10.7. CUDA

10.7.1. How do I build Open MPI with CUDA-aware support?

CUDA-aware support means that the MPI library can send and receive GPU buffers directly. CUDA support is being continuously updated so different levels of support exist in different versions. We recommend you use the latest version of Open MPI for best support.

Open MPI offers two flavors of CUDA support:

  1. Via UCX.

    This is the preferred mechanism. Since UCX will be providing the CUDA support, it is important to ensure that UCX itself is built with CUDA support.

    To see if your ucx was built with CUDA support run the following command:

    # Check if ucx was built with CUDA support
    shell$ ucx_info -v
    
    # configured with: --build=powerpc64le-redhat-linux-gnu --host=powerpc64le-redhat-linux-gnu --program-prefix= --disable-dependency-tracking --prefix=/usr --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64 --libexecdir=/usr/libexec --localstatedir=/var --sharedstatedir=/var/lib --mandir=/usr/share/man --infodir=/usr/share/info --disable-optimizations --disable-logging --disable-debug --disable-assertions --enable-mt --disable-params-check --enable-cma --without-cuda --without-gdrcopy --with-verbs --with-cm --with-knem --with-rdmacm --without-rocm --without-xpmem --without-ugni --without-java
    

    If you need to build ucx yourself to include CUDA support, please see the UCX documentation for building ucx with Open MPI:

    It should look something like:

    # Configure UCX this way
    shell$ ./configure --prefix=/path/to/ucx-cuda-install --with-cuda=/usr/local/cuda --with-gdrcopy=/usr
    
    # Configure Open MPI this way
    shell$ ./configure --with-cuda=/usr/local/cuda --with-ucx=/path/to/ucx-cuda-install <other configure params>
    
  2. Via internal Open MPI CUDA support

Regardless of which flavor of CUDA support (or both) you plan to use, Open MPI should be configured using the --with-cuda=<path-to-cuda> configure option to build CUDA support into Open MPI.

This affects the smcuda shared memory btl, as well as the uct btl.


10.7.2. How do I verify that Open MPI has been built with CUDA support?

Verify that Open MPI has been built with cuda using ompi_info

# Use ompi_info to verify cuda support in Open MPI
shell$ ./ompi_info |grep "MPI extensions"
       MPI extensions: affinity, cuda, pcollreq

10.7.3. How do I run Open MPI with applications that pass CUDA buffers to MPI?

Open MPI will detect and enable CUDA enabled components at runtime with no additional mpirun parameters.


10.7.4. How do I build Open MPI with CUDA-aware support using PGI?

With CUDA 6.5, you can build all versions of CUDA-aware Open MPI without doing anything special. However, with CUDA 7.0 and CUDA 7.5, you need to pass in some specific compiler flags for things to work correctly. Add the following to your configure line.

# For PGI 15.9 and later (Also called NVCC):
shell$ ./configure --with-wrapper-cflags=-ta:tesla

# For earlier versions of PGI:
shell$ ./configure CFLAGS=-D__LP64__ --with-wrapper-cflags="-D__LP64__ -ta:tesla"

10.7.5. What kind of CUDA support exists in Open MPI?

CUDA-aware support is defined as Open MPI automatically detecting that the argument pointer being passed to an MPI routine is a CUDA device memory pointer.

See this FAQ entry for more details on which APIs are CUDA-aware.

Error

CUDA 4.0 is SUPER OLD! End users dont care about the differences between cuda-aware, cuda-ipc, gpu-direct, and gpu-direct-rdma

Open MPI depends on various features of CUDA 4.0, so one needs to have at least the CUDA 4.0 driver and toolkit. The new features of interest are the Unified Virtual Addressing (UVA) so that all pointers within a program have unique addresses. In addition, there is a new API that allows one to determine if a pointer is a CUDA device pointer or host memory pointer. This API is used by the library to decide what needs to be done with each buffer. In addition, CUDA 4.1 also provides the ability to register host memory with the CUDA driver, which can improve performance. CUDA 4.1 also added CUDA IPC support for fast communication between GPUs on the same node.

Note that derived datatypes — both contiguous and non-contiguous — are supported. However, the non-contiguous datatypes currently have high overhead because of the many calls to the CUDA function cuMemcpy() to copy all the pieces of the buffer into the intermediate buffer.

CUDA-aware support is available in:

  • The UCX (ucx) PML

  • The PSM2 (psm2) MTL with the CM (cm) PML.

  • Both CUDA-ized shared memory (smcuda) and TCP (tcp) BTLs with the OB1 (ob1) PML.

  • The HCOLL (hcoll) COLL


10.7.6. PSM2 support for CUDA

CUDA-aware support is present in PSM2 MTL. When running CUDA-aware Open MPI on Intel Omni-path, the PSM2 MTL will automatically set PSM2_CUDA environment variable which enables PSM2 to handle GPU buffers. If the user wants to use host buffers with a CUDA-aware Open MPI, it is recommended to set PSM2_CUDA to 0 in the execution environment. PSM2 also has support for the NVIDIA GPUDirect support feature. To enable this, users will need to set PSM2_GPUDIRECT to 1 in the execution environment.

Note: The PSM2 library and hfi1 driver with CUDA support are requirements to use GPUDirect support on Intel Omni-Path. The minimum PSM2 build version required is PSM2 10.2.175.

For more information refer to the Intel Omni-Path documentation.


10.7.7. How can I tell if Open MPI was built with CUDA support?

Use the ompi_info command:

shell$ ompi_info --parsable --all | grep mpi_built_with_cuda_support:value
mca:mpi:base:param:mpi_built_with_cuda_support:value:true

10.7.8. Can I get additional CUDA debug-level information at run-time?

Yes, by enabling some vebosity flags.

  • The opal_cuda_verbose parameter has only one level of verbosity:

    shell$ mpirun --mca opal_cuda_verbose 10 ...
    
  • The mpi_common_cuda_verbose parameter provides additional information about CUDA-aware related activities. This can be set to a variety of different values. There is really no need to use these unless you have strange problems:

    # A bunch of CUDA debug information
    shell$ mpirun --mca mpi_common_cuda_verbose 10 ...
    # Even more CUDA debug information
    shell$ mpirun --mca mpi_common_cuda_verbose 20 ...
    # Yet more CUDA debug information
    shell$ mpirun --mca mpi_common_cuda_verbose 100 ...
    
  • The smcuda BTL has three MCA parameters related to the use of CUDA IPC. By default, CUDA IPC is used where possible. But the user can now turn it off if they prefer.

    shell$ mpirun --mca btl_smcuda_use_cuda_ipc 0 ...
    

    In addition, it is assumed that CUDA IPC is possible when running on the same GPU, and this is typically true. However, there is the ability to turn it off.

    shell$ mpirun --mca btl_smcuda_use_cuda_ipc_same_gpu 0 ...
    

    Last, to get some insight into whether CUDA IPC is being used, you can turn on some verbosity that shows whether CUDA IPC gets enabled between two GPUs.

    shell$ mpirun --mca btl_smcuda_cuda_ipc_verbose 100 ...
    

10.7.9. NUMA Node Issues

When running on a node that has multiple GPUs, you may want to select the GPU that is closest to the NUMA node on which your process is running. One way to do this is to make use of the hwloc library. The following is a C code snippet that can be used in your application to select a GPU that is close. It will determine on which CPU it is running and then look for the closest GPU. There could be multiple GPUs that are the same distance away. This is dependent on having hwloc somewhere on your system.

/**
 * Test program to show the use of hwloc to select the GPU closest to the CPU
 * that the MPI program is running on.  Note that this works even without
 * any libpciacces or libpci support as it keys off the NVIDIA vendor ID.
 * There may be other ways to implement this but this is one way.
 * January 10, 2014
 */
#include <assert.h>
#include <stdio.h>
#include "cuda.h"
#include "mpi.h"
#include "hwloc.h"

#define ABORT_ON_ERROR(func) \
  { CUresult res; \
    res = func; \
    if (CUDA_SUCCESS != res) { \
        printf("%s returned error=%d\n", #func, res); \
        abort(); \
    } \
  }
static hwloc_topology_t topology = NULL;
static int gpuIndex = 0;
static hwloc_obj_t gpus[16] = {0};

/**
 * This function searches for all the GPUs that are hanging off a NUMA
 * node.  It walks through each of the PCI devices and looks for ones
 * with the NVIDIA vendor ID.  It then stores them into an array.
 * Note that there can be more than one GPU on the NUMA node.
 */
static void find_gpus(hwloc_topology_t topology, hwloc_obj_t parent, hwloc_obj_t child) {
    hwloc_obj_t pcidev;
    pcidev = hwloc_get_next_child(topology, parent, child);
    if (NULL == pcidev) {
        return;
    } else if (0 != pcidev->arity) {
        /* This device has children so need to look recursively at them */
        find_gpus(topology, pcidev, NULL);
        find_gpus(topology, parent, pcidev);
    } else {
        if (pcidev->attr->pcidev.vendor_id == 0x10de) {
            gpus[gpuIndex++] = pcidev;
        }
        find_gpus(topology, parent, pcidev);
    }
}

int main(int argc, char *argv[])
{
    int rank, retval, length;
    char procname[MPI_MAX_PROCESSOR_NAME+1];
    const unsigned long flags = HWLOC_TOPOLOGY_FLAG_IO_DEVICES | HWLOC_TOPOLOGY_FLAG_IO_BRIDGES;
    hwloc_cpuset_t newset;
    hwloc_obj_t node, bridge;
    char pciBusId[16];
    CUdevice dev;
    char devName[256];

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    if (MPI_SUCCESS != MPI_Get_processor_name(procname, &length)) {
        strcpy(procname, "unknown");
    }

    /* Now decide which GPU to pick.  This requires hwloc to work properly.
     * We first see which CPU we are bound to, then try and find a GPU nearby.
     */
    retval = hwloc_topology_init(&topology);
    assert(retval == 0);
    retval = hwloc_topology_set_flags(topology, flags);
    assert(retval == 0);
    retval = hwloc_topology_load(topology);
    assert(retval == 0);
    newset = hwloc_bitmap_alloc();
    retval = hwloc_get_last_cpu_location(topology, newset, 0);
    assert(retval == 0);

    /* Get the object that contains the cpuset */
    node = hwloc_get_first_largest_obj_inside_cpuset(topology, newset);

    /* Climb up from that object until we find the HWLOC_OBJ_NODE */
    while (node->type != HWLOC_OBJ_NODE) {
        node = node->parent;
    }

    /* Now look for the HWLOC_OBJ_BRIDGE.  All PCI busses hanging off the
     * node will have one of these */
    bridge = hwloc_get_next_child(topology, node, NULL);
    while (bridge->type != HWLOC_OBJ_BRIDGE) {
        bridge = hwloc_get_next_child(topology, node, bridge);
    }

    /* Now find all the GPUs on this NUMA node and put them into an array */
    find_gpus(topology, bridge, NULL);

    ABORT_ON_ERROR(cuInit(0));
    /* Now select the first GPU that we find */
    if (gpus[0] == 0) {
        printf("No GPU found\n");
    } else {
        sprintf(pciBusId, "%.2x:%.2x:%.2x.%x", gpus[0]->attr->pcidev.domain, gpus[0]->attr->pcidev.bus,
        gpus[0]->attr->pcidev.dev, gpus[0]->attr->pcidev.func);
        ABORT_ON_ERROR(cuDeviceGetByPCIBusId(&dev, pciBusId));
        ABORT_ON_ERROR(cuDeviceGetName(devName, 256, dev));
        printf("rank=%d (%s): Selected GPU=%s, name=%s\n", rank, procname, pciBusId, devName);
    }

    MPI_Finalize();
    return 0;
}

10.7.10. How do I develop CUDA-aware Open MPI applications?

Developing CUDA-aware applications is a complex topic, and beyond the scope of this document. CUDA-aware applications often have to take machine-specific considerations into account, including the number of GPUs installed on each node and how the GPUs are connected to the CPUs and to each other. Often, when using a particular transport layer (such as OPA/PSM2) there will be run-time decisions to make about which CPU cores will be used with which GPUs.

A good place to start is the NVIDIA CUDA Toolkit Documentation including the Programming Guide and the Best Practices Guide. For examples of how to write CUDA-aware MPI applications, the NVIDIA developers blog offers examples and the OSU Micro-Benchmarks offer an excellent example of how to write CUDA-aware MPI applications.


10.7.11. Which MPI APIs work with CUDA-aware?

  • MPI_Allgather

  • MPI_Allgatherv

  • MPI_Allreduce

  • MPI_Alltoall

  • MPI_Alltoallv

  • MPI_Alltoallw

  • MPI_Bcast

  • MPI_Bsend

  • MPI_Bsend_init

  • MPI_Exscan

  • MPI_Ibsend

  • MPI_Irecv

  • MPI_Isend

  • MPI_Irsend

  • MPI_Issend

  • MPI_Gather

  • MPI_Gatherv

  • MPI_Get

  • MPI_Put

  • MPI_Rsend

  • MPI_Rsend_init

  • MPI_Recv

  • MPI_Recv_init

  • MPI_Reduce

  • MPI_Reduce_scatter

  • MPI_Reduce_scatter_block

  • MPI_Scan

  • MPI_Scatter

  • MPI_Scatterv

  • MPI_Send

  • MPI_Send_init

  • MPI_Sendrecv

  • MPI_Ssend

  • MPI_Ssend_init

  • MPI_Win_create


10.7.12. Which MPI APIs do NOT work with CUDA-aware?

  • MPI_Accumulate

  • MPI_Compare_and_swap

  • MPI_Fetch_and_op

  • MPI_Get_Accumulate

  • MPI_Iallgather

  • MPI_Iallgatherv

  • MPI_Iallreduce

  • MPI_Ialltoall

  • MPI_Ialltoallv

  • MPI_Ialltoallw

  • MPI_Ibcast

  • MPI_Iexscan

  • MPI_Rget

  • MPI_Rput


10.7.13. How do I use CUDA-aware UCX for Open MPI?

Example of running osu_latency from the OSU benchmarks with CUDA buffers using Open MPI and UCX CUDA support:

shell$ mpirun -n 2 --mca pml ucx \
    -x UCX_TLS=rc,sm,cuda_copy,gdr_copy,cuda_ipc ./osu_latency D D

10.7.14. Which MPI APIs work with CUDA-aware UCX?

  • MPI_Send

  • MPI_Bsend

  • MPI_Ssend

  • MPI_Rsend

  • MPI_Isend

  • MPI_Ibsend

  • MPI_Issend

  • MPI_Irsend

  • MPI_Send_init

  • MPI_Bsend_init

  • MPI_Ssend_init

  • MPI_Rsend_init

  • MPI_Recv

  • MPI_Irecv

  • MPI_Recv_init

  • MPI_Sendrecv

  • MPI_Bcast

  • MPI_Gather

  • MPI_Gatherv

  • MPI_Allgather

  • MPI_Reduce

  • MPI_Reduce_scatter

  • MPI_Reduce_scatter_block

  • MPI_Allreduce

  • MPI_Scan

  • MPI_Exscan

  • MPI_Allgatherv

  • MPI_Alltoall

  • MPI_Alltoallv

  • MPI_Alltoallw

  • MPI_Scatter

  • MPI_Scatterv

  • MPI_Iallgather

  • MPI_Iallgatherv

  • MPI_Ialltoall

  • MPI_Iialltoallv

  • MPI_Ialltoallw

  • MPI_Ibcast

  • MPI_Iexscan


10.7.15. Which MPI APIs do NOT work with CUDA-aware UCX?

  • All one-sided operations such as MPI_Put, MPI_Get, MPI_Accumulate, MPI_Rget, MPI_Rput, MPI_Get_Accumulate, MPI_Fetch_and_op, MPI_Compare_and_swap, etc

  • All window creation calls such as MPI_Win_create

  • All non-blocking reduction collectives like MPI_Ireduce, MPI_Iallreduce, etc


10.7.16. Can I tell at compile time or runtime whether I have CUDA-aware support?

There is both a compile time check and a run-time check available. You can use whichever is the most convenient for your program. To access them, you need to include mpi-ext.h. Note that mpi-ext.h is specific to Open MPI. The following program shows an example of using the CUDA-aware macro and run-time check.

/*
 * Program that shows the use of CUDA-aware macro and runtime check.
 */
#include <stdio.h>
#include "mpi.h"

#if !defined(OPEN_MPI) || !OPEN_MPI
#error This source code uses an Open MPI-specific extension
#endif

/* Needed for MPIX_Query_cuda_support(), below */
#include "mpi-ext.h"

int main(int argc, char *argv[])
{
    printf("Compile time check:\n");
#if defined(MPIX_CUDA_AWARE_SUPPORT) && MPIX_CUDA_AWARE_SUPPORT
    printf("This MPI library has CUDA-aware support.\n", MPIX_CUDA_AWARE_SUPPORT);
#elif defined(MPIX_CUDA_AWARE_SUPPORT) && !MPIX_CUDA_AWARE_SUPPORT
    printf("This MPI library does not have CUDA-aware support.\n");
#else
    printf("This MPI library cannot determine if there is CUDA-aware support.\n");
#endif /* MPIX_CUDA_AWARE_SUPPORT */

    printf("Run time check:n");
#if defined(MPIX_CUDA_AWARE_SUPPORT)
    if (1 == MPIX_Query_cuda_support()) {
        printf("This MPI library has CUDA-aware support.\n");
    } else {
        printf("This MPI library does not have CUDA-aware support.\n");
    }
#else /* !defined(MPIX_CUDA_AWARE_SUPPORT) */
    printf("This MPI library cannot determine if there is CUDA-aware support.\n");
#endif /* MPIX_CUDA_AWARE_SUPPORT */

    return 0;
}

10.7.17. How do I limit how much CUDA IPC memory is held in the registration cache?

As mentioned earlier, the Open MPI library will make use of CUDA IPC support where possible to move the GPU data quickly between GPUs that are on the same node and same PCI root complex. The library holds on to registrations even after the data transfer is complete as it is expensive to make some of the CUDA IPC registration calls. If you want to limit how much memory is registered, you can use the mpool_rgpusm_rcache_size_limit MCA parameter. For example, this sets the limit to 1000000 bytes:

shell$ mpirun --mca mpool_rgpusm_rcache_size_limit 1000000 ...

When the cache reaches this size, it will kick out the least recently used until it can fit the new registration in.

There also is the ability to have the cache empty itself out when the limit is reached:

shell$ mpirun --mca mpool_rgpusm_rcache_empty_cache 1 ...

10.7.18. What are some guidelines for using CUDA and Open MPI with Omni-Path?

When developing CUDA-aware Open MPI applications for OPA-based fabrics, the PSM2 transport is preferred and a CUDA-aware version of PSM2 is provided with all versions of the Intel Omni-Path IFS software suite.

Error

TODO Are Intel/OPA references still correct?

The PSM2 library provides a number of settings that will govern how it will interact with CUDA, including PSM2_CUDA and PSM2_GPUDIRECT, which should be set in the environment before MPI_Init() is called. For example:

shell$ mpirun -x PSM2_CUDA=1 -x PSM2_GPUDIRECT=1 --mca mtl psm2 mpi_hello

In addition, each process of the application should select a specific GPU card to use before calling MPI_Init(), by using cudaChooseDevice(), cudaSetDevice() and similar. The chosen GPU should be within the same NUMA node as the CPU the MPI process is running on. You will also want to use the mpirun --bind-to-core or --bind-to-socket option to ensure that MPI processes do not move between NUMA nodes. See the section on NUMA Node Issues, for more information.

For more information see the Intel Performance Scaled Messaging 2 (PSM2) Programmer’s Guide and the Intel Omni-Path Performance Tuning Guide, which can be found on the Intel Omni-Path web site.

Error

TODO Are Intel/OPA references still correct?


10.7.19. When do I need to select a CUDA device?

“mpi-cuda-dev-selection”

OpenMPI requires CUDA resources allocated for internal use. These are allocated lazily when they are first needed, e.g. CUDA IPC mem handles are created when a communication routine first requires them during a transfer. So, the CUDA device needs to be selected before the first MPI call requiring a CUDA resource. MPI_Init and most communicator related operations do not create any CUDA resources (guaranteed for MPI_Init, MPI_Comm_rank, MPI_Comm_size, MPI_Comm_split_type and MPI_Comm_free). It is thus possible to use those routines to query rank information and use those to select a GPU, e.g. using

int local_rank = -1;
{
    MPI_Comm local_comm;
    MPI_Comm_split_type(MPI_COMM_WORLD, MPI_COMM_TYPE_SHARED, rank, MPI_INFO_NULL, &local_comm);
    MPI_Comm_rank(local_comm, &local_rank);
    MPI_Comm_free(&local_comm);
}
int num_devices = 0;
cudaGetDeviceCount(&num_devices);
cudaSetDevice(local_rank % num_devices);

MPI internal CUDA resources are released during MPI_Finalize. Thus it is an application error to call cudaDeviceReset before MPI_Finalize is called.


10.7.20. How do I enable CUDA support in HCOLL collective component

HCOLL component supports CUDA GPU buffers for the following collectives:

MPI_Allreduce MPI_Bcast MPI_Allgather MPI_Ibarrier MPI_Ibcast MPI_Iallgather MPI_Iallreduce

To enable CUDA GPU buffer support in these collectives pass the following environment variables via mpirun:

shell$ mpirun -x HCOLL_GPU_ENABLE=1 -x HCOLL_ENABLE_NBC=1 ..

See nVidia HCOLL documentation for more information.