RuntimeError: Expected to mark a variable ready only once.

在进行深度学习模型训练时,我们可能会遇到以下错误:

RuntimeError: Expected to mark a variable ready only once. This error is caused by one of the following reasons: 1) Use of a module parameter outside the `forward` function. Please make sure model parameters are not shared across multiple concurrent forward-backward passes. or try to use _set_static_graph() as a workaround if this module graph does not change during training loop.2) Reused parameters in multiple reentrant backward passes. For example, if you use multiple `checkpoint` functions to wrap the same part of your model, it would result in the same set of parameters been used by different reentrant backward passes multiple times, and hence marking a variable ready multiple times. DDP does not support such use cases in default. You can try to use _set_static_graph() as a workaround if your module graph does not change over iterations.

 这个错误的主要原因可能是,我们的模型在分布式训练过程中,对某个网络层计算了两次loss. 想要找到问题出在哪一层,可以用以下代码调试:

TORCH_DISTRIBUTED_DEBUG=DETAIL python xxx.py

这是可以看到出错的网络层:

 但,即使找到这个网络层,其实也比较难解决该问题。

简单粗暴的解决方案有以下两种:

方法一:

torch.nn.parallel.DistributedDataParallel(model, device_ids=[args.gpu], find_unused_parameters=False)

方法二:

ddp_model = DistributedDataParallel(model) 

ddp_model._set_static_graph()

方法二一般可以解决该问题。

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