Re: [Starpu-devel] data_unregister_submit() does not flush data from GPU to CPU

[Samuel Thibault <samuel.thibault@inria.fr>]

Hello,

Aleksandr Mikhalev, le mar. 24 juil. 2018 16:18:36 +0300, a ecrit:
> If I understood correctly, idea of “submit” functions is to hint StarPU to 
> flush data from GPU to CPU,

No, "submit" means that it is asynchronous, i.e. it retuns immediately
without waiting for the result.

> StarPU does not copy output data from GPU to CPU if corresponding handle is 
> destroyed by starpu_data_unregister_submit().

Well, yes, and that is what is documented.

I don't actually really see how it would be useful to copy output
back from GPU to CPU: since it is asynchronous, you have no way of
knowing when that will be effective. If you want to bring back data
to the CPU, either use starpu_data_acquire(STARPU_R) (synchronous) or
starpu_data_acquire_cb(STARPU_R) (asynchronous but with a callback
called when the data up to date on the CPU) and starpu_data_release()

> The same holds true for starpu_invalidate_data_submit().

Well, yes, invalidate_data_submit() is explicitly exactly meant to
discard the value of a data.

Samuel