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 - is_secure: Specify whether the tunable should be read for setuid binaries. True allows the tunable to be read for setuid binaries while false disables it. Note that even if this is set as true and the value is read, it may not be used if it does not validate against the acceptable values or is not considered safe by the module. 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