Chapter 16 Options and Tunables

To allow for customization, DTrace affords its consumers several important degrees of freedom. To minimize the likelihood of requiring specific tuning, DTrace is implemented using reasonable default values and flexible default policies. However, situations may arise that require tuning the behavior of DTrace on a consumer-by-consumer basis. This chapter describes the DTrace options and tunables and the interfaces you can use to modify them.

Consumer Options

DTrace is tuned by setting or enabling options. The available options are described in the table below. For some options, dtrace(1M) provides a corresponding command-line option.

Table 16–1 DTrace Consumer Options


Option Name	Value	dtrace(1M) Alias	Description	See Chapter
`aggrate`	`time`		Rate of aggregation reading	Chapter 9, Aggregations
`aggsize`	`size`		Aggregation buffer size	Chapter 9, Aggregations
`bufresize`	`auto` or `manual`		Buffer resizing policy	Chapter 11, Buffers and Buffering
`bufsize`	`size`	`-b`	Principal buffer size	Chapter 11, Buffers and Buffering
`cleanrate`	`time`		Cleaning rate. Must be specified in number-per-second with the `hz` suffix.	Chapter 13, Speculative Tracing
`cpu`	`scalar`	`-c`	CPU on which to enable tracing	Chapter 11, Buffers and Buffering
`defaultargs`	—		Allow references to unspecified macro arguments	Chapter 15, Scripting
`destructive`	—	`-w`	Allow destructive actions	Chapter 10, Actions and Subroutines
`dynvarsize`	`size`		Dynamic variable space size	Chapter 3, Variables
`flowindent`	—	`-F`	Indent function entry and prefix with `->`; unindent function return and prefix with `<-`	Chapter 14, dtrace(1M) Utility
`grabanon`	—	`-a`	Claim anonymous state	Chapter 36, Anonymous Tracing
`jstackframes`	`scalar`		Number of default stack frames `jstack()`	Chapter 10, Actions and Subroutines
`jstackstrsize`	`scalar`		Default string space size for `jstack()`	Chapter 10, Actions and Subroutines
`nspec`	`scalar`		Number of speculations	Chapter 13, Speculative Tracing
`quiet`	—	`-q`	Output only explicitly traced data	Chapter 14, dtrace(1M) Utility
`specsize`	`size`		Speculation buffer size	Chapter 13, Speculative Tracing
`strsize`	`size`		String size	Chapter 6, Strings
`stackframes`	`scalar`		Number of stack frames	Chapter 10, Actions and Subroutines
`stackindent`	`scalar`		Number of whitespace characters to use when indenting `stack()` and `ustack()` output	Chapter 10, Actions and Subroutines
`statusrate`	`time`		Rate of status checking
`switchrate`	`time`		Rate of buffer switching	Chapter 11, Buffers and Buffering
`ustackframes`	`scalar`		Number of user stack frames	Chapter 10, Actions and Subroutines

Values that denote sizes may be given an optional suffix of k, m, g, or t to denote kilobytes, megabytes, gigabytes, and terabytes respectively. Values that denote times may be given an optional suffix of ns, us, ms, s or hz to denote nanoseconds, microseconds, milliseconds, seconds, and number-per-second, respectively.

Modifying Options

Options may be set in a D script by using #pragma D followed by the string option and the option name. If the option takes a value, the option name should be followed by an equals sign (=) and the option value. The following examples are all valid option settings:

#pragma D option nspec=4
#pragma D option grabanon
#pragma D option bufsize=2g
#pragma D option switchrate=10hz
#pragma D option aggrate=100us
#pragma D option bufresize=manual

The dtrace(1M) command also accepts option settings on the command-line as an argument to the -x option. For example:

# dtrace -x nspec=4 -x grabanon -x bufsize=2g \
    -x switchrate=10hz -x aggrate=100us -x bufresize=manual

If an invalid option is specified, dtrace indicates that the option name is invalid and exits:

# dtrace -x wombats=25
dtrace: failed to set option -x wombats: Invalid option name
#

Similarly, if an option value is not valid for the given option, dtrace will indicate that the value is invalid:

# dtrace -x bufsize=100wombats
dtrace: failed to set option -x bufsize: Invalid value for specified option
#

If an option is set more than once, subsequent settings overwrite earlier settings. Some options, such as grabanon, may only be set. The presence of such an option sets it, and you cannot subsequently unset it.

Options that are set for an anonymous enabling will be honored by the DTrace consumer that claims the anonymous state. See Chapter 36, Anonymous Tracing for information about enabling anonymous tracing.