[Bugfix] Fix custom all reduce nvlink check on multi node

[esmeetu]

pynvml.nvmlDeviceGetHandleByIndex will throw error when index is larger than the node's max device count index.

For example. Serving using two nodes:
node1: [0,1,2,3]
node2: [0,1]
and the final physical_device_ids will be [0,1,2,3,0,1]. Node 2 will throw error when it encounter 2 and 3 which is larger than its max device index(1).

Another example:
node1: [0,1]
node2: [0,1]
the final physical_device_ids will be [0,1,0,1]. So the NVLINK status will be repeatedly checked.

I think it is unnecessary to using all_gather to get physical_device_ids where we can get that from device_ids directly. Correct me if i miss anything. @youkaichao

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[youkaichao]

This is not true. Sometimes, e.g. in the testing code, we manually remove cuda visible devices variables, and the process can see all device ids. However, we only need to check nvlink topology within several gpus, rather than all.

[youkaichao]

What is your intended usecase? I think no one uses a TP group that crosses machine boundary. The designed use case is to use TP inside a node.

[esmeetu]

@youkaichao I want to serve a big model across multi nodes by using tensor parallelism. One node's memory cannot afford a model.

The designed use case is to use TP inside a node.

Do you mean custom all reduce feature? If so, we should check if using tp inside a node before nvlink status check.

[youkaichao]

we should check if using tp inside a node before nvlink status check

I would vote for this. maybe with another all_gather to get all host ips, and disable custom allreduce when we have multiple host ips.

[esmeetu]

@youkaichao I revert changes and check device_ids length with word_size to identify if serving on multi nodes before nvlink status check. Is that ok?

[youkaichao]

This might be wrong. We cannot assume CUDA_VISIBLE_DEVICES is always set. If not set, device_ids will be all devices (e.g. 8), but we should be able to use custom allreduce with world_size 2 or 4.

[esmeetu]

@youkaichao Please take a look again.

[youkaichao]

This is still not safe. It is possible, that one node uses gpu 0, 1 , while the other node uses 2, 3, and the world size is 4. In this case, we cannot use custom allreduce. However, this code will not capture it.

[youkaichao]

we should check if using tp inside a node before nvlink status check

If you want to detect whether a tp group spans across a node, all_gather to get all host ips is a feasible and sound idea. Why do you try so many ways to approximate it?

[esmeetu]

@youkaichao I think i was not clear about this part code. Your suggestion is good. Thanks for your careful review!

[youkaichao]

close as #5369 is a better alternative.