Maximum Size of Data Transfers with MPI?

Hey guys,

Does anyone happen to know the maximum amount of data that can be send in one MPI_Send and received in one MPI_Recv transfer?

I think that I had read once that the amount was not limited by MPI itself, but by other factors. That does make sense, but I'm having an annoying issue:

I have a variable length array that takes a message size that the user specifies on the command line. This array is then filled up with a bunch of 'a' characters.

After that, the character message is sent to another process. The receiving process then simply prints the specified amount of 'a' characters.

When I run the program, if I specify any message size from 1-64, it receives and prints fine.

If I try, say, 100 the program receives the message but only prints out 64 characters.

A rough idea of my sending process code is:

Code:

	
long long double TEST;
	
printf("How large should the message be?\n");
    	
scanf("%lld", &TEST);
	
char test_string[TEST];
	
	for(long long double i = 0; i <= TEST; i++){
    	    test_string[i] = 'a';
       }	
		
	MPI_Status	status;
	//MPI_Request	req;
	
	MPI_Init(&argc, &argv);
	MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
	MPI_Comm_size(MPI_COMM_WORLD, &mpi_size);
	
	
	if (my_rank == 0){
	// Send the string
	MPI_Send(test_string, /*4*/TEST, MPI_CHAR, 1, 99, MPI_COMM_WORLD);

And the receiving process is:

Code:

	
long long double TEST;
char test_str[TEST];
	
MPI_Status	status;
	
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
MPI_Comm_size(MPI_COMM_WORLD, &mpi_size);
	
	// Send the string
	MPI_Recv(test_str, /*4*/UPPER_LIMIT, MPI_CHAR, 0, 99, MPI_COMM_WORLD, &status);
	
	// Print received string
	printf("\n%s\n",test_str);

Where UPPER_LIMIT is a #define of 50000000. I will eventually write this code so that the size of TEST is send first, and then the message is sent.

A sample output is:

How large should the message be?
2

aa

And if I put 100:

How large should the message be?
100

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaa

Any ideas?

Edit - By the way, I am on a 64-bit architecture.

Code:

long long double TEST;
char test_str[TEST];

That shouldn't even compile.

Originally Posted by CornedBee

That shouldn't even compile.

It does!

Why do you think it shouldn't compile?

1) long long double is not a valid type
2) TEST is not initialized, so it does not form an integral constant expression, which makes it invalid as an array size specifier
3) If TEST is supposed to be long double, it's not even integral, so it's even more invalid.

What absurd compiler does this compile on, and what warnings do you get if you push up the warning level a bit?

I am using gcc. I have switched to long long int, and now the process 0 program (the sending program) exists with signal 15.

It gets as far as this statement:

Code:

printf("How large should the message be?\n");

Prints that to the screen then the signal 15 occurs.

Any idea what might be wrong?

Before or after you enter the number?

Originally Posted by CornedBee

Before or after you enter the number?

Before I enter the number.

I have no time to do anything -- right after I run the program it prints that statement and then exits.