Fast and wide modes

You can gather data for events in the following modes:

Wide mode
The instrumented kernel uses as many buffer slots as are necessary to fully log the event. The amount of space is theoretically unlimited and can span several kilobytes for a single event. Most of the time, it doesn't exceed four 16-byte spaces.
Fast mode
The instrumented kernel uses only one buffer slot per event.

In general, wide mode generates several times more data than fast mode.

Note: Fast mode doesn't simply clip the tail end of the event data that you'd get in wide mode; fast mode summarizes the most important aspects of the event in a single buffer slot. Thus, the first element of an event in wide mode might not be the same as the same event in fast mode.

You can set fast and wide mode for all classes, specific classes, and even specific events in a class; some can be fast while others are wide. We'll describe how to set this in the Capturing Trace Data chapter.

For the specific output differences between fast and wide mode, see the Current Trace Events and Data appendix.