Optimize Graphs

with nGraph Compiler fusions

The nGraph Compiler is an optimizing compiler. As such, it provides a way to capture a given function graph and perform a series of optimization passes over that graph. The result is a semantically-equivalent graph that, when executed using any backend, has optimizations inherent at the hardware level: superior runtime characteristics to increase training performance or reduce inference latency.

There are several ways to describe what happens when we capture and translate the framework’s output of ops into an nGraph graph. Fusion is the term we shall use in our documentation; the action also can be described as: combining, folding, squashing, collapsing, or merging of graph functions.

Optimization passes may include algebraic simplifications, domain-specific simplifications, and fusion. Most passes share the same mode of operation (or the same operational structure) and consist of various stages (each one a step) where a developer can experiment with the intercepted or dynamic graph. These steps may be cycled or recycled as needed:

  1. Locate a list of potentially-transformable subgraphs in the given graph.
  2. Transform the selected candidates into semantically-equivalent subgraphs that execute faster, or with less memory (or both).
  3. Verify that the optimization pass performs correctly, with any or all expected transformations, with the NGRAPH_SERIALIZE_TRACING option, which serializes a graph in the json format after a pass.
  4. Measure and evaluate your performance improvements with NGRAPH_CPU_TRACING, which produces timelines compatible with chrome://tracing.

Optimizations can be experimented upon without using any backend by registering a pass with pass manager (Manager), calling run_passes on a function, and then inspecting the transformed graph.

Optimization passes can be programmed ahead of time if you know or can predict what your graph will look like when it’s ready to be executed (in other words: which ops can be automatically translated into nGraph Core ops).

The Interpreter is simply a backend providing reference implementations of ngraph ops in C++, with the focus on simplicity over performance.