Run NCCL on Compute Engine instances

This page describes how to run NCCL tests with NCCL/gIB on A4X Max, A4X, A4, and A3 Ultra instances. The example in this page is for a two node NCCL test.

If you use schedulers such as Cluster Director, Cluster Toolkit, or GKE, you don't need to follow the steps on this page. Instead, see the following:

To run NCCL tests on A3 Mega and A3 High, see Create an A3 Mega or A3 High instance with GPUDirect enabled.

Before you begin

Before you run NCCL tests, make sure you install NCCL/gIB in your runtime environment.

Run NCCL tests

To run NCCL tests in a non-scheduled environment:

  1. Install CUDA 12.8 (or later).
  2. Install Open MPI.
  3. Set up non-interactive ssh logins among the compute instances.
  4. Build nccl-tests with MPI enabled. When building nccl-tests, set NCCL_HOME=/usr/local/gib.

To run the script shipped with the NCCL/gIB package:

# The script assumes binaries at /opt/nccl-tests/build/
$ /usr/local/gib/scripts/run_nccl_tests.sh -d /opt/nccl-tests/build/ -p 22 -t all_gather -m 0x0 -b 4K -e 16G a4-vm-1 a4-vm-2

We also publish a diagnostic container image with everything included at http://us-docker.pkg.dev/gce-ai-infra/gpudirect-gib/nccl-plugin-gib-diagnostic:latest.

The following example shows output on two A4 instances:

NCCL version 2.25.1+cuda12.8
#
#                                                              out-of-place                       in-place
#       size         count      type   redop    root     time   algbw   busbw #wrong     time   algbw   busbw #wrong
#        (B)    (elements)                               (us)  (GB/s)  (GB/s)            (us)  (GB/s)  (GB/s)
        4096            64     float    none      -1    59.97    0.07    0.06      0    57.49    0.07    0.07      0
        8192           128     float    none      -1    58.17    0.14    0.13      0    58.36    0.14    0.13      0
       16384           256     float    none      -1    59.07    0.28    0.26      0    59.03    0.28    0.26      0
       32768           512     float    none      -1    60.93    0.54    0.50      0    60.79    0.54    0.51      0
       65536          1024     float    none      -1    61.93    1.06    0.99      0    62.17    1.05    0.99      0
      131072          2048     float    none      -1    64.62    2.03    1.90      0    64.48    2.03    1.91      0
      262144          4096     float    none      -1    66.50    3.94    3.70      0    67.05    3.91    3.67      0
      524288          8192     float    none      -1    69.37    7.56    7.09      0    67.83    7.73    7.25      0
     1048576         16384     float    none      -1    117.2    8.95    8.39      0    113.7    9.22    8.64      0
     2097152         32768     float    none      -1    118.8   17.65   16.55      0    118.1   17.75   16.64      0
     4194304         65536     float    none      -1    122.2   34.32   32.17      0    122.6   34.22   32.08      0
     8388608        131072     float    none      -1    132.2   63.44   59.48      0    130.7   64.20   60.18      0
    16777216        262144     float    none      -1    139.2  120.49  112.96      0    139.7  120.07  112.56      0
    33554432        524288     float    none      -1    152.0  220.81  207.01      0    152.1  220.59  206.81      0
    67108864       1048576     float    none      -1    227.6  294.87  276.44      0    225.9  297.08  278.51      0
   134217728       2097152     float    none      -1    431.7  310.87  291.44      0    438.0  306.41  287.26      0
   268435456       4194304     float    none      -1    728.6  368.44  345.41      0    735.9  364.79  341.99      0
   536870912       8388608     float    none      -1   1404.2  382.33  358.44      0   1418.4  378.51  354.85      0
  1073741824      16777216     float    none      -1   2795.8  384.06  360.05      0   2768.9  387.79  363.55      0
  2147483648      33554432     float    none      -1   5440.1  394.75  370.08      0   5418.7  396.31  371.54      0
  4294967296      67108864     float    none      -1    10754  399.40  374.43      0    10746  399.67  374.69      0
  8589934592     134217728     float    none      -1    21434  400.77  375.72      0    21421  401.01  375.95      0
 17179869184     268435456     float    none      -1    42679  402.53  377.38      0    42792  401.48  376.38      0

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