[starpu-devel] Follow-Up on Data Handle Scope and Task Scheduling

[Xinbo Li <lix34545@myumanitoba.ca>]

Hi Samuel,

Thank you for your reply.

Just to confirm my understanding — when a data handle is about to go out of scope, it needs to be explicitly unregistered using starpu_data_unregister(), since that deallocates the memory internally tracked by StarPU.

In that case, if we define a data handle inside a local block and unregister it immediately after task submission, any subsequent tasks — even if unrelated — would be forced to wait for the earlier tasks to complete, due to the synchronization imposed by the unregister operation. This could potentially reduce efficiency.

For example,

DataType data; {    preparation(&data);    starpu_data_handle_t handle;    starpu_custom_data_register(&handle, STARPU_MAIN_RAM, &data);    // Task 1    starpu_task_insert(        &some_codelet,        STARPU_RW, handle,        0);    starpu_data_unregister(handle); // unregister at end of block } // Task 2 starpu_task_insert(    &some_other_codelet,    STARPU_RW, some_other_handle,    0);

In this case, would Task 2 be forced to wait for Task 1 to finish, even if they operate on completely independent data?

Looking forward to your clarification, and thanks again for your support!

Best regards,
Xinbo

---

From: Samuel Thibault
Sent: Thursday, April 17, 2025 2:34 PM
To: Xinbo Li
Cc: starpu-devel@inria.fr
Subject: Re: [starpu-devel] StarPU Question: Using Local Variable as Data Handles?

Caution! This message was sent from outside the University of Manitoba.


Hello,

Xinbo Li, le jeu. 17 avril 2025 21:20:56 +0000, a ecrit:
> Is it safe to define a starpu_data_handle_t as a local variable for the purpose
> of task submission?
[...]
> My concern is whether StarPU may run into undefined behavior or errors
> when the scheduled task is executed, since it may no longer be able to
> access the handle:

starpu_data_handle_t is just a pointer, so losing it is "safe", sure,
but you need to deallocate it first :)

> For example, in the code snippet below, the handle is
> declared within a local scope. Once the block ends, the variable handle is no
> longer accessible.

So you need to unregister it. Either waiting for tasks with
starpu_data_unregister or without waiting for tasks with
starpu_data_unregister_submit.

Samuel

> DataType data;
>
> {
>     preparation(&data);
>
>     starpu_data_handle_t handle;
>     starpu_custom_data_register(&handle, STARPU_MAIN_RAM, &data);
>
>     starpu_task_insert(
>         &some_codelet,
>         STARPU_RW, handle,
>         0);
> }