiexec:

shell$ mpiexec -f hosts -n 4 ./app

The host file can also be specified as follows:

shell$ cat hosts

In this case, the first 2 processes are scheduled on "donner", the next 3 on "foo" and the last 2 on "shakey". Comments in the host file start with a "#" character.

shell$ cat hosts
   # This is a sample host file
   donner:2     # The first 2 procs are scheduled to run here
   foo:3        # The next 3 procs run on this host
   shakey:2     # The last 2 procs run on this host

Environment Settings

HYDRA_HOST_FILE: This variable points to the default host file to use, when the "-f" option is not provided to mpiexec.

  For bash:
    export HYDRA_HOST_FILE=<path_to_host_file>/hosts

  For csh/tcsh:
    setenv HYDRA_HOST_FILE <path_to_host_file>/hosts

HYDRA_DEBUG: Setting this to "1" enables debug mode; set it to "0" to disable.

HYDRA_ENV: Setting this to "all" will pass all the environment to the application processes.

HYDRA_PROXY_PORT: The port to use for the proxies.

Bootstrap Servers

A bootstrap server is the basic remote node access mechanism that is provided on any system. Hydra supports multiple bootstrap servers including ssh, rsh, fork, and slurm to launch processes. All of these are compiled in by default, so you can pick any one of them at runtime using the mpiexec option -bootstrap:

shell$ mpiexec -bootstrap ssh -f hosts -n 4 ./app


shell$ mpiexec -bootstrap fork -f hosts -n 4 ./app

This can also be controlled by using the HYDRA_BOOTSTRAP environment variable.

The default bootstrap server is ssh.

The executable to use as the bootstrap server can be specified using the option -bootstrap-exec:

 $ mpiexec -bootstrap ssh -bootstrap-exec /usr/bin/ssh -f hosts -n 4 ./app

This can also be specified using the HYDRA_BOOTSTRAP_EXEC environment variable. If the bootstrap executable is not specified, Hydra will automatically look for it in your path and other known locations.

Process-core Binding

On supported platforms, Hydra automatically configures available process-core binding capability (currently using PLPA). We support three models of allocation strategies:

  1. Basic allocation strategies: This just allocates processes using the OS specified processor IDs. Currently, only round-robin scheme is provided here.
  2. Topology-aware allocation strategies: These are a bit more intelligent in that they try to understand the system topology and assign processes in that order. Currently, "buddy" and "pack" schemes are provided. The "buddy" scheme loops between all the available sockets, allocating one process per socket; this tries to minimize the inter-process resource sharing (assuming the closer the processes are the more resources that they share). The "pack" scheme packs everything as closely as it can; this tries to maximize resource sharing hoping that the communication library can take advantage of this packing for better performance.
  3. User-defined allocation strategies: Two schemes are provided---command-line and host-file based. The command-line scheme lets the user specify a common-mapping for all physical nodes on the command line. The host-file scheme is the most general and lets the user specify the mapping for each node separately.

The modes of process-core binding are: round-robin ("rr"), buddy-allocation ("buddy"), closest packing ("pack") and user-defined ("user"). These can be selected as follows:

shell$ mpiexec -binding rr -f hosts -n 8 ./app


shell$ mpiexec -binding pack -f hosts -n 8 ./app

Consider the following layout of processing elements in the system (e.g., two nodes, each with two processors, and each processor with two cores). Suppose the Operating System assigned processor IDs for each of these processing elements are as shown below:

__________________________________________      __________________________________________
|  _________________    _________________  |    |  _________________    _________________  | 
| |  _____   _____  |  |  _____   _____  | |    | |  _____   _____  |  |  _____   _____  | |
| | |     | |     | |  | |     | |     | | |    | | |     | |     | |  | |     | |     | | |
| | |     | |     | |  | |     | |     | | |    | | |     | |     | |  | |     | |     | | | 
| | |  0  | |  2  | |  | |  1  | |  3  | | |    | | |  0  | |  2  | |  | |  1  | |  3  | | |
| | |     | |     | |  | |     | |     | | |    | | |     | |     | |  | |     | |     | | |
| | |_____| |_____| |  | |_____| |_____| | |    | | |_____| |_____| |  | |_____| |_____| | |
| |_________________|  |_________________| |    | |_________________|  |_________________| |
|__________________________________________|    |__________________________________________|

In this case, the binding options are as follows:

  1. RR: 0, 1, 2, 3 (use the order provided by the OS)
  2. Buddy: 0, 1, 2, 3 (increasing sharing of resources)
  3. Pack: 0, 2, 1, 3 (closest packing)
  4. User: as defined by the user

Within the user-defined binding, two modes are supported: command-line and host-file based. The command-line based mode can be used as follows:

shell$ mpiexec -binding user:0,3 -f hosts -n 4 ./app

If a machine has 4 processing elements, and only two bindings are provided (as in the above example), the rest are padded with (-1), which refers to no binding. Also, the mapping is the same for all machines; so if the application is run with 8 processes, the first 2 processes on "each machine" are bound to processing elements as specified.

The host-file based mode for user-defined binding can be used by the "map=" argument on each host line. E.g.:

shell$ cat hosts
   donner:4    map=0,-1,-1,3
   foo:4       map=3,2

Using this method, each host can be given a different mapping. Any unspecified mappings are treated as (-1), referring to no binding.

Command-line based mappings are given a higher priority than the host-file based mappings. So, if a mapping is given at both places, the host-file mappings are ignored.

Binding options can also be controlled with the environment variable HYDRA_BINDING.

X Forwarding

X-forwarding is specific to each bootstrap server. Some servers do it by default, while some don't. For ssh, this is disabled by default. To force-enable it, you should use the option -enable-x to mpiexec.

shell$ mpiexec -enable-x -f hosts -n 4 ./app

Persistent Proxies

Hydra also supports proxies to be launched in persistent mode on the system (e.g., by a system administrator). To launch in persistent mode, use:

shell$ mpiexec -boot-proxies -f hosts

shell$ mpiexec -use-persistent -f hosts -n 4 ./app1

shell$ mpiexec -use-persistent -f hosts -n 4 ./app2

shell$ mpiexec -use-persistent -f hosts -n 4 ./app3

shell$ mpiexec -shutdown-proxies -f hosts

Persistent mode can also be picked using the environment setting HYDRA_LAUNCH_MODE=persistent.

The option "-boot-foreground-proxies" can be used to prevent persistent proxies from spawning a child process and exiting. This option is useful for debugging. This option can also be picked using the environment setting HYDRA_BOOT_FOREGROUND_PROXIES=1.

shell$ mpiexec -boot-foreground-proxies -f hosts

shell$ mpiexec -use-persistent -f hosts -n 4 ./app1

shell$ mpiexec -shutdown-proxies -f hosts

Communication sub-systems

Hydra supports different communication sub-systems to connect proxies in the persistent mode. The default is "none", which means that the proxies are not connected. You can pick these through the mpiexec option -css:

shell$ mpiexec -css ib -f hosts -n 4 ./app


shell$ mpiexec -css mx -f hosts -n 4 ./app

This can also be controlled by using the HYDRA_CSS environment variable.

Resource Manager Integration

Hydra provides capability to integrate with different resource managers. The default is "dummy", which means no resource manager. You can pick these through the mpiexec option -rmk:

shell$ mpiexec -rmk lsf -f hosts -n 4 ./app

This can also be controlled by using the HYDRA_RMK environment variable.

Hydra in hybrid environments

Hydra can be used to launch other process managers as well, such as a UPC launcher, for example:

shell$ mpiexec -n 2 -ranks-per-proc=4 upcrun -n 4 ./app

This launches two instances of upcrun, each of which is expected to launch 4 application processes (two subgroups of processes). Hydra needs the -ranks-per-proc argument to tell it how many MPI ranks it needs to allocate to each group of processes.

If the internal nested environment also needs to use Hydra as a launcher, but not as a process manager, this can be set using:

shell$ mpiexec -n 2 -ranks-per-proc=4 mpiexec -n 4 -disable-pm-env ./app


shell$ mpiexec -n 2 -ranks-per-proc=4 HYDRA_PM_ENV=0 mpiexec -n 4 ./app