7.8 Speculation Options and Tuning

If a speculative buffer is full when a speculative tracing action is attempted, no data is stored in the buffer and a drop count is incremented. In this situation, dtrace generates a message similar to the following:

dtrace: 38 speculative drops

Speculative drops do not prevent the full speculative buffer from being copied into the principal buffer when it is committed. Similarly, speculative drops can occur even if drops were experienced on a speculative buffer that were ultimately discarded. Speculative drops can be reduced by increasing the speculative buffer size, which is tuned by using the specsize option. The specsize option can be specified with any size suffix. The resizing policy of this buffer is dictated by the bufresize option.

Speculative buffers might be unavailable when speculation is called. If buffers that have not yet been committed or discards exist, dtrace generates a message similar to the following:

dtrace: 1 failed speculation (no speculative buffer available)

You can reduce the likelihood of failed speculations of this nature by increasing the number of speculative buffers by specifying the nspec option. The value of nspec defaults to 1.

Also, speculation can fail if all speculative buffers are busy. In this case, an error message similar to the following is displayed:

dtrace: 1 failed speculation (available buffer(s) still busy)

This error message indicates that speculation was called after commit was called for a speculative buffer, but before that buffer was actually committed on all CPUs. You can reduce the likelihood of failed speculations of this nature by increasing the rate at which CPUs are cleaned by using the cleanrate option. The value of cleanrate defaults to 101.