Re: Question about shared-memory-handler

From: Volker Springel <vspringel_at_MPA-Garching.MPG.DE>
Date: Sun, 15 Nov 2020 13:18:47 +0100

Hi Leonardo,

If you want to asynchronously modify particle data on another MPI rank, you need to distinguish between two situations:

1) If the other MPI rank is on the same shared memory node, you can access the corresponding data directly (via shared memory) and modify it.

2) If the other MPI rank is on a different node, you need to send a request to the dedicated listener on the other node, which can then access and modify the particle data on the target rank via shared memory access. To make this feasible, you need to implement a corresponding handler routine in the function shmem::shared_memory_handler() in file src/mpi_utils/shared_mem_handler.cc (this is the code which the designated listener executes all the time). In the normal tree routines, it is mostly the function tree_fetch_foreign_nodes() that reads remote data via these handler routines.

Further comments:

-- For both to work, you will need to make sure that the current base-address of all the particle data is known to all MPI ranks on a local shared memory node. This is normally done by calling the function prepare_shared_memory_access() before an access period, and then to release this again with cleanup_shared_memory_access() when the access period is over.

-- It would not be good to try to avoid (1) in favour of using (2) always (i.e. to let the ghost rank also deal with other MPI ranks that are local), because then the code would not work any more if executed on a single node (because in the latter case, no listener ranks are created at all).

-- The change of particle data belonging to a different process can in general easily create race conditions. If needed, this therefore needs to be protected by a spin lock on the particle's data to serialize all changes acting on the same particle. Look at the function simparticles::drift_particle() in src/time_integration/predict.cc for an example of how a spin-lock on a particle can be set and released.

Hope this helps at least a bit... Especially if you are new to MPI, the changes you are attempting are rather non-trivial.

Regards,
Volker


> On 13. Nov 2020, at 10:55, Leonard Romano <leonard.romano_at_tum.de> wrote:
>
>
> Dear Gagdet-community,
>
> I am working with Gadget-4 and am trying to develop a function that sends a request
> from a local computing rank to a foreign memory rank telling it to update a value that's stored
> in the instance of particle_data on the foreign one.
> I am using a data-structure which is analogous to the neighbor tree for SPH particles, which is equipped with a structure
> similar to "foreign_sphpoint_data".
> I figured that the easiest way to do is would be to simply provide it with all the access data I need to call "get_Pp" in addition to the
> shared memory rank of the processor node it resides on.
> The problem is, I am not sure where that last bit of information can be found.
> Is it by any chance simply "shmem::MyShmRankInGlobal"? In that case I could send my request to the destination "MyShmRankInGlobal" for the communicator "MPI_COMM_WORLD" or am I mistaken?
> I am new to MPI and have not fully understood how all of this works, so if someone could help, I would be very grateful!
>
> All the best,
> Leonard Romano
>
>
>
>
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Received on 2020-11-15 13:18:48

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