DPA All-to-all

NVIDIA DOCA DPA All-to-all Application Guide

This guide explains the all-to-all collective operation example when accelerated using the DPA in NVIDIA® BlueField®-3 DPU.

1. Introdution

This example shows how the MPI all-to-all collective can be accelerated on the DPA. In an MPI collective, all processes in the same job call the collective routine.

Given a communicator of n ranks, the example performs a collective operation in which all the processes send and receive the same amount of data from all the processes (hence all-to-all).

This document describes how to run the all-to-all example using DOCA DPA.

All-to-all is a message passing interface (MPI) method. MPI is a standardized and portable message-passing standard designed to function on parallel computing architectures. An MPI program is one where several processes run in parallel.

Each process in the diagram divides its local sendbuf into n blocks (4 in this example), each containing sendcount (4 in this example) elements. Process i sends the k-th block of its local sendbuf to process k which places the data in the i-th block of its local recvbuf.

Implementing the all-to-all method using DOCA DPA offloads the copying of the elements from the srcbuf to the recvbufs to the DPA, and leaves the CPU free to perform other computations.

3. Application Architecture

The following diagram describes the differences between the host-based all-to-all and DPA all-to-all.

In DPA all-to-all, DPA threads perform the all-to-all and the CPU is free to do other computations
In host-based all-to-all, the CPU must still perform the all-to-all at some point and is not completely free for other computations

4. DOCA Libraries

This application leverages the following DOCA driver:

DPA library

5. Configuration Flow

This section lists the application's configuration flow which includes different FlexIO functions and wrappers.

Initialize MPI.

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            MPI_Init(&argc, &argv);

Parse application argument.
1. Initialize arg parser resources and register DOCA general parameters.
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            doca_argp_init();
        
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2. Register the application's parameters.
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            register_all_to_all_params();
        
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3. Parse the arguments.
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            doca_argp_start();
        
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  1. The msgsize parameter is the size of the sendbuf and recvbuf (in bytes). It must be in multiples of an integer and at least the number of processes times an integer size.
  2. The devices_param parameter is the names of the InfiniBand devices to use (must support DPA). It can include up to two devices names.
4. Only let the first process (of rank 0) parse the parameters to then broadcast them to the rest of the processes.
Check and prepare the needed resources for the all_to_all call:
- Check the number of processes (maximum is 16).
- Check the msgsize. It must be in multiples of integer size and at least the number of processes times integer size.
- Allocate the sendbuf and recvbuf according to msgsize.

Prepare the resources required to perform the all-to-all method using DOCA DPA:

Initialize DOCA DPA context:

Open DOCA DPA device (DOCA device that supports DPA).

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            open_dpa_device();

Create DOCA DPA context using the opened device.

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            doca_dpa_create();

Create the required events for the all-to-all: One completion event for the kernel launch (wait location CPU and update location DPA) and kernel events (wait location remote and update location DPA) as the number of processes.

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            create_dpa_a2a_events() {
	doca_dpa_event_create(doca_dpa, DOCA_DPA_EVENT_ACCESS_DPA, DOCA_DPA_EVENT_ACCESS_CPU, DOCA_DPA_EVENT_WAIT_DEFAULT, &comp_event, 0);
	for (i = 0; i < resources->num_ranks; i++)
		doca_dpa_event_create(doca_dpa, DOCA_DPA_EVENT_ACCESS_REMOTE, DOCA_DPA_EVENT_ACCESS_DPA, DOCA_DPA_EVENT_WAIT_DEFAULT, &(kernel_events[i]), 0);
}

Create DOCA DPA worker (for the endpoints).

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            doca_dpa_worker_create();

Prepare DOCA DPA endpoints:

Create DOCA DPA endpoints as the number of processes/ranks.

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            for (i = 0; i < resources->num_ranks; i++)
    doca_dpa_ep_create();

Connect the local process' endpoints to the other processes' endpoints.

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            connect_dpa_a2a_endpoints();

Export the endpoints to DOCA DPA device endpoints (so they can be used by the DPA) and copy them to DPA heap memory.

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            for (int i = 0; i < resources->num_ranks; i++) {
      result = doca_dpa_ep_dev_export();
      doca_dpa_mem_alloc();
      doca_dpa_h2d_memcpy();
}

Prepare the memory required to perform the all-to-all method using DOCA DPA. This includes creating memory handlers for the sendbuf and recvbuf, getting the other processes' recvbufs handlers, and copying these memory handlers and their remote keys and the events' handlers to the DPA heap memory.
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            prepare_dpa_a2a_memory();
        
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Launch the alltoall_kernel using DOCA DPA kernel launch with all the required parameters:
1. Every MPI rank launches a kernel of up to MAX_NUM_THREADS. This example defines MAX_NUM_THREADS as 16.
2. Launch alltoall_kernel using kernel_launch.
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            doca_dpa_kernel_launch();
        
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3. Using the doca_dpa_dev_put_signal_nb() function, copy the relevant sendbuf to the correct recvbuf (according to the process' rank) for every OS process. Remember that multithreading is also used inside of the DPA threads.
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            for (i = thread_rank; i < num_ranks; i += num_threads)
    doca_dpa_dev_put_signal_nb();
        
```
4. Wait until the alltoall_kernel has finished.
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            doca_dpa_event_wait_until();
        
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  Note:
  Add an MPI barrier after waiting for the event to make sure that all of the processes have finished executing the alltoall_kernel.
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  MPI_Barrier();
  After the alltoall_kernel is finished, the recvbuf of all the processes now contain the expected output of the all-to-all method.

Destroy the a2a_resources:

Free all the DOCA DPA memories.

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            doca_dpa_mem_free();

Unregister all the DOCA DPA host memories.

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            doca_dpa_mem_unregister();

Destroy all the DOCA DPA endpoints.

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            doca_dpa_ep_destroy();

Destroy the DOCA DPA worker.

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            doca_dpa_worker_destroy();

Destroy all the DOCA DPA events.

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            doca_dpa_event_destroy();

Destroy the DOCA DPA context.

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            doca_dpa_destroy();

Close the DOCA device.

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            doca_dev_close();

6. Dependencies

BlueField-3 or later
Ubuntu 18.04/20.04/22.04 on the host (x86)
Open MPI version 4.1.5rc2 or greater (included in DOCA installation)

7. Running the Application

Refer to the following documents:
- NVIDIA DOCA Installation Guide for Linux for details on how to install BlueField-related software.
- NVIDIA DOCA Troubleshooting Guide for any issue you may encounter with the installation, compilation, or execution of DOCA applications.
- NVIDIA DOCA Applications Overview for additional compilation instructions and development tips of DOCA applications.
The doca_dpa_all_to_all binary is located under /opt/mellanox/doca/applications/dpa_all_to_all/bin/doca_dpa_all_to_all. To build all the applications together, run:
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            cd /opt/mellanox/doca/applications/
meson build
ninja -C build
        
```

To build only the dpa_all_to_all application:

Edit the following flags in /opt/mellanox/doca/applications/meson_options.txt:
- Set enable_all_applications to false
- Set enable_dpa_all_to_all to true
Run the commands in step 2.

Note:

doca_dpa_all_to_all is created under ./build/dpa_all_to_all/src/host/.

Application usage:

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            Usage: doca_dpa_all_to_all [DOCA Flags] [Program Flags]
DOCA Flags:
  -h, --help                          Print a help synopsis
  -v, --version                       Print program version information
  -l, --log-level                     Set the log level for the program <CRITICAL=20, ERROR=30, WARNING=40, INFO=50, DEBUG=60>
 
Program Flags:
  -m, --msgsize <Message size>        The message size - the size of the sendbuf and recvbuf (in bytes). Must be in multiplies of integer size. Default is size of one integer times the number of processes.
  -d, --devices <IB device names>     IB devices names that supports DPA, separated by comma without spaces (max of two devices). If not provided then a random IB device will be chosen.

Pre-run setup:
- Make sure that MPI is installed in your setup (openmpi is provided as part of the doca-tools metapackage). Do not forget to update your LD_LIBRARY_PATH and PATH environment variable to include MPI. For example, if MPI is installed under /usr/mpi/gcc/openmpi-4.1.5rc2/ then run:
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            export PATH=/usr/mpi/gcc/openmpi-4.1.5rc2/bin:$PATH
export LD_LIBRARY_PATH=/usr/mpi/gcc/openmpi-4.1.5rc2/lib:$LD_LIBRARY_PATH
        
```
CLI example for running the application. Remember, this is an MPI program, so use mpirun to run the application (with the -np flag to specify the number of processes to run). The same command works for running the application on the host (x86) and the BlueField (Arm cores).
- The following runs the DPA all-to-all application with 8 processes using the default message size (the number of processes, which is 8, times the size of 1 integer) with a random InfiniBand device:
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            mpirun -np 8 /opt/mellanox/doca/applications/dpa_all_to_all/bin/doca_dpa_all_to_all
        
```
- The following runs DPA all-to-all application with 8 processes, with 128 bytes as the message size and with mlx5_0 and mlx5_1 as the IB devices:
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            mpirun-np 8 /opt/mellanox/doca/applications/dpa_all_to_all/bin/doca_dpa_all_to_all -m 128 -d "mlx5_0,mlx5_1"
        
```
  Note:
  
  The application supports running with a maximum of 16 processes. If you try to run with more processes, then an error is printed and the application exits.

To run doca_all_to_all_dpa using a JSON file:

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            doca_dpa_all_to_all --json [json_file]

For example:

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            cd /opt/mellanox/doca/applications/dpa_all_to_all/bin
./doca_dpa_all_to_all --json ./dpa_all_to_all_params.json

8. Arg Parser DOCA Flags

Refer to NVIDIA DOCA Arg Parser Programming Guide for more information.

Flag Type	Short Flag	Long Flag/JSON Key	Description	JSON Content
General flags	`l`	`log-level`	Sets the log level for the application: CRITICAL=20 ERROR=30 WARNING=40 INFO=50 DEBUG=60	Copy Copied! `"log-level": 60`
	`v`	`version`	Prints program version information	N/A
	`h`	`help`	Prints a help synopsis	N/A
Program flags	`m`	`msgsize`	The message size. The size of the sendbuf and recvbuf (in bytes). Must be in multiples of an integer. The default is size of 1 integer times the number of processes.	Copy Copied! `"msgsize": -1` Note: The value -1 is a placeholder to use the default size, which is only known at run time (because it depends on the number of processes).
Program flags	`d`	`device`	InfiniBand devices names that support DPA, separated by comma without spaces (max of two devices). If `NOT_SET` then a random InfiniBand device is chosen.	Copy Copied! `"devices": "NOT_SET"`

9. References

/opt/mellanox/doca/applications/all_to_all_dpa/src
/opt/mellanox/doca/applications/all_to_all_dpa/bin/all_to_all_dpa_params.json

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