Houdini 20.0 Nodes TOP nodes

In Process Scheduler TOP node

Controls the scheduling of in-process work items.

Since 18.0

By default, PDG automatically runs in-process work items as part of the graph cook, and it also runs as many in-process items as it can based on the value of HOUDINI_MAX_THREADS.

You can use this scheduler node to manually control the number of in-process work items that can run at the same time. For example, you can use this node to limit the number of work items that can cook simultaneously in an Invoke node.

This node is most commonly used to reduce the number of concurrent in-process work items to a value that is less than the number of threads in Houdini. The settings in this scheduler are also limited by the value of HOUDINI_MAX_THREADS. For example, if your Houdini is configured to use 16 threads, then this scheduler would only able to cook a maximum of 16 work items.

If you want to use an instance of this scheduler to control in-process work items, then you must specify this scheduler as the TOP Scheduler Override on the node to which the work items belong. Otherwise, that node would continue to use the default in-process scheduling.

Examples of TOP nodes that create in-process work items include: the Invoke node, the File Copy node, and the Python Script node when Evaluate In Process is turned on in its parameter settings.


You cannot use the In Process Scheduler node to run out-of-process work items.



The global parameters for all work items using this scheduler.

Max Work Items

Determines the maximum number of work items that can be scheduled at the same time by this scheduler.

By default, the number of work items that can execute concurrently is limited by the number of available threads in your Houdini session as set by HOUDINI_MAX_THREADS.

Equal to CPU Count

When selected, all in-process work items are scheduled automatically as part of the PDG graph evaluation.

Custom Count

Specifies a custom limit for the number of concurrent in-process work items.


Working Directory

Specifies the relative directory where the work generates intermediate files and output. The intermediate files are placed in a subdirectory. For the Local Scheduler or HQueue, typically $HIP is used. For other schedulers, this should be a relative directory to Local Shared Root Path and Remote Shared Root Path; this path is then appended to these root paths.

Validate Outputs When Recooking

When on, PDG validates the output files of the scheduler’s cooked work items when the graph is recooked to see if the files still exist on disk. Work items that are missing output files are then automatically dirtied and cooked again. If any work items are dirtied by parameter changes, then their cache files are also automatically invalidated. Validate Outputs When Recooking is on by default.

Check Expected Outputs on Disk

When on, PDG looks for any unexpected outputs (for example, like outputs that can result from custom output handling internal logic) that were not explicitly reported when the scheduler’s work items finished cooking. This check occurs immediately after the scheduler marks work items as cooked, and expected outputs that were reported normally are not checked. If PDG finds any files that are different from the expected outputs, then they are automatically added as real output files.

Temp Directory


Determines where the local temporary files are written to. Files that are written to this location are needed for the PDG cook, but are not typically the end product and can be removed when the cook completes.

For example, log files and python scripts are some of the files that are usually written during the cook.

Working Directory

Use pdgtemp subdirectory specified in the Working Directory field.

Houdini Temp

Use pdgtemp subdirectory of $HOUDINI_TEMP_DIR.


Use the custom directory specified by in the Custom field.

Append PID

When on, a subdirectory is added to the location specified by the Location parameter and is named after the value of your Houdini session’s PID (Process Identifier). The PID is typically a 3-5 digit number.

This is necessary when multiple sessions of Houdini are cooking TOP graphs at the same time.


The full path to the custom temporary directory.

This parameter is only available when Directory Location is set to Custom.

Delete Temp Dir

Determines when PDG should automatically delete the temporary file directory associated with the scheduler.


PDG never automatically deletes the temp file directory.

When Scheduler is Deleted

PDG automatically deletes the temp file directory when the scheduler is deleted or when Houdini is closed.

When Cook Completes

PDG automatically deletes the temp file directory each time a cook completes.

Job Parms

The job-specific parameters.


You can override these parameters per node with the Edit Parameter Interface. For more information, see Scheduler Job Parms / Properties.



When on, the scheduler is only be able to run one work item at a time. Work items in other nodes where Single is turned off are still able to run concurrently.

Run on Main Thread

When on, the scheduler will run the work item on the main thread instead of in the background. While the work item is running the UI will be unresponsive, however this allows you to safely run work items that modify the .hip file, node graph or UI.


When an in-process work item encounters an error while cooking, it is marked as failed by default. These parameters allow you to change that behavior.

On Task Failure

Determines what happens when a work item fails.

Report Error

The work item fails and an error message is added to the scheduler.

Report Warning

The work item succeeds and a warning message is added to the scheduler.

Retry Task

The work item restarts immediately according to the current Maximum Retries and Retry Count Attribute parameter settings.


The work item succeeds and no message is issued.

Maximum Retries

Sets the maximum number of times the work item will be restarted.

This parameter is only available when On Task Failure is set to Retry Task.

Retry Count Attribute

When on, adds an int attribute set to the number of times the task was restarted.

See also

TOP nodes