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0014-pipeline-executor.md

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1. Summary

This proposal introduces Pipeline Executor: A runtime executor that schedules a list of Relay modules in pipeline to achieve task level parallelism to improve computation throughput.

2. Motivation

Currently more and more edge device inference deployments happen on SOC devices. Since SOC devices have heterogeneous chipset like GPU, FPGA, CPU, DSP, etc. To reach the best performance, there is a requirement to run an ML network in these heterogeneous chipsets. However, currently graph executor does not have parallelism logic, and the existing data parallelism solution only supports parallel on homogeneous chipset(device). Then, the only way to do batch processing on heterogeneous devices with TVM is to treat a whole ML network as a schedule unit and run it on different heterogeneous devices, but that would cause latency issue (low speed chipset becomes the latency bottleneck for single data processing).

Therefore, we need a runtime executor that can provide parallel scheduling functionality with a finer-grained schedule unit like subgraph (a group of operator with dependency relation) to be more efficient to use SOC heterogeneous hardware resource to achieve a better performance.

Benefits of Pipeline Executor

There are three benefits for Pipeline Executor

Pipeline Executor provides:

  • Compute a single network on multiple backends in parallel to improve performance.

  • Use RPC to perform distributed computation cross multiple remote devices.

  • Pipeline executor provide the capability to integrate non-DNN model function.

3. Guide-level explanation

Pipeline Executor is a runtime executor which implements pipeline execution logic for multiple subgraphs and relies on graph_executor for operator storage and execution.

This section introduces the use case for Pipeline Executor.

    1. Manually split/partition Relay module to a list of Relay modules and generate modules configuration (automatic module splitting is out of scope of this RFC and will be a future work).
    1. Use pipeline_executor to build a pipeline module with the subgraphs and configuration.
    1. Use pipeline_executor to load the pipeline module to run network in pipeline parallelism mode.

3.1. Manually Split relay module a list relay modules and generate modules configuration.

mod1, mod2, mod3 = my_manual_partitioner()
pipe_cfg = PipelineModuleConfig()

# Define pipeline inputs. Here I assume two inputs of mod1 and one input of mod3 are the pipeline inputs.
pipe_config.connect(pipe_config.pipe_input("data_0"), pipe_config[mod1].input("data_0"))
pipe_config.connect(pipe_config.pipe_input("data_1"), pipe_config[mod1].input("data_1"))
pipe_config.connect(pipe_config.pipe_input("data_2"), pipe_config[mod3].input("data_1"))

# Define pipeline outputs to be the first output of mod3.
pipe_config.connect(pipe_config[mod3].output(0), pipe_config.pipe_output("0"))

# Define connections.
pipe_config.connect(pipe_config[mod1].output(0), pipe_config[mod2].input("data_0")) # mod1.output(0) -> mod2.data_0
pipe_config.connect(pipe_config[mod2].output(0), pipe_config[mod3].input("data_1")) # mod2.output(0) -> mod3.data_1

# Print config for debugging
print(str(pipe_cfg))
# Inputs:
#   |- data_0: mod1.data_0
#   |- data_1: mod1.data_1
#   |- data_2: mod3.data_0
# Outputs:
#   |- output(0): mod3.output(0)
# Connections:
#   |- mod1.output(0) -> mod2.data_0
#   |- mod2.output(0) -> mod3.data_1

3.2. Use pipeline_executor to build pipeline module with the said subgraph and configuration.

The interface is mostly the same as the graph executor but accepts a pipeline configuration instead of a Relay module. Here is an example.

# Use the config to build a pipeline executor
with relay.build_config(opt_level=3):
    lib = pipeline_executor.build_pipeline(pipe_cfg)

3.3. Use pipeline_executor to load pipeline module to run network in pipeline parallism mode.

Pipeline executor works asynchronously. Unlike the blocking run API in graph executor, run API in pipeline executor is non-blocking. As a result, we could have the following scenario:

  1. set_input(): Push the input to the queue.
  2. run(): Launch a task with the first input in the queue.
  3. set_input(): Push the second input to the queue.
  4. set_input(): Push the third input to the queue.
  5. run(): Launch a task with the second input.
  6. get_output(): Get the output of the first input.
  7. run(): Launch a task with the third input.
  8. get_output(): Get the output of the second input.
  9. get_output(): Get the output of the third input.

As can be seen, get_output() can be called anytime to get the first available output in the result queue, and it will return an empty array if no output is ready.

Following is one example:

#...

datas = []
for _ in range(5):
    # Each data includes 3 tensors (i.e., data_0, data_1, data_2 for the pipeline).
    datas.append([np.full(shape[i], 0).astype("float32") for i in range(3)])

# Feed all available inputs.
for data in datas:
    pipeline_module.set_input("data_0", data[0])
    pipeline_module.set_input("data_1", data[1])
    pipeline_module.set_input("data_2", data[2])
    pipeline_module.run()

# Get all outputs.
while pipeline_module.has_next_output():
  pipeline_outputs.append(pipeline_module.get_output())

4 Reference-level explanation

This section introduces the underlying techniques for the pipeline executor. The figure below briefly illustrates the workflow of the system

Pipeline executor architecture meta-schedule-workflow

Manually construct the subgraph meta-schedule-workflow

How pipeline executor runtime work meta-schedule-workflow

The pipeline executor schedule logic meta-schedule-workflow

The network pipeline compute effect meta-schedule-workflow

5. Drawbacks

Pipeline executor currently needs manually subgraph splitting and configuration construction. Further graph splitting feature would do automatically split.

6. Rationale and alternative

whithout pipeline executor, current tvm still can run network in Heterogeneous hardware but that running is serialized instead of parallel run operator in different hardware

7. Prior art

Schedule Primtive like Vectorize etc the schedule primtive implement data parallism on same device.

8. Unresolved questions

Automatically split compute graph

9. Future possibilities

Using Automatic Graph Split feature to construct pipeline subgraph and configuration.

This feature not in this RFC scope. the logic as following.

this future solution include 3 steps, 1. Operator Auto tune, 2. Graph dependency tree build and balance, 3. Graph Auto Tune. following are more detail.

1. Operator Auto Tune :

  • a. In operator Auto tune tune section, user would using existing tuning logic to tune the every operator, but the tune would separately and serialized happen in all target involved by pipeline executor.

  • b. After operator tune done , here can get performance data, for example , con2d_0 best perf in GPU is 3ms, in VTA is 2ms etc, this perf data would get used in later Graph dependency tree build balance step.

2. Graph dependency tree build balance

  • a. Initialize a DAG, the node of the DAG is subgraph, initially for a N node DAG, first [1, N -1] node mapping to [1 , N-1] layer(compute density operator and others) of original compute graph, the number N node is mapping to [N, M] layer , M here is the original compute layer number.

  • b. by using the perf data generated in 3.1.1.b , every dependency tree node can get a time consume value, the time consume value for difference node not at beginning is not same, then we call this DAG is not balanced in weight of node, by using the way to adjust the node(subgraph) scope(how many operator in this node), we make every node of the DAG become same or value closed on weight(same time consume), then such DAG is a graph split solution, here we use DAG is to record the parent/child relation that child only can run after parent runned, and the scope adjustment only can hapen between parent and child.

3. Graph Auto Tune.

  • a. 2 can generate more than one subgraph split solution DAG, in this step, Graph Auto Tune would try these multiple solution to get best configuration.

after 1. 2. 3. , here can get an automatic graph split configuration.