Tip 15: drops and dynvardrops

If the DTrace main buffer overflows because of the system’s inability to drain it quickly enough, “drops” warnings will be printed (for example, dtrace: 710 drops on CPU 0). To eliminate these warnings, the principal buffer size can be increased with either the -b option at the command line or the bufsize tunable option. The default is 4MB per CPU. Another fix may be to increase the switchrate tunable option to flush the buffers more quickly than the default of once per second.

Another type of warning is dynvardrops, when the dynamic variable buffer overflows. The dynvarsize tunable can be used to increase the size of this buffer. This often happens when assigning dynamic variables and then forgetting to free them after they are no longer needed, and over time they can fill the dynamic variable buffer to the point of dynvardrops. Rather than increase the dynvarsize variable, first inspect your D script to determine whether there are variables you should be freeing but are not. Note dynvardrops must be eliminated for correct results.