Houdini 17.5 Nodes TOP nodes

Tractor Scheduler TOP node

Schedules work items using Pixar’s Tractor.

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This scheduler allows the use of Pixar’s Tractor to schedule work items on a farm of remote machines.

This requires Pixar’s Tractor to be installed and set up on the local machine. The installed tractor directory containing Python scripts must be specified in PYTHONPATH. For example, on Windows this directory is typically located at: C:\Program Files\Pixar\Tractor-2.2\lib\python2.7\Lib\site-packages

TOP Attributes



When the schedule submits a work item to Tractor, it will add this attribute to the work item in order to track the Tractor Job and Task IDs. The first element is the Job jid and the second element is the Task tid.



These are global parameters for all work items using this scheduler.

Submit Graph As Job

Called when the scheduler should cook the entire TOP Network as a standalone job. Displays the status URI for the submitted job.

Tractor Server Hostname

The Tractor server address.

Tractor Server Port

The Tractor server port.

Tractor User

User name for Tractor server login.

Tractor Password

Password for Tractor server login.

Job Title

The title of the top-level Job for submitted cooks.

Job Priority

The priority of cook Jobs.


List of valid site-wide tiers, where each tier represents a particular global job priority and scheduling discipline.


Names of project affiliations for this job.

Max Active Tasks

Maximum number of Tasks that the PDG Cook Job is allowed to run concurrently.

Working Directory

The relative directory where the work will be generating intermediate files and output. The intermediate files will be placed in a subdirectory. Typically $HIP is used. For running jobs on the farm, the working directory would typically be a mounted directory on all client machines, including the machine scheduling the work.

Python Executable

The full path to the python executable on the farm machines. This is used to execute the job wrapper script for PDG work items.

Shared File Root Path


The path to the shared file root for farm machines in the NFS zone.

UNC (Windows)

The path to the shared file root for farm machines in the UNC zone.

$HFS Path


The path to the houdini installation for farm machines in the NFS zone.

UNC (Windows)

The path to the houdini installation for farm machines in the UNC zone.

Message Queue

Task Callback Port

Set the TCP Port used by the Message Queue Server for the XMLRPC callback API. The port must be accessible between farm blades.

Relay Port

Set the TCP Port used by the Message Queue Server connection between PDG and the blade that is running the Message Queue Command. The port must be reachable on farm blades by the PDG/user machine.

Job Parms

These job specific parameters can be overridden by each node under the Tractor Scheduler tab on the node’s Schedulers parameter interface.

Service Key Expression

The job service key expression. Used to specify the type of blade that supports running this job.

Limit Tags

The job limit tags. This is a space-separated list of strings representing the tags to be associated with every command of the job.

At Least Slots

The minimum number of free slots that must be available on a Tractor blade in order to execute this command.

At Most Slots

If enabled, the maximum number of free slots that this command will use when launched. This will be used as the default value of Houdini Max Threads unless explicitly set.

Houdini Max Threads

Set the HOUDINI_MAXTHREADS environment to the given value. By default HOUDINI_MAXTHREADS is set to the value of At Most Slots, if enabled.

The default of 0 means to use all available processors.

Positive values will limit the number of threads that can be used. A value of 1 will disable multithreading entirely (limiting to only one thread). Positive values will be clamped to the number of CPU cores available.

If the value is negative, the value is added to the maximum number of processors to determine the threading limit. For example, a value of -1 will use all CPU cores except 1.

Inherit Local Environment

When enabled, environment variables in the current session of PDG will be copied into the Job’s environment.

Tractor Env Keys

Space separated list of environment keys which are defined in the blade profiles.


An arbitrary string that will be attached to the task definition.

Job Environment

Additional work item environment variables can be specified here.

Pre Shell

Shell script to be executed/sourced before command is executed.

Post Shell

Shell script to be executed/sourced after command is executed.

Pre Python

Python script to be exec'd in wrapper script before command process is spawned.

Post Python

Python script to be exec'd in wrapper script after command process exits.

See also

TOP nodes