https://github.com/zelmoghazy/mpi-basics-c
https://github.com/zelmoghazy/mpi-basics-c
Last synced: 3 months ago
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- Host: GitHub
- URL: https://github.com/zelmoghazy/mpi-basics-c
- Owner: Zelmoghazy
- Created: 2023-05-26T19:20:14.000Z (about 2 years ago)
- Default Branch: main
- Last Pushed: 2023-05-31T22:19:45.000Z (about 2 years ago)
- Last Synced: 2025-01-15T12:14:43.713Z (5 months ago)
- Language: C
- Size: 29.3 KB
- Stars: 1
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
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README
# MPI_BASICS_C
- No shared memory between processes in different machines only way of communication is through message passing
- Used in distributed systems and high performance computing
- Not targeted for shared memory machines but still works although rarely used as it degrades performance due to overhead of send and recieve of message passing
- Running as a set of processes No shared Variables
- All communication, synchronization require subroutine calls
---
- **MPI** : Message Passing Interface
* Processes running on different CPUs communicate with each other
* Various communication patterns that are supported by MPI are abstracted in a "communicator"
* **Communicator type :** `MPI_Comm` -> A communicator is a group of processes that can communicate and interact with each other using MPI functions.
* **Predefined default communicator :** `MPI_COMM_WORLD` -> predefined constant representing the communicator that encompasses all processes in a parallel program.
* Number of concurrent processes to be run is defined at **runtime**.
* MPI functions return error codes or `MPI_SUCCESS` it doesnt return values.
## MPI Fuctions
- `int MPI_Init(int *argc, char ***argv);` : Configure our program to run MPI, must be called before any MPI functions.
- Takes the address of command line arguments (in/out argument), search for specific commands to MPI, use it to configure, and remove them from the list of command line arguments.
- `int MPI_Finalize();` : Called at the end of the program, close communication and free allocated memory.
- `int MPI_Comm_size(MPI_Comm comm, int *size);` : get the number of processes that are communicating inside our communicator
- Takes a pointer to an integer `int* size` (out argument), we should define a variable `size` and pass its address to this function to be updated.
(Each process has its own unique identifier, called a "rank", which is used to distinguish it from other processes)
- `int MPI_Comm_rank(MPI_Comm comm, int *rank);` : retrieve the rank of the calling process within a specified communicator.
- Takes a pointer to an integer `int* rank` (out argument), we should define a variable `rank` and pass its address to this function to be updated
(Transmit from one process to another process)
```c
int MPI_Send( void *buf, // Generic pointer to anything in memory to fetch and send
int count, // number of primitive element in this buffer determined by datatype.
MPI_Datatype datatype, // constants defined like MPI_CHAR as an abstraction to actual data type that allows comm. bet. different machines.
int dest, // rank of process which we want to transmit the buffer to.
int tag, // Generic integer used to distinguish one type of a message from another to same destination.
MPI_Comm comm ); // Communicator -> MPI_COMM_WORLD.
int MPI_Recv( void *buf, // Generic pointer to place in memory where you want to recieve the incoming data (out argument).
int count, // number of primitive element in this buffer determined by datatype.
MPI_Datatype datatype, // constants defined like MPI_CHAR as an abstraction to actual data type that allows comm. bet. different machines.
int source, // rank of process which we want to recieve from.
int tag, // select kind of message I want to recieve from source.
MPI_Comm comm // Communicator -> MPI_COMM_WORLD.
MPI_Status *status ); // Info about success or failure of communication from another process.
```
(Reciever can get a message without knowing the amount of data, the sender `MPI_ANY_SOURCE` or the tag `MPI_ANY_TAG`)
```c
typedef struct MPI_Status {
int MPI_SOURCE; //: rank of the process where the information came from
int MPI_TAG; //: tag value sent by the sender
int MPI_ERROR; //: indication that anything gone wrong
};
```
- Example use of `MPI_Status`
```c
MPI_Status status;
MPI_Recv(...,&status);
tag = status.MPI_TAG;
source = status.MPI_SOURCE
error = status.MPI_ERROR;
```
- `MPI_WTIME()` : returns the current time with a double float.
- to time a program segment :
```c
start_time = MPI_Wtime();
end_time = MPI_Wtime();
Time spent = end_time - start_time;
```
## Compilation
- Compiled using WSL2 `sudo apt install libopenmpi-dev`
- run using `make n=4`
- type required number of processes (4 processes chosen here)
- Makefile is included