Adds distributed row gatherer #1589

MarcelKoch · 2024-04-04T10:49:19Z

This PR adds a distributed row gatherer. This operator essentially provides the communication required in our matrix apply.

Besides the normal apply (which is blocking), it also provides two asynchronous calls. One version has an additional workspace parameter which is used as send buffer. This version can be called multiple times without restrictions, if different workspaces are used for each call. The other version doesn't have a workspace parameter, and instead uses an internal buffer. As a consequence, this function can only be called a second time, if the request of the previous call has been waited on. Otherwise, this function will throw.

This is the second part of splitting up #1546.

It also introduces some intermediate changes, which could be extracted out beforehand:

~~a type-erased DenseCache~~
~~makingdetail::run easier to use~~ now part of Use index_map in distributed::matrix #1544

PR Stack:

MarcelKoch · 2024-04-22T08:29:49Z

One issue that I have is the constructor. It takes a collective_communicator and an index_map. The index_map already defines the communication pattern, so the collective_communicator has to match that.
One option might be to have a virtual function like

std::unique_ptr<collective_communicator> create_with_same_type(communicator, index_map);

If I can't come up with anything better, I guess I will use that.

pratikvn · 2024-04-26T09:50:57Z

Do we need to have the std::future setup for the release ? Can we remove that for now and just use a normal synchronous approach ? I think that is a significant change that maybe needs more thought and probably a separate PR.

pratikvn

Really nice work! LGTM!

core/distributed/row_gatherer.cpp

pratikvn · 2024-07-23T08:01:34Z

core/distributed/row_gatherer.cpp

+    int is_inactive;
+    MPI_Status status;
+    GKO_ASSERT_NO_MPI_ERRORS(
+        MPI_Request_get_status(req_listener_, &is_inactive, &status));


Can we maybe move this MPI function into mpi.hpp and create a wrapper for it ?

That doesn't really work here, since this function would be a member function of request, but I'm using a bare MPI_Request (and can't use request, because it will try to free the request in the destructor), so it would not be applicable.

pratikvn · 2024-07-23T08:08:09Z

include/ginkgo/core/distributed/row_gatherer.hpp

+
+    mutable array<char> send_workspace_;
+
+    mutable MPI_Request req_listener_{MPI_REQUEST_NULL};


This can be of type mpi::request ?

No, because the destructor of mpi::request will try to free the request. But req_listenser_ doesn't own any requests, so the program would crash.

pratikvn · 2024-07-23T09:10:12Z

core/distributed/row_gatherer.cpp

+template <typename LocalIndexType>
+void RowGatherer<LocalIndexType>::apply_impl(const LinOp* alpha, const LinOp* b,
+                                             const LinOp* beta, LinOp* x) const
+    GKO_NOT_IMPLEMENTED;


I think you can also implement the advanced apply by replacing b_local->row_gather(idxs, buffer) by b_local->row_gather(alpha, idxs, beta, buffer) ?

core/distributed/matrix.cpp

yhmtsai · 2024-10-07T13:26:19Z

core/multigrid/pgm.cpp

-                      send_sizes.data(), send_offsets.data(), type, recv_ptr,
-                      recv_sizes.data(), recv_offsets.data(), type);
+    coll_comm
+        ->i_all_to_all_v(use_host_buffer ? exec->get_master() : exec, send_ptr,


any difference between using all_to_all_v vs i_all_to_all_v? I assume all_to_all_v also update the interface

all_to_all_v is a blocking call, while i_all_to_all_v is non-blocking. Right now the collective_communicator only provides the non-blocking interface, since it is more general.

include/ginkgo/core/distributed/row_gatherer.hpp

yhmtsai · 2024-10-07T13:28:56Z

include/ginkgo/core/distributed/row_gatherer.hpp

+ * auto x = matrix::Dense<double>::create(...);
+ *
+ * auto future = rg->apply_async(b, x);
+ * // do some computation that doesn't modify b, or access x


I think it access x but it is unclear when it will be accessed before the wait

I guess this just meant to say that you can't expect any meaningful data when accessing x before the wait has completed.

I think I get it wrong.
Is the comment here to describe that user can do something safely after the call or the apply_async behavior?
My comment was based on that it is the behavior of the apply_async because apply_async definitely accesses x.
If it is for user action during async and wait, then it is correct.

core/distributed/row_gatherer.cpp

yhmtsai · 2024-10-23T08:37:11Z

core/distributed/row_gatherer.cpp

+                    workspace.set_executor(mpi_exec);
+                    if (send_size_in_bytes > workspace.get_size()) {
+                        workspace.resize_and_reset(sizeof(ValueType) *
+                                                   send_size[0] * send_size[1]);
+                    }


combining them to assign the workspace directly?

Combine how? Do you mean like

workspace = array<char>(mpi_exec, sizeof(ValueType) * send_size[0] * send_size[1]);

yhmtsai · 2024-10-23T08:41:27Z

core/distributed/row_gatherer.cpp

+                    req = coll_comm_->i_all_to_all_v(
+                        mpi_exec, send_ptr, type.get(), recv_ptr, type.get());


send_buffer might be on the host but the recv_ptr(x_local) might be on the device

I have a check above to ensure that the memory space of the recv buffer is accessible from the mpi executor. So if GPU aware MPI is used, it should work (even if send buffer is on the host and recv buffer in the device or vice versa). Otherwise an exception will be thrown.

core/test/mpi/distributed/row_gatherer.cpp

- only allocate if necessary - synchronize correct executor Co-authored-by: Pratik Nayak <pratik.nayak@kit.edu>

- split tests into core and backend part - fix formatting - fix openmpi pre 4.1.x macro Co-authored-by: Pratik Nayak <pratik.nayak4@gmail.com> Co-authored-by: Yu-Hsiang M. Tsai <yhmtsai@gmail.com>

MarcelKoch self-assigned this Apr 4, 2024

ginkgo-bot added reg:build This is related to the build system. reg:testing This is related to testing. mod:core This is related to the core module. type:matrix-format This is related to the Matrix formats labels Apr 4, 2024

MarcelKoch requested a review from pratikvn April 4, 2024 10:49

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MarcelKoch mentioned this pull request Apr 19, 2024

Adds sparse communicator class #1546

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This was referenced Apr 22, 2024

Add segmented array type #1545

Merged

Distributed Index Map #1543

Merged

Adds Index map device kernels #1579

Merged

Use index_map in distributed::matrix #1544

Merged

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MarcelKoch and others added 9 commits October 30, 2024 16:28

[dist-rg] adds distributed row-gatherer

cbf79ee

[dist-rg] handle copy to host buffer

90a151e

[dist-rg] use mpi request instead of future

8f61b09

[dist-rg] adds distributed row-gatherer tests

de9e47a

[dist-mat] use row-gatherer

64304bf

[pgm] use row-gatherer from matrix

be777a3

[core] enable array conversion between ints

9cd1ec7

[dist-rg] review update:

0b64def

- only allocate if necessary - synchronize correct executor Co-authored-by: Pratik Nayak <pratik.nayak@kit.edu>

[dist-rg] review updates:

2a54c3e

- split tests into core and backend part - fix formatting - fix openmpi pre 4.1.x macro Co-authored-by: Pratik Nayak <pratik.nayak4@gmail.com> Co-authored-by: Yu-Hsiang M. Tsai <yhmtsai@gmail.com>

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Adds distributed row gatherer #1589

Adds distributed row gatherer #1589

MarcelKoch commented Apr 4, 2024 •

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		mutable array<char> send_workspace_;

		mutable MPI_Request req_listener_{MPI_REQUEST_NULL};

		req = coll_comm_->i_all_to_all_v(
		mpi_exec, send_ptr, type.get(), recv_ptr, type.get());

Adds distributed row gatherer #1589

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MarcelKoch commented Apr 4, 2024 •

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