@Marvin I have a very long process where i have hundreds of tasks to run and have set up a concurrency limit. It locks up regularly not using up the concurrency limit and also not running the tasks. How can I tune it to work more consistently.

Your issue with the concurrency limit not being fully utilized and tasks not running consistently can be influenced by a few factors. Here are some insights and potential solutions: 1. Slot Decay: Prefect uses a concept called "slot decay" which affects how quickly concurrency slots become available again after being used. You can adjust the

slot_decay_per_second

parameter to control how fast slots refresh. A higher value means slots refresh quickly, allowing tasks to run more frequently. You can read more about this and adjust it as needed in the Prefect documentation on global concurrency limits. 2. Kubernetes and Task Cancellation: There is a known issue where tasks running in a Kubernetes environment do not release their concurrency slots if they are canceled. This could lead to slots being occupied indefinitely, causing the behavior you're observing. You can track this issue and any progress on it here on GitHub. 3. Concurrency and Task Submission: There might be a delay between the release and acquisition of concurrency slots, especially when using

.submit

for multiple tasks. This could lead to a batch of tasks running, completing, and then a pause before the next batch starts. This behavior has been noted in this GitHub issue. To address these issues, consider the following steps: - Adjust the

slot_decay_per_second

to see if a higher value improves the task throughput. - If running on Kubernetes, be cautious of task cancellation behavior and monitor the related GitHub issue for updates. - Review your task submission logic to ensure that tasks are being queued and executed as expected. If you have further questions or need more specific guidance, feel free to ask.

@Marvin can i force a stalled task to restart in some way

To address a stalled task in Prefect and potentially restart it, consider the following options: 1. Retries: If your task is configured with retries, Prefect can automatically retry the task if it fails. You can set the number of retries and the delay between retries using the

retries

and

retry_delay_seconds

parameters in your task definition. This won't help if the task is stuck indefinitely, but it's useful for transient issues. Here's an example of how retries work in Prefect's task engine:

python
   if self.retries < self.task.retries and self.can_retry(exc):
       # Logic for retrying the task

Source Code Reference 2. Task Worker: For tasks that are deferred or run in the background, ensure that your task worker is running and properly configured to handle and restart tasks. You can start a task worker using the

prefect.task_worker.serve()

method. This setup is similar to using a Celery worker. Deferred Tasks Documentation 3. Handling Crashes: Prefect has mechanisms to detect and handle crashes. If a task crashes, it can be set to a "crashed" state, and you can configure your flow to handle these states appropriately.

python
   def handle_crash(self, exc: BaseException) -> None:
       state = run_coro_as_sync(exception_to_crashed_state(exc))
       # Logic to handle crashes

Source Code Reference If these options do not address your issue, you may need to manually intervene by canceling the stalled task and triggering it again. Unfortunately, there isn't a built-in feature to forcefully restart a task without some form of manual intervention or configuration for retries.