### 39.4 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.