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38.5 POSIX Thread Tunables

Tunable namespace: glibc.pthread

The behavior of POSIX threads can be tuned to gain performance improvements according to specific hardware capabilities and workload characteristics by setting the following tunables in the pthread namespace:

Tunable: glibc.pthread.mutex_spin_count

The glibc.pthread.mutex_spin_count tunable sets the maximum number of times a thread should spin on the lock before calling into the kernel to block. Adaptive spin is used for mutexes initialized with the PTHREAD_MUTEX_ADAPTIVE_NP GNU extension. It affects both pthread_mutex_lock and pthread_mutex_timedlock.

The thread spins until either the maximum spin count is reached or the lock is acquired.

The default value of this tunable is ‘100’.

Tunable: glibc.pthread.stack_cache_size

This tunable configures the maximum size of the stack cache. Once the stack cache exceeds this size, unused thread stacks are returned to the kernel, to bring the cache size below this limit.

The value is measured in bytes. The default is ‘41943040’ (forty mibibytes).

Tunable: glibc.pthread.rseq

The glibc.pthread.rseq tunable can be set to ‘0’, to disable restartable sequences support in the GNU C Library. This enables applications to perform direct restartable sequence registration with the kernel. The default is ‘1’, which means that the GNU C Library performs registration on behalf of the application.

Restartable sequences are a Linux-specific extension.

Tunable: glibc.pthread.stack_hugetlb

This tunable controls whether to use Huge Pages in the stacks created by pthread_create. This tunable only affects the stacks created by the GNU C Library, it has no effect on stack assigned with pthread_attr_setstack.

The default is ‘1’ where the system default value is used. Setting its value to 0 enables the use of madvise with MADV_NOHUGEPAGE after stack creation with mmap.

This is a memory utilization optimization, since internal glibc setup of either the thread descriptor and the guard page might force the kernel to move the thread stack originally backup by Huge Pages to default pages.

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