15.3 Using Cacheable Predicates

You use DTrace predicates to filter unwanted data from the experiment by tracing data only if a specified condition is found to be true. When enabling many probes, you generally use predicates of a form that identifies a specific thread, or threads of interest, such as /self->traceme/ or /pid == 12345/. Although many of these predicates evaluate to a false value for most threads in most probes, the evaluation itself can become costly when done for many thousands of probes. To reduce this cost, DTrace caches the evaluation of a predicate if it includes only thread-local variables, such as /self->traceme/, or for immutable variables, such as /pid == 12345/. The cost of evaluating a cached predicate is much less than the cost of evaluating a non-cached predicate, especially if the predicate involves thread-local variables, string comparisons, or other relatively costly operations. While predicate caching is transparent to the user, it does require some guidelines for constructing optimal predicates. Some guidelines for constructing optimal predicates are outlined in the following table.

Cacheable	Uncacheable
`self->mumble`	`mumblecurthread` `mumblepid` `tid`
`execname`	`curpsinfo->pr_fname` `((struct task_struct *)curthread)->comm`
`pid`	`curpsinfo->pr_pid` `((struct task_struct *)curthread)->pid`
`tid`	`curlwpsinfo->pr_lwpid` `((struct task_struct *)curthread)->pid`
`curthread`	`curthread->any_member` `curlwpsinfo->any_member` `curpsinfo->any_member`

The following example uses an associative array in the predicate and is not cacheable:

syscall::read:entry
{
  follow[pid, tid] = 1;
}

lockstat:::
/follow[pid, tid]/
{}

syscall::read:return
/follow[pid, tid]/
{
  follow[pid, tid] = 0;
}

Using a cacheable, thread-local variable, per the following example, is preferable:

syscall::read:entry
{
  self->follow = 1;
}

lockstat:::
/self->follow/
{}

syscall::read:return
/self->follow/
{
  self->follow = 0;
}

For a predicate to be cacheable, it must consist exclusively of cacheable expressions. All of the following predicates all cacheable:

/execname == "myprogram"/

/execname == $$1/

/pid == 12345/

/pid == $1/

/self->traceme == 1/

The following examples, which use global variables, are not cacheable:

/execname == one_to_watch/

/traceme[execname]/

/pid == pid_i_care_about/

/self->traceme == my_global/