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37.3 Hardware Capability Tunables

Tunable namespace: glibc.tune

Behavior of the GNU C Library can be tuned to assume specific hardware capabilities by setting the following tunables in the tune namespace:

Tunable: glibc.tune.hwcap_mask

This tunable supersedes the LD_HWCAP_MASK environment variable and is identical in features.

The AT_HWCAP key in the Auxilliary Vector specifies instruction set extensions available in the processor at runtime for some architectures. The glibc.tune.hwcap_mask tunable allows the user to mask out those capabilities at runtime, thus disabling use of those extensions.

Tunable: glibc.tune.hwcaps

The glibc.tune.hwcaps=-xxx,yyy,-zzz... tunable allows the user to enable CPU/ARCH feature yyy, disable CPU/ARCH feature xxx and zzz where the feature name is case-sensitive and has to match the ones in sysdeps/x86/cpu-features.h.

This tunable is specific to i386 and x86-64.

Tunable: glibc.tune.cpu

The glibc.tune.cpu=xxx tunable allows the user to tell the GNU C Library to assume that the CPU is xxx where xxx may have one of these values: generic, thunderxt88.

This tunable is specific to aarch64.

Tunable: glibc.tune.x86_data_cache_size

The glibc.tune.x86_data_cache_size tunable allows the user to set data cache size in bytes for use in memory and string routines.

This tunable is specific to i386 and x86-64.

Tunable: glibc.tune.x86_shared_cache_size

The glibc.tune.x86_shared_cache_size tunable allows the user to set shared cache size in bytes for use in memory and string routines.

Tunable: glibc.tune.x86_non_temporal_threshold

The glibc.tune.x86_non_temporal_threshold tunable allows the user to set threshold in bytes for non temporal store.

This tunable is specific to i386 and x86-64.


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