Because DTrace buffers some aggregation data in the kernel, space might not be available when a new key is added to an aggregation. In this case, the data will be dropped, a counter will be incremented, and dtrace will generate a message indicating an aggregation drop. This situation rarely occurs because DTrace keeps long-running state (consisting of the aggregation's key and intermediate result) at user-level where space may grow dynamically. In the unlikely event that aggregation drops occur, you can increase the aggregation buffer size with the aggsize option to reduce the likelihood of drops. You can also use this option to minimize the memory footprint of DTrace. As with any size option, aggsize may be specified with any size suffix. The resizing policy of this buffer is dictated by the bufresize option. For more details on buffering, see Chapter 11, Buffers and Buffering. For more details on options, see Chapter 16, Options and Tunables.
An alternative method to eliminate aggregation drops is to increase the rate at which aggregation data is consumed at user-level. This rate defaults to once per second, and may be explicitly tuned with the aggrate option. As with any rate option, aggrate may be specified with any time suffix, but defaults to rate-per-second. For more details on the aggsize option, see Chapter 16, Options and Tunables.