Performance

The performance of the APM instance running in the Linux machine can be monitored using the SCHED_DEBUG APM parameter.

According to the wiki:

Set to non-zero to enable scheduler debug messages. When set to show “Slips” the scheduler will display a message whenever a scheduled task is delayed due to too much CPU load. When set to ShowOverruns the scheduled will display a message whenever a task takes longer than the limit promised in the task table.

Setting up option 3 using param set SCHED_DEBUG 3 outputs something like:

Scheduler overrun task[15] (34/20)
Scheduler overrun task[16] (26/20)
PERF: 4/1000 21647
PERF: 3/1000 23464
Flight battery warning
PERF: 5/1000 21938
Scheduler overrun task[5] (15/10)
PERF: 4/1000 23464
PERF: 4/1000 21506
Scheduler overrun task[15] (24/20)
PERF: 5/1000 22517
PERF: 3/1000 23606
Scheduler overrun task[5] (14/10)
Scheduler overrun task[16] (23/20)
PERF: 4/1000 21146
Flight battery warning

Understanding the output

PERF

PERF: 4/1000 23464

should be understood as:

4 out of 1000 tasks took longer than expected (20000 us since we are running at 50 Hz)

Overrun task

Scheduler overrun task[16] (23/20)

task 16 was overrun by 23 versus the 20 that was expected to take.

Refer to http://dev.ardupilot.com/wiki/code-overview-scheduling-your-new-code-to-run-intermittently/ for understanding more about tasks.