Specifying Host Nodes

For example,

prun -H aa,aa,bb ./a.out

launches two processes on node aa and one on bb.

Or, consider the hostfile

% cat myhostfile aa slots=2 bb slots=2 cc slots=2

Here, we list both the host names (aa, bb, and cc) but also how many "slots" there are for each. Slots indicate how many processes can potentially execute on a node. For best performance, the number of slots may be chosen to be the number of cores on the node or the number of processor sockets. If the hostfile does not provide slots information, PSRVR will attempt to discover the number of cores (or hwthreads, if the use-hwthreads-as-cpus option is set) and set the number of slots to that value. This default behavior also occurs when specifying the -host option with a single hostname. Thus, the command

prun -H aa ./a.out

launches a number of processes equal to the number of cores on node aa.

prun -hostfile myhostfile ./a.out

will launch two processes on each of the three nodes.

prun -hostfile myhostfile -host aa ./a.out

will launch two processes, both on node aa.

prun -hostfile myhostfile -host dd ./a.out

will find no hosts to run on and abort with an error. That is, the specified host dd is not in the specified hostfile.

When running under resource managers (e.g., SLURM, Torque, etc.), PSRVR will obtain both the hostnames and the number of slots directly from the resource manger.

Specifying Number of Processes

The number of processes launched can be specified as a multiple of the number of nodes or processor sockets available. For example,

prun -H aa,bb -npersocket 2 ./a.out

launches processes 0-3 on node aa and process 4-7 on node bb, where aa and bb are both dual-socket nodes. The -npersocket option also turns on the -bind-to-socket option, which is discussed in a later section.

prun -H aa,bb -npernode 2 ./a.out

launches processes 0-1 on node aa and processes 2-3 on node bb.

prun -H aa,bb -npernode 1 ./a.out

launches one process per host node.

prun -H aa,bb -pernode ./a.out

is the same as -npernode 1.

Another alternative is to specify the number of processes with the -np option. Consider now the hostfile

% cat myhostfile aa slots=4 bb slots=4 cc slots=4

Now,

prun -hostfile myhostfile -np 6 ./a.out

will launch processes 0-3 on node aa and processes 4-5 on node bb. The remaining slots in the hostfile will not be used since the -np option indicated that only 6 processes should be launched.

Mapping Processes to Nodes: Using Policies

Consider the same hostfile as above, again with -np 6:

node aa node bb node cc

prun 0 1 2 3 4 5

prun --map-by node 0 3 1 4 2 5

prun -nolocal 0 1 2 3 4 5

The --map-by node option will load balance the processes across the available nodes, numbering each process in a round-robin fashion.

The -nolocal option prevents any processes from being mapped onto the local host (in this case node aa). While prun typically consumes few system resources, -nolocal can be helpful for launching very large jobs where prun may actually need to use noticeable amounts of memory and/or processing time.

Just as -np can specify fewer processes than there are slots, it can also oversubscribe the slots. For example, with the same hostfile:

prun -hostfile myhostfile -np 14 ./a.out

will launch processes 0-3 on node aa, 4-7 on bb, and 8-11 on cc. It will then add the remaining two processes to whichever nodes it chooses.

One can also specify limits to oversubscription. For example, with the same hostfile:

prun -hostfile myhostfile -np 14 -nooversubscribe ./a.out

will produce an error since -nooversubscribe prevents oversubscription.

Limits to oversubscription can also be specified in the hostfile itself: % cat myhostfile aa slots=4 max_slots=4 bb max_slots=4 cc slots=4

The max_slots field specifies such a limit. When it does, the slots value defaults to the limit. Now:

prun -hostfile myhostfile -np 14 ./a.out

causes the first 12 processes to be launched as before, but the remaining two processes will be forced onto node cc. The other two nodes are protected by the hostfile against oversubscription by this job.

Using the --nooversubscribe option can be helpful since PSRVR currently does not get "max_slots" values from the resource manager.

Of course, -np can also be used with the -H or -host option. For example,

prun -H aa,bb -np 8 ./a.out

launches 8 processes. Since only two hosts are specified, after the first two processes are mapped, one to aa and one to bb, the remaining processes oversubscribe the specified hosts.

And here is a MIMD example:

prun -H aa -np 1 hostname : -H bb,cc -np 2 uptime

will launch process 0 running hostname on node aa and processes 1 and 2 each running uptime on nodes bb and cc, respectively.

Mapping, Ranking, and Binding: Oh My!

mapping

Assigns a default location to each process

ranking

Assigns a rank value to each process

binding

Constrains each process to run on specific processors

The mapping step is used to assign a default location to each process based on the mapper being employed. Mapping by slot, node, and sequentially results in the assignment of the processes to the node level. In contrast, mapping by object, allows the mapper to assign the process to an actual object on each node.

Note: the location assigned to the process is independent of where it will be bound - the assignment is used solely as input to the binding algorithm.

The mapping of process processes to nodes can be defined not just with general policies but also, if necessary, using arbitrary mappings that cannot be described by a simple policy. One can use the "sequential mapper," which reads the hostfile line by line, assigning processes to nodes in whatever order the hostfile specifies. Use the -pmca rmaps seq option. For example, using the same hostfile as before:

prun -hostfile myhostfile -pmca rmaps seq ./a.out

will launch three processes, one on each of nodes aa, bb, and cc, respectively. The slot counts don't matter; one process is launched per line on whatever node is listed on the line.

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

The second phase focuses on the ranking of the process within the job. PSRVR separates this from the mapping procedure to allow more flexibility in the relative placement of processes. This is best illustrated by considering the following two cases where we used the —map-by ppr:2:socket option:

node aa node bb

rank-by core 0 1 ! 2 3 4 5 ! 6 7

rank-by socket 0 2 ! 1 3 4 6 ! 5 7

rank-by socket:span 0 4 ! 1 5 2 6 ! 3 7

Ranking by core and by slot provide the identical result - a simple progression of ranks across each node. Ranking by socket does a round-robin ranking within each node until all processes have been assigned a rank, and then progresses to the next node. Adding the span modifier to the ranking directive causes the ranking algorithm to treat the entire allocation as a single entity - thus, the MCW ranks are assigned across all sockets before circling back around to the beginning.

The binding phase actually binds each process to a given set of processors. This can improve performance if the operating system is placing processes suboptimally. For example, it might oversubscribe some multi-core processor sockets, leaving other sockets idle; this can lead processes to contend unnecessarily for common resources. Or, it might spread processes out too widely; this can be suboptimal if application performance is sensitive to interprocess communication costs. Binding can also keep the operating system from migrating processes excessively, regardless of how optimally those processes were placed to begin with.

The processors to be used for binding can be identified in terms of topological groupings - e.g., binding to an l3cache will bind each process to all processors within the scope of a single L3 cache within their assigned location. Thus, if a process is assigned by the mapper to a certain socket, then a —bind-to l3cache directive will cause the process to be bound to the processors that share a single L3 cache within that socket.

To help balance loads, the binding directive uses a round-robin method when binding to levels lower than used in the mapper. For example, consider the case where a job is mapped to the socket level, and then bound to core. Each socket will have multiple cores, so if multiple processes are mapped to a given socket, the binding algorithm will assign each process located to a socket to a unique core in a round-robin manner.

Alternatively, processes mapped by l2cache and then bound to socket will simply be bound to all the processors in the socket where they are located. In this manner, users can exert detailed control over relative MCW rank location and binding.

Finally, --report-bindings can be used to report bindings.

As an example, consider a node with two processor sockets, each comprising four cores. We run prun with -np 4 --report-bindings and the following additional options:

% prun ... --map-by core --bind-to core [...] ... binding child [...,0] to cpus 0001 [...] ... binding child [...,1] to cpus 0002 [...] ... binding child [...,2] to cpus 0004 [...] ... binding child [...,3] to cpus 0008

% prun ... --map-by socket --bind-to socket [...] ... binding child [...,0] to socket 0 cpus 000f [...] ... binding child [...,1] to socket 1 cpus 00f0 [...] ... binding child [...,2] to socket 0 cpus 000f [...] ... binding child [...,3] to socket 1 cpus 00f0

% prun ... --map-by core:PE=2 --bind-to core [...] ... binding child [...,0] to cpus 0003 [...] ... binding child [...,1] to cpus 000c [...] ... binding child [...,2] to cpus 0030 [...] ... binding child [...,3] to cpus 00c0

% prun ... --bind-to none

Here, --report-bindings shows the binding of each process as a mask. In the first case, the processes bind to successive cores as indicated by the masks 0001, 0002, 0004, and 0008. In the second case, processes bind to all cores on successive sockets as indicated by the masks 000f and 00f0. The processes cycle through the processor sockets in a round-robin fashion as many times as are needed. In the third case, the masks show us that 2 cores have been bound per process. In the fourth case, binding is turned off and no bindings are reported.

PSRVR's support for process binding depends on the underlying operating system. Therefore, certain process binding options may not be available on every system.

Process binding can also be set with MCA parameters. Their usage is less convenient than that of prun options. On the other hand, MCA parameters can be set not only on the prun command line, but alternatively in a system or user mca-params.conf file or as environment variables, as described in the MCA section below. Some examples include:

prun option MCA parameter key value

--map-by core rmaps_base_mapping_policy core --map-by socket rmaps_base_mapping_policy socket --rank-by core rmaps_base_ranking_policy core --bind-to core hwloc_base_binding_policy core --bind-to socket hwloc_base_binding_policy socket --bind-to none hwloc_base_binding_policy none

Rankfiles

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

For example:

$ cat myrankfile rank 0=aa slot=1:0-2 rank 1=bb slot=0:0,1 rank 2=cc slot=1-2 $ prun -H aa,bb,cc,dd -rf myrankfile ./a.out

Means that

Rank 0 runs on node aa, bound to logical socket 1, cores 0-2. Rank 1 runs on node bb, bound to logical socket 0, cores 0 and 1. Rank 2 runs on node cc, bound to logical cores 1 and 2.

Rankfiles can alternatively be used to specify physical processor locations. In this case, the syntax is somewhat different. Sockets are no longer recognized, and the slot number given must be the number of the physical PU as most OS's do not assign a unique physical identifier to each core in the node. Thus, a proper physical rankfile looks something like the following:

$ cat myphysicalrankfile rank 0=aa slot=1 rank 1=bb slot=8 rank 2=cc slot=6

This means that

Rank 0 will run on node aa, bound to the core that contains physical PU 1 Rank 1 will run on node bb, bound to the core that contains physical PU 8 Rank 2 will run on node cc, bound to the core that contains physical PU 6

Rankfiles are treated as logical by default, and the MCA parameter rmaps_rank_file_physical must be set to 1 to indicate that the rankfile is to be considered as physical.

The hostnames listed above are "absolute," meaning that actual resolveable 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=1:0-2 rank 1=+n1 slot=0:0,1 rank 2=+n2 slot=1-2 $ prun -H aa,bb,cc,dd -rf myrankfile ./a.out

All socket/core slot locations are be specified as logical indexes. You can use tools such as HWLOC's "lstopo" to find the logical indexes of socket and cores.

Application Context or Executable Program?

Locating Files

If a relative directory is specified, it must be relative to the initial working directory determined by the specific starter used. For example when using the rsh or ssh starters, the initial directory is $HOME by default. Other starters may set the initial directory to the current working directory from the invocation of prun.

Current Working Directory

If the -wdir option appears both in a context file and on the command line, the context file directory will override the command line value.

If the -wdir option is specified, prun will attempt to change to the specified directory on all of the remote nodes. If this fails, prun will abort.

If the -wdir option is not specified, prun will send the directory name where prun was invoked to each of the remote nodes. The remote nodes will try to change to that directory. If they are unable (e.g., if the directory does not exist on that node), then prun will use the default directory determined by the starter.

All directory changing occurs before the user's program is invoked.

Standard I/O

PSRVR directs UNIX standard output and error from remote nodes to the node that invoked prun and prints it on the standard output/error of prun. Local processes inherit the standard output/error of prun and transfer to it directly.

Thus it is possible to redirect standard I/O for applications by using the typical shell redirection procedure on prun.

% prun -np 2 my_app < my_input > my_output

Note that in this example only the rank 0 process will receive the stream from my_input on stdin. The stdin on all the other nodes will be tied to /dev/null. However, the stdout from all nodes will be collected into the my_output file.

Signal Propagation

SIGUSR1 and SIGUSR2 signals received by prun are propagated to all processes in the job.

A SIGTSTOP signal to prun will cause a SIGSTOP signal to be sent to all of the programs started by prun and likewise a SIGCONT signal to prun will cause a SIGCONT sent.

Other signals are not currently propagated by prun.

Process Termination / Signal Handling

Process Environment

See the "Remote Execution" section for more details.

Remote Execution

However, it is not always desirable or possible to edit shell startup files to set PATH and/or LD_LIBRARY_PATH. The --prefix option is provided for some simple configurations where this is not possible.

The --prefix option takes a single argument: the base directory on the remote node where PSRVR is installed. PSRVR will use this directory to set the remote PATH and LD_LIBRARY_PATH before executing any user applications. This allows running jobs without having pre-configured the PATH and LD_LIBRARY_PATH on the remote nodes.

PSRVR adds the basename of the current node's "bindir" (the directory where PSRVR's executables are installed) to the prefix and uses that to set the PATH on the remote node. Similarly, PSRVR adds the basename of the current node's "libdir" (the directory where PSRVR's libraries are installed) to the prefix and uses that to set the LD_LIBRARY_PATH on the remote node. For example:

Local bindir:

/local/node/directory/bin

Local libdir:

/local/node/directory/lib64

If the following command line is used:

% prun --prefix /remote/node/directory

PSRVR will add "/remote/node/directory/bin" to the PATH and "/remote/node/directory/lib64" to the D_LIBRARY_PATH on the remote node before attempting to execute anything.

The --prefix option is not sufficient if the installation paths on the remote node are different than the local node (e.g., if "/lib" is used on the local node, but "/lib64" is used on the remote node), or if the installation paths are something other than a subdirectory under a common prefix.

Note that executing prun via an absolute pathname is equivalent to specifying --prefix without the last subdirectory in the absolute pathname to prun. For example:

% /usr/local/bin/prun ...

is equivalent to

% prun --prefix /usr/local

Exported Environment Variables

Setting MCA Parameters

The -pmca switch takes two arguments: <key> and <value>. The <key> argument generally specifies which MCA module will receive the value. For example, the <key> "btl" is used to select which BTL to be used for transporting messages. The <value> argument is the value that is passed. For example:

prun -pmca btl tcp,self -np 1 foo

Tells PSRVR to use the "tcp" and "self" BTLs, and to run a single copy of "foo" on an allocated node.

prun -pmca btl self -np 1 foo

Tells PSRVR to use the "self" BTL, and to run a single copy of "foo" on an allocated node.

The -pmca switch can be used multiple times to specify different <key> and/or <value> arguments. If the same <key> is specified more than once, the <value>s are concatenated with a comma (",") separating them.

Note that the -pmca switch is simply a shortcut for setting environment variables. The same effect may be accomplished by setting corresponding environment variables before running prun. The form of the environment variables that PSRVR sets is:

PMIX_MCA_<key>=<value>

Thus, the -pmca switch overrides any previously set environment variables. The -pmca settings similarly override MCA parameters set in the $OPAL_PREFIX/etc/psrvr-mca-params.conf or $HOME/.psrvr/mca-params.conf file.

Unknown <key> arguments are still set as environment variable -- they are not checked (by prun) for correctness. Illegal or incorrect <value> arguments may or may not be reported -- it depends on the specific MCA module.

To find the available component types under the MCA architecture, or to find the available parameters for a specific component, use the pinfo command. See the pinfo(1) man page for detailed information on the command.

Setting MCA parameters and environment variables from file.

A valid line in the file may contain zero or many "-x", "-pmca", or “--pmca” arguments. The following patterns are supported: -pmca var val -pmca var "val" -x var=val -x var. If any argument is duplicated in the file, the last value read will be used.

MCA parameters and environment specified on the command line have higher precedence than variables specified in the file.

Running as root

Reflecting this advice, prun will refuse to run as root by default. To override this default, you can add the --allow-run-as-root option to the prun command line.

Exit status

•

if all processes in the primary job normally terminate with exit status 0, we return 0

•

if one or more processes in the primary job normally terminate with non-zero exit status, we return the exit status of the process with the lowest rank to have a non-zero status

•

if all processes in the primary job normally terminate with exit status 0, and one or more processes in a secondary job normally terminate with non-zero exit status, we (a) return the exit status of the process with the lowest rank in the lowest jobid to have a non-zero status, and (b) output a message summarizing the exit status of the primary and all secondary jobs.

•

if the cmd line option --report-child-jobs-separately is set, we will return -only- the exit status of the primary job. Any non-zero exit status in secondary jobs will be reported solely in a summary print statement.

By default, PSRVR records and notes that processes exited with non-zero termination status. This is generally not considered an "abnormal termination" - i.e., PSRVR will not abort a job if one or more processes return a non-zero status. Instead, the default behavior simply reports the number of processes terminating with non-zero status upon completion of the job.

However, in some cases it can be desirable to have the job abort when any process terminates with non-zero status. For example, a non-PMIx job might detect a bad result from a calculation and want to abort, but doesn't want to generate a core file. Or a PMIx job might continue past a call to PMIx_Finalize, but indicate that all processes should abort due to some post-PMIx result.

It is not anticipated that this situation will occur frequently. However, in the interest of serving the broader community, PSRVR now has a means for allowing users to direct that jobs be aborted upon any process exiting with non-zero status. Setting the MCA parameter "orte_abort_on_non_zero_status" to 1 will cause PSRVR to abort all processes once any process exits with non-zero status.

Terminations caused in this manner will be reported on the console as an "abnormal termination", with the first process to so exit identified along with its exit status.