When you run your usual script, instructions are executed in order. Using 🤗 Accelerate to deploy your script on several GPUs at the same time introduces a complication: while each process executes all instructions in order, some may be faster than others.
You might need to wait for all processes to have reached a certain point before executing a given instruction. For instance, you shouldn’t save a model before being sure every process is done with training, and you wouldn’t want to continue training before all the model weights have been loaded in. To do this, just write the following line in your code:
This instruction will block all the processes that arrive first until all the other processes have reached that point (if you run your script on just one GPU or CPU, this won’t do anything).
A few example cases of when to use this utility are listed below:
Some of these are utilized with the main_process_first() context manager, which utilizes wait_for_everyone() to run a particular set of code on the main process beforehand before triggering and launching the other processes
When downloading a dataset, you should download it first on the main process and then load the cached dataset afterward
load_dataset will perform a lock under the hood to stop multiple downloads from happening at once, but if you are downloading something
not using this library you should use this method.
with accelerator.main_process_first(): datasets = load_dataset("glue", "mrpc")
Under the hood this is the same as calling:
# First do something on the main process if accelerator.is_main_process: datasets = load_dataset("glue", "mrpc") else: accelerator.wait_for_everyone() # And then send it to the rest of them if not accelerator.is_main_process: datasets = load_dataset("glue", "mrpc") else: accelerator.wait_for_everyone()
When saving the
state_dict of the model, since you would normally save one file on just the main process
you should specify that:
if accelerator.is_main_process: model = accelerator.unwrap_model(model) torch.save(model.state_dict(), "weights.pth")
When loading in the
state_dict to a model, optimizer, or scheduler, you should wait
for all workers to have the weights loaded in before moving on to training
with accelerator.main_process_first(): state = torch.load("weights.pth") model.load_state_dict(state)
map() operation on multiple workers, such as tokenizing should be done on the
main process first, and then propagated to each one.
datasets = load_dataset("glue", "mrpc") with accelerator.main_process_first(): tokenized_datasets = datasets.map( tokenize_function, batched=True, remove_columns=["idx", "sentence1", "sentence2"], )
To have a check that works with a flag set by a particular process, the
check_trigger API should be used. Useful examples
for doing so can include situations such as using early stopping and monitoring the loss (as each loss slightly differs on each process).
for (x,y) in data_loader: logits = model(x) loss = loss_func(logits, y) # Assume `should_do_early_stopping` is a custom defined function that returns a conditional if should_do_early_stopping(loss): accelerator.set_trigger() # Later in the training script when we need to check for the breakpoint if accelerator.check_trigger(): break