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Using the Intercept Layer for OpenCL Applications with Chrome Tracing

The Intercept Layer for OpenCL Applications includes support for generating JSON files compatible with the Google Chrome built-in profiler front end. This document describes how to use the Intercept Layer for OpenCL Applications to visualize how an OpenCL application executes using Chrome Tracing.

Background and Setup

The Chrome Tracing Format is described here.

To start up the Chrome Tracing front end, open Chrome and enter chrome://tracing as your "URL". This should give you a Window that looks like this one:

Empty Chrome Tracing Window

Configuring Chrome Tracing

There are (currently) two Chrome Tracing-related controls for the Intercept Layer for OpenCL Applications:

  • ChromeCallLogging: This is the control for tracing OpenCL host APIs. It will plot OpenCL calls for each thread of the host application, similar to those dumped when CallLogging is enabled.
  • ChromePerformanceTiming: This is the control for tracing OpenCL device commands. It will plot OpenCL commands for each command queue created by the application, similar to those dumped when DevicePerformanceTiming is enabled.

Collecting Chrome Tracing Data

After setting one of these two controls (or both!), run your application, and you should see a "CLIntercept_trace.json" file in your CLIntercept_Dump directory.

Visualizing Chrome Tracing Data

After collecting a "CLIntercept_Trace.json" file, simply click the "load" button in the "chrome::tracing" UI, or drag your file into Chrome.

If all goes well you will see a timegraph like this one:

Chrome Tracing Example

You can navigate around the timegraph with 'wasd' controls similar to many popular games: w zooms in, s zooms out, a goes backwards in time, and d moves forwards in time.

Let's zoom in a bit and look at the what's in the timegraph:

Chrome Tracing Detail

Overhead

Empirically, the overhead of Chrome Tracing is very low. The difference in scores between a run of LuxMark without Chrome Tracing vs. enabling ChromeCallLogging and ChromePerformanceTiming was less than 1%, and the trace file size for the 2+ minutes of execution was 26MB.

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