Dynamic Parallelism in TorchScript¶
In this tutorial, we introduce the syntax for doing dynamic inter-op parallelism in TorchScript. This parallelism has the following properties:
- dynamic - The number of parallel tasks created and their workload can depend on the control flow of the program.
- inter-op - The parallelism is concerned with running TorchScript program fragments in parallel. This is distinct from intra-op parallelism, which is concerned with splitting up individual operators and running subsets of the operator’s work in parallel.
The two important APIs for dynamic parallelism are:
torch.jit.fork(fn : Callable[..., T], *args, **kwargs) -> torch.jit.Future[T]
torch.jit.wait(fut : torch.jit.Future[T]) -> T
A good way to demonstrate how these work is by way of an example:
import torch def foo(x): return torch.neg(x) @torch.jit.script def example(x): # Call `foo` using parallelism: # First, we "fork" off a task. This task will run `foo` with argument `x` future = torch.jit.fork(foo, x) # Call `foo` normally x_normal = foo(x) # Second, we "wait" on the task. Since the task may be running in # parallel, we have to "wait" for its result to become available. # Notice that by having lines of code between the "fork()" and "wait()" # call for a given Future, we can overlap computations so that they # run in parallel. x_parallel = torch.jit.wait(future) return x_normal, x_parallel print(example(torch.ones(1))) # (-1., -1.)
fork() takes the callable
fn and arguments to that callable
kwargs and creates an asynchronous task for the execution of
fn can be a function, method, or Module instance.
fork() returns a
reference to the value of the result of this execution, called a
fork returns immediately after creating the async task,
not have been executed by the time the line of code after the
is executed. Thus,
wait() is used to wait for the async task to complete
and return the value.
These constructs can be used to overlap the execution of statements within a function (shown in the worked example section) or be composed with other language constructs like loops:
import torch from typing import List def foo(x): return torch.neg(x) @torch.jit.script def example(x): futures : List[torch.jit.Future[torch.Tensor]] =  for _ in range(100): futures.append(torch.jit.fork(foo, x)) results =  for future in futures: results.append(torch.jit.wait(future)) return torch.sum(torch.stack(results)) print(example(torch.ones()))
When we initialized an empty list of Futures, we needed to add an explicit
type annotation to
futures. In TorchScript, empty containers default
to assuming they contain Tensor values, so we annotate the list constructor
# as being of type
This example uses
fork() to launch 100 instances of the function
waits on the 100 tasks to complete, then sums the results, returning
Applied Example: Ensemble of Bidirectional LSTMs¶
Let’s try to apply parallelism to a more realistic example and see what sort of performance we can get out of it. First, let’s define the baseline model: an ensemble of bidirectional LSTM layers.
import torch, time # In RNN parlance, the dimensions we care about are: # # of time-steps (T) # Batch size (B) # Hidden size/number of "channels" (C) T, B, C = 50, 50, 1024 # A module that defines a single "bidirectional LSTM". This is simply two # LSTMs applied to the same sequence, but one in reverse class BidirectionalRecurrentLSTM(torch.nn.Module): def __init__(self): super().__init__() self.cell_f = torch.nn.LSTM(input_size=C, hidden_size=C) self.cell_b = torch.nn.LSTM(input_size=C, hidden_size=C) def forward(self, x : torch.Tensor) -> torch.Tensor: # Forward layer output_f, _ = self.cell_f(x) # Backward layer. Flip input in the time dimension (dim 0), apply the # layer, then flip the outputs in the time dimension x_rev = torch.flip(x, dims=) output_b, _ = self.cell_b(torch.flip(x, dims=)) output_b_rev = torch.flip(output_b, dims=) return torch.cat((output_f, output_b_rev), dim=2) # An "ensemble" of `BidirectionalRecurrentLSTM` modules. The modules in the # ensemble are run one-by-one on the same input then their results are # stacked and summed together, returning the combined result. class LSTMEnsemble(torch.nn.Module): def __init__(self, n_models): super().__init__() self.n_models = n_models self.models = torch.nn.ModuleList([ BidirectionalRecurrentLSTM() for _ in range(self.n_models)]) def forward(self, x : torch.Tensor) -> torch.Tensor: results =  for model in self.models: results.append(model(x)) return torch.stack(results).sum(dim=0) # For a head-to-head comparison to what we're going to do with fork/wait, let's # instantiate the model and compile it with TorchScript ens = torch.jit.script(LSTMEnsemble(n_models=4)) # Normally you would pull this input out of an embedding table, but for the # purpose of this demo let's just use random data. x = torch.rand(T, B, C) # Let's run the model once to warm up things like the memory allocator ens(x) x = torch.rand(T, B, C) # Let's see how fast it runs! s = time.time() ens(x) print('Inference took', time.time() - s, ' seconds')
On my machine, this network runs in
2.05 seconds. We can do a lot better!
Parallelizing Forward and Backward Layers¶
A very simple thing we can do is parallelize the forward and backward layers
BidirectionalRecurrentLSTM. For this, the structure of the computation
is static, so we don’t actually even need any loops. Let’s rewrite the
BidirectionalRecurrentLSTM like so:
def forward(self, x : torch.Tensor) -> torch.Tensor: # Forward layer - fork() so this can run in parallel to the backward # layer future_f = torch.jit.fork(self.cell_f, x) # Backward layer. Flip input in the time dimension (dim 0), apply the # layer, then flip the outputs in the time dimension x_rev = torch.flip(x, dims=) output_b, _ = self.cell_b(torch.flip(x, dims=)) output_b_rev = torch.flip(output_b, dims=) # Retrieve the output from the forward layer. Note this needs to happen # *after* the stuff we want to parallelize with output_f, _ = torch.jit.wait(future_f) return torch.cat((output_f, output_b_rev), dim=2)
In this example,
forward() delegates execution of
cell_f to another thread,
while it continues to execute
cell_b. This causes the execution of both the
cells to be overlapped with each other.
Running the script again with this simple modification yields a runtime of
1.71 seconds for an improvement of
Aside: Visualizing Parallelism¶
We’re not done optimizing our model but it’s worth introducing the tooling we have for visualizing performance. One important tool is the PyTorch profiler.
Let’s use the profiler along with the Chrome trace export functionality to visualize the performance of our parallelized model:
This snippet of code will write out a file named
parallel.json. If you
navigate Google Chrome to
chrome://tracing, click the
Load button, and
load in that JSON file, you should see a timeline like the following:
The horizontal axis of the timeline represents time and the vertical axis
represents threads of execution. As we can see, we are running two
instances at a time. This is the result of our hard work parallelizing the
Parallelizing Models in the Ensemble¶
You may have noticed that there is a further parallelization opportunity in our
code: we can also run the models contained in
LSTMEnsemble in parallel with
each other. The way to do that is simple enough, this is how we should change
forward method of
def forward(self, x : torch.Tensor) -> torch.Tensor: # Launch tasks for each model futures : List[torch.jit.Future[torch.Tensor]] =  for model in self.models: futures.append(torch.jit.fork(model, x)) # Collect the results from the launched tasks results : List[torch.Tensor] =  for future in futures: results.append(torch.jit.wait(future)) return torch.stack(results).sum(dim=0)
Or, if you value brevity, we can use list comprehensions:
def forward(self, x : torch.Tensor) -> torch.Tensor: futures = [torch.jit.fork(model, x) for model in self.models] results = [torch.jit.wait(fut) for fut in futures] return torch.stack(results).sum(dim=0)
Like described in the intro, we’ve used loops to fork off tasks for each of the models in our ensemble. We’ve then used another loop to wait for all of the tasks to be completed. This provides even more overlap of computation.
With this small update, the script runs in
1.4 seconds, for a total speedup
32%! Pretty good for two lines of code.
We can also use the Chrome tracer again to see where’s going on:
We can now see that all
LSTM instances are being run fully in parallel.
In this tutorial, we learned about
wait(), the basic APIs
for doing dynamic, inter-op parallelism in TorchScript. We saw a few typical
usage patterns for using these functions to parallelize the execution of
functions, methods, or
Modules in TorchScript code. Finally, we worked through
an example of optimizing a model using this technique and explored the performance
measurement and visualization tooling available in PyTorch.