Limitations of PyTorch on Cerebras
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Limitations of PyTorch on Cerebras¶
Floating Point Precision¶
Only mixed precision is supported on the Cerebras system. This means that the
weights are stored as
float32 but the computations happens using
float16. Casts are automatically inserted and becomes unnecessary to
insert them manually.
This is due to the architecture of the system itself and thus, there are no plans to support other precision modes at this time.
As of the 1.7.0 software release, it is not permitted to reprogram the fabric after initial programming. This means that multiple compiles are not supported and therefore the PyTorch compute graph must not change between iterations.
This means that there are a number of caveats as to how the training loop is allowed to be constructed, all of which are already addressed in our custom PyTorch runner classses. Refer to our implementations of the various hooks mentioned in PyTorch Runners.
mode = "train" and
mode = "eval" are supported in this release.
mode = "train_and_eval" is not supported due to the fact that it is not
permitted to reprogram the fabric after initial programming. So, the fabric
cannot be reprogrammed for
eval mode after having been programmed for
train in the same run.
Even when Cerebras supports reprogramming the fabric after initial programming in the future, it will still be ideal to avoid recompiling if possible, as recompiling and reprogramming the fabric can take a very long time.
Learning Rate Scheduler¶
Currently, we do not support the typical PyTorch learning rate scheduler paradigm. A typical PyTorch learning scheduler would compute a learning rate scalar and set the values of the learning rates in the optimizer parameter groups. However, due to current limitations of the system requiring static graphs, we cannot support this behaviour.
The supported PyTorch learning rate schedulers are listed on this page: Supported PyTorch Learning Rate Schedulers.
There are two different ways of scheduling a learning rate, depending on whether you are running in pipeline mode or in weight streaming mode.
We provide custom LR scheduler classes which pass in the scheduler information directly to the system’s communication manager so that the scheduler can be programmed into the fabric.
modelzoo.common.pytorch.optim.lr_scheduler for more details.
Weight Streaming mode¶
For weight streaming mode, we must specify the entire learning rate schedule
as a function of the global step. This means that the learning rate becomes less
of a scalar value and more of a tensor that depends on the value of the global
modelzoo.common.pytorch.optim.lr_scheduler for examples of this.
This does also mean that any optimizers being used need to be written in a way
such that the learning rate is not treated as a scalar value, but rather as a
modelzoo.common.pytorch.optim.AdamBase for an example of
Eval metrics are only allowed to return a single result value. Therefore, only the final metric value should be returned. No intermediate state or values can be retrieved at this time.