Interpreting Perfetto trace visualization

[lucaslingle]

When collecting a trace using the Jax profiler, the Perfetto UI shows an expanded view and a compact view.
For example, in the attached screenshot, I have expanded the visualization for TPU 7 only.

Notably, the compact view looks very different, and appears to show a 1-second-long period of inactivity on all the TPUs between 00:00:01 and 00:00:02, whereas the expanded view does not.

What does the compact view purport to show? Thanks for any insights you can provide.