Implement Connectivity.java example using Java bindings #13201

vdhyasani17 · 2025-04-18T23:22:43Z

[WIP] This adds a Java version of the connectivity example using Open MPI's Java bindings. It demonstrates basic point-to-point communication and serves as a reference for Java-based MPI applications. The functionality will be the same as connectivity_c.c.

github-actions · 2025-04-18T23:23:13Z

Hello! The Git Commit Checker CI bot found a few problems with this PR:

a46c371: Create Connectivity.java and be able to compile/ru...

check_signed_off: does not contain a valid Signed-off-by line

Please fix these problems and, if necessary, force-push new commits back up to the PR branch. Thanks!

github-actions · 2025-04-18T23:23:37Z

Hello! The Git Commit Checker CI bot found a few problems with this PR:

a46c371: Create Connectivity.java and be able to compile/ru...

check_signed_off: does not contain a valid Signed-off-by line

Please fix these problems and, if necessary, force-push new commits back up to the PR branch. Thanks!

github-actions · 2025-04-18T23:28:30Z

Hello! The Git Commit Checker CI bot found a few problems with this PR:

a46c371: Create Connectivity.java and be able to compile/ru...

check_signed_off: does not contain a valid Signed-off-by line

Please fix these problems and, if necessary, force-push new commits back up to the PR branch. Thanks!

vdhyasani17 · 2025-04-30T16:05:00Z

Hi @jsquyres, can you review my PR when you have the chance? Thank you!

jsquyres

I'm not quite sure how the extra mpi.h.in / python changes came into this PR. Can you rebase to the current tip of main and avoid including Howard's commit here?

jsquyres · 2025-05-01T12:43:46Z

docs/man-openmpi/man3/MPI_T_pvar_get_info.3.rst

-For more information see MPI-3 section 14.3.2.
+* ``MPI_T_BIND_MPI_SESSION``: MPI session
+
+For more information see MPI-4 section 14.3.2.


The MPI tools chapter moved in MPI-4.

Suggested change

For more information see MPI-4 section 14.3.2.

For more information see MPI-4.1 section 15.3.2.

jsquyres · 2025-05-01T12:44:14Z

examples/Connectivity.java

@@ -0,0 +1,66 @@
+/*
+ * Test the connectivity between all processes
+ * WIP


Remove WIP.

jsquyres · 2025-05-01T12:45:10Z

examples/Connectivity.java

+        MPI.Init(args);
+
+        /*
+         * MPI_COMM_WORLD is the communicator provided when MPI is


Suggested change

* MPI_COMM_WORLD is the communicator provided when MPI is

* MPI.COMM_WORLD is the communicator provided when MPI is

jsquyres · 2025-05-01T12:45:36Z

examples/Connectivity.java

+        int myRank = MPI.COMM_WORLD.getRank();
+        int numProcesses = MPI.COMM_WORLD.getSize();
+        Status status;
+        int verbose = 0;


Should this be a boolean instead of an integer?

Yes. Not sure why the C example uses an integer. Is that standard C convention?

jsquyres · 2025-05-01T12:46:12Z

examples/Connectivity.java

+        String processorName = MPI.getProcessorName();
+
+        for (int i = 0; i < numProcesses; i++) {
+            if (myRank == i) { /* Find current process */


I don't know the Java ecosystem at all -- is it more common for single-line comments to be // instead of /* ... */?

Other Java programs here that I've seen use both, but mostly use /*.

jsquyres · 2025-05-01T12:47:31Z

examples/Connectivity.java

+                        System.out.printf("Checking connection between rank %d on %s and rank %d\n", i, processorName,
+                                j);
+
+                    int[] dataBuffer = { myRank }; /*


There's mixing of styles here: end-of-line comments and before-the-line comments. We should probably be consistent.

In the majority of the rest of the Open MPI code base (and in examples/connectivity.c), we use before-the-line comments. That might be a good choice here, for consistency.

Ok, I will fix that to strictly use before-the-line comments.

jsquyres · 2025-05-01T12:49:12Z

examples/Connectivity.java

+                                                    * Data buffer to be sent to next process (in this case, only send
+                                                    * rank num)
+                                                    */
+                    int[] recBuffer = new int[1]; /* Data buffer that current process receives from previous process */


In connectivity_c.c, the variables are named rank and peer. If this example is meant to be the Java version of the C example, it might be good to make them similar in spirit -- e.g., have the variable names and meanings agree.

Got it. I will rename them to rank and peer.

jsquyres · 2025-05-01T12:51:11Z

examples/Connectivity.java

+
+                    /* Process must send message first then wait to receive to avoid deadlock */
+                    MPI.COMM_WORLD.send(dataBuffer, 1, MPI.INT, j, myRank);
+                    MPI.COMM_WORLD.recv(recBuffer, 1, MPI.INT, j, j);


Heh. I guess we hadn't really looked closely at connectivity_c.c before, because technically this is a deadlock -- using a blocking send to send to yourself might block and might not. In this case, it (most likely) won't because the message is so short -- but it definitely could.

This example -- and the C example -- should likely be fixed.

Ok, interesting. Should I use non-blocking iSend and iRecv instead to fix this?

jsquyres · 2025-05-01T12:51:44Z

examples/Connectivity.java

+                }
+            } else if (myRank > i) {
+                int[] dataBuffer = { myRank };
+                int[] recBuffer = new int[1];


Is there a reason to use a fixed array for dataBuffer and an alloc'ed buffer for recBuffer? I.e., why the difference?

I used a fixed array for data buffer because I thought it would be fine prepopulating the array with myRank since I knew I would be sending that, but I will keep it consistent in this next push. Also, I will be using IntBuffer instead of int[] for iSend and iRecv methods.

Signed-off-by: vdhyasani17 <[email protected]>

Add comments to explain the functionality Signed-off-by: vdhyasani17 <[email protected]>

Fix comment semantics and structure Add -v verbose check Signed-off-by: vdhyasani17 <[email protected]>

vdhyasani17 · 2025-05-01T16:16:06Z

Sorry, I had some issues with rebasing, so all of my commits are now underneath the comments.

github-actions bot added the Target: main label Apr 18, 2025

vdhyasani17 force-pushed the java-connectivity-example branch from dad9cfe to 448388b Compare April 19, 2025 00:02

jsquyres marked this pull request as draft April 21, 2025 12:41

vdhyasani17 marked this pull request as ready for review April 30, 2025 15:57

jsquyres requested changes May 1, 2025

View reviewed changes

vdhyasani17 added 3 commits May 1, 2025 08:57

Create Connectivity.java and be able to compile/run java files

354e8b7

Signed-off-by: vdhyasani17 <[email protected]>

Implement the code for Connectivity.java

b01b5a3

Add comments to explain the functionality Signed-off-by: vdhyasani17 <[email protected]>

Make communication non-blocking by using iSend and iRecv

64badb5

Fix comment semantics and structure Add -v verbose check Signed-off-by: vdhyasani17 <[email protected]>

vdhyasani17 force-pushed the java-connectivity-example branch from af303de to 64badb5 Compare May 1, 2025 16:11

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Implement Connectivity.java example using Java bindings #13201

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vdhyasani17 commented May 1, 2025

	For more information see MPI-4 section 14.3.2.
	For more information see MPI-4.1 section 15.3.2.

	* MPI_COMM_WORLD is the communicator provided when MPI is
	* MPI.COMM_WORLD is the communicator provided when MPI is

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vdhyasani17 commented May 1, 2025

vdhyasani17 commented Apr 18, 2025 •

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