Selecting and Enabling Probes

Use the standard prex list command to list kernel probes and their attributes. For example, to list the name and keys attributes of the probe named pagein:

prex> list name=pagein keys
name=pagein keys=vm pageio io

To list the name and keys attributes of all probes in the io group:

prex> list name keys probes io 
name=biodone keys=io blockio
name=physio_start keys=io rawio
name=pagein keys=vm pageio io
name=pageout keys=vm pageio io
name=physio_end keys=io rawio
name=strategy keys=io blockio
prex>

The next step is to trace and enable the probes you need. For example, to select all probes whose keys specify thread, vm, and io, and prepare them for tracing:

prex> trace thread vm io    	# attach trace function to probes 	
prex> enable thread vm io	   # enable probes
prex> 

Unlike user-level probes, kernel probes are not automatically `traced' (in prex terminology) when prex attaches to the kernel. You must explicitly issue the trace and enable commands as shown above.

At this point the relevant probes are ready for tracing, but kernel tracing has not been globally enabled so no trace records are being written.