NODE_RANK causes DDP jobs to hang at `initializing ddp: GLOBAL_RANK: 0, MEMBER: 1/8`

[ajtao]

Hello,

In my compute cluster, all pytorch lightning code will hang when using more than 1 GPU.
It hangs right at "initializing ddp: GLOBAL_RANK: 0, MEMBER: 1/8"

Some relevant stats:

• 8-gpu ddp training
• NODE_RANK=0
• WORLD_SIZE=8

I have found that training does work if i unset NODE_RANK.

By instrumenting the pytorch-lightning code, i have observed that:

1. in the case when things hang, i.e. NODE_RANK is set, i've observed that select_accelerator() calls DDPHPCAccelerator every time, for all ranks. I guess this is because setting NODE_RANK seems to force use_torchelastic_ddp for all ranks.
2. in the case when NODE_RANK isn't set, then training works. In this case, it appears that the first call to select_accelerator() will call DDPAccelerator() for the first rank but then will call DDPHPCAccelerator for subsequent ranks.

My questions:

• Why is NODE_RANK being set causing my code to hang?
• Could the dependence on environment variables be explicitly stated / defined somewhere?
• I also would love to know: where is the spawn for DDP happening? I was trying to read the code, but just can't find where it happens.

What's your environment?

• Ubuntu 18.04
• pip
• 1.0.8 pytorch-lightning
• torch 1.6.0