The GNU C library has a number of magic constants that were decided based on performance and resource data available when they were first introduced. Those constants may be suboptimal for some loads and may have even been rendered incorrect due to advances in other components or hardware. Further, there are a number of global configuration variables that were added over the years to work around the problems posed by such magic constants (the MMAP_THRESHOLD in malloc is one such example). These variables have ad hoc names and each have their own scheme of initialization and maintenance. A tunables framework aims to provide a layer that manages such global configuration and provides a unified interface to programmers, system administrators, and integrators to tweak this configuration. This talk describes the architecture of this layer and the interface it provides and also describes the potential roadmap for this feature in glibc.

1. Presented by
Date
Event
Tunables
The Future of Platform Optimization
Selection in glibc
Siddhesh Poyarekar
BK16-111 March 7, 2016
Linaro Connect BKK16

2. Outline
● What problem are we trying to solve?
● Goals
− Tunables for users
− Tunables for distribution maintainers
● Roadmap (a.k.a The future of the future)

3. The Problem
● Constants in glibc
− Decided years ago based on the state of the art
− May not be relevant anymore
− May be heuristic (64k ought to be enough for anybody!)
● Naming consistency
− MALLOC_MMAP_THRESHOLD_: what's with the trailing underscore?!
− Distribution-specific tunables

4. Goals
● For Users
− Per-process tunable values
− Per-user tunable values
− System-wide tunable values
− Tweak tunables at runtime
● For Distribution maintainers
− Add distribution-specific tunables
− Distribution defaults for tunables

5. Tunables for users
● One environment variable to rule them all!
− GLIBC_TUNABLES
− Colon-separated name=value pairs
− e.g. GLIBC_TUNABLES=”glibc.malloc.check=3:glibc.malloc.arena_max=1”

6. Tunables for users – the other plan
● One environment variable per tunable
− Standard environment variable names
− e.g. GLIBC_MALLOC_CHECK=2

7. Tunables for users – the future
Coming up, stay tuned!

8. Tunables for distributions
● Add distribution-specific tunables
− Tunables relevant to customer base
− More importantly, NOT relevant upstream
− Users know which tunables are supported across distributions
− e.g. redhat.malloc.allocator=tcmalloc
● Distribution defaults
− e.g. disable per-thread arenas for cloud images with less than 1GB RAM

9. Roadmap (cue hand-waving...)
● System-wide and per-user configuration files
− ~/.sysctl.user.conf
− /etc/sysctl.user.conf
− Figure out performance cost
− Used to set distribution defaults
● Tweaking tunables at runtime
− echo foo > /proc/PID/top_namespace/tunable_namespace/tunable

10. Thank you!
siddhesh@linaro.org