Houdini 18.5 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.

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 enabled, PDG will check the output files of work items when the graph recooks, to see if the files still exist on disk. Work items that are missing output files will be dirtied and cook again.

Check Expected Outputs on Disk

When enabled, PDG will look on disk for any expected work items outputs that were not explicitly reported when the work item cooked. Expected outputs for a work item are checkd immediately after the scheduler marks the work item as cooked. Output files that were reported by the work item normally while cooking will not be checked.

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.

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 items encounters an error while cooking, it’s marked as failed by default. These parameters 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 node.

Report Warning

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

Retry Task

The work item restarts immediately, according to the 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.

Retry Count Attribute

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

See also

TOP nodes