Horovod is an open-source distributed deep learning framework that makes it easy to train models across multiple GPUs and nodes. Developed by Uber, it is designed to improve the speed and efficiency of training large-scale deep learning models by leveraging data parallelism. Horovod is particularly popular for its ease of integration with existing deep learning frameworks like TensorFlow, Keras, and PyTorch.
One common issue that users may encounter when using Horovod is that the system hangs during the initialization phase. This can be frustrating as it prevents the training process from starting, leaving users with no clear error message or indication of what went wrong.
When Horovod hangs during initialization, the process appears to be stuck, and no progress is made. This typically occurs right after launching the training script, and no further output is generated in the console or logs.
The primary cause of Horovod hanging during initialization is often a mismatch between the number of processes specified and the number of available GPUs. Horovod requires that each process be assigned to a GPU, and if there is a discrepancy, it can lead to the system hanging as it waits for resources that are not available.
When launching a Horovod job, you specify the number of processes using the --np flag. This number should match the number of GPUs available on the nodes being used. If there are more processes than GPUs, some processes will not be able to find a GPU to execute on, causing the initialization to hang.
To fix the issue of Horovod hanging during initialization, follow these steps:
First, check the number of GPUs available on your system. You can use the nvidia-smi command to list all available GPUs:
nvidia-smi
This command will display a list of GPUs along with their status. Ensure that the number of GPUs matches the number of processes you intend to run.
When launching your Horovod job, ensure that the --np flag matches the number of available GPUs. For example, if you have 4 GPUs, your command should look like this:
horovodrun -np 4 -H localhost:4 python train.py
Replace train.py with the name of your training script.
If you are running Horovod across multiple nodes, ensure that each host is correctly configured with the appropriate number of GPUs. The -H flag should reflect the correct distribution of processes across hosts.
For example, if you have two hosts each with 2 GPUs, your command should be:
horovodrun -np 4 -H host1:2,host2:2 python train.py
For more information on configuring and troubleshooting Horovod, refer to the following resources:
By ensuring that your process allocation matches the available resources, you can prevent initialization hangs and ensure smooth operation of your Horovod jobs.
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