glibc/README.tunables

			TUNABLE FRAMEWORK
			=================

Tunables is a feature in the GNU C Library that allows application authors and
distribution maintainers to alter the runtime library behaviour to match their
workload.

The tunable framework allows modules within glibc to register variables that
may be tweaked through an environment variable.  It aims to enforce a strict
namespace rule to bring consistency to naming of these tunable environment
variables across the project.  This document is a guide for glibc developers to
add tunables to the framework.

ADDING A NEW TUNABLE
--------------------

The TOP_NAMESPACE macro is defined by default as 'glibc'.  If distributions
intend to add their own tunables, they should do so in a different top
namespace by overriding the TOP_NAMESPACE macro for that tunable.  Downstream
implementations are discouraged from using the 'glibc' top namespace for
tunables they don't already have consensus to push upstream.

There are two steps to adding a tunable:

1. Add a tunable ID:

Modules that wish to use the tunables interface must define the
TUNABLE_NAMESPACE macro.  Following this, for each tunable you want to
add, make an entry in elf/dl-tunables.list.  The format of the file is as
follows:

TOP_NAMESPACE {
  NAMESPACE1 {
    TUNABLE1 {
      # tunable attributes, one per line
    }
    # A tunable with default attributes, i.e. string variable.
    TUNABLE2
    TUNABLE3 {
      # its attributes
    }
  }
  NAMESPACE2 {
    ...
  }
}

The list of allowed attributes are:

- type:			Data type.  Defaults to STRING.  Allowed types are:
			INT_32, SIZE_T and STRING.

- minval:		Optional minimum acceptable value.  For a string type
			this is the minimum length of the value.

- maxval:		Optional maximum acceptable value.  For a string type
			this is the maximum length of the value.

- env_alias:		An alias environment variable

- security_level:	Specify security level of the tunable.  Valid values:

			SXID_ERASE: (default) Don't read for AT_SECURE binaries and
				    removed so that child processes can't read it.
			SXID_IGNORE: Don't read for AT_SECURE binaries, but retained for
				     non-AT_SECURE subprocesses.
			NONE: Read all the time.

2. Call either the TUNABLE_SET_VALUE and pass into it the tunable name and a
   pointer to the variable that should be set with the tunable value.
   If additional work needs to be done after setting the value, use the
   TUNABLE_SET_VALUE_WITH_CALLBACK instead and additionally pass a pointer to
   the function that should be called if the tunable value has been set.

FUTURE WORK
-----------

The framework currently only allows a one-time initialization of variables
through environment variables and in some cases, modification of variables via
an API call.  A future goals for this project include:

- Setting system-wide and user-wide defaults for tunables through some
  mechanism like a configuration file.

- Allow tweaking of some tunables at runtime