Tunable parameters

Autotuned parameters

Autotuned parameters are kernel variables whose default values are automatically tuned according to the amount of memory that the system has.

For large memory systems (those with more than 1GB of general purpose memory) a distinction is made between autouned parameters which consume kernel virtual memory and those that simply manage physical memory.

Parameters related to virtual memory are calculated according to the amount of general purpose memory (memory that is managed by page tables, not dedicated memory) present on the system. These parameters are defined by a line in the Autotune(4dsp), file, where the first field is the parameter name, and the second field is ``PHYS''. In this documentation, these parameters are identified in tables by ``PHYS'' in the ``Auto'' column.
Parameters that are used for calculations that consume kernel virtual memory are also calculated according to the amount of general purpose memory available, but on systems that have greater than 1GB of memory, these parameters are limited by the amount of kernel virtual memory available. If there is no ``PHYS'' entry for a parameter in the Autotune(4dsp) file, the autotune value is calculated using this method. In this documentation, these parameters are identified in tables by ``KVM'' in the ``Auto'' column.

For autotuned parameters, a table of information is provided from which mtune entries are created. This table is passed to the kernel to allow autotuning of these parameters when the system is initialized.

The mtune lines are recalculated whenever idtune or idbuild is run. The command idtune -g tunable-name provides the current, default, minimum, and maximum information for the tunable based on the amount of memory currently available.

The files specifying default, minimum and maximum values are in /etc/conf/autotune.d. The format for a line in an autotune file is:

   ``tunable-name''  ``record-type''  ``LINEAR|STEP''  ``memsize (in MB)''  ``value''

The fields are:

``tunable-name'': The name of the tunable parameter.
``record-type'': A ``PHYS'' entry in this field indicates that the parameter is autotuned using a method based on the amount of general purpose memory available. If the ``record type'' is ``PHYS'', subsequent fields are not used. Otherwise, the parameter is autotuned using a (default) method that takes both general purpose and kernel virtual memory into account. In this case, information is provided about whether the line pertains to the default value (``DEF''), minimum value (``MIN''), or maximum value (``MAX'').
``LINEAR | STEP'': Indicates whether values for larger amounts of memory should be calculated linearly or as a step function If the entry in the ``record type'' field is ``PHYS'' this field is not used.
``memsize'': The number of megabytes this line specifies information about. If the entry in the ``record type'' field is ``PHYS'' this field is not used.
``value'': The value of the parameter for the specified amount of memory. If the entry in the ``record type'' field is ``PHYS'' this field is not used.

The following is an example from /etc/conf/autotune.d/s5:

BUFHWM DEF LINEAR 4 200 BUFHWM DEF LINEAR 8 400 BUFHWM DEF LINEAR 16 1024 BUFHWM DEF LINEAR 32 2048

BUFHWM MIN STEP 4 10

BUFHWM MAX LINEAR 4 512 BUFHWM MAX LINEAR 8 1024 BUFHWM MAX LINEAR 16 4096 BUFHWM MAX LINEAR 32 8192

Do not edit these files directly; instead, use the System Tuner or idtune(1M). If you add memory after tuning one of these tunables, perform an idtune -d tunable_name to reset autotuning.

For some parameters, the default and maximum values are autotuned; for others, only the maximum value is autotuned.