Contended locks are usually slow and can lead to performance and scalability
issues in multithread code. Lock elision will use memory transactions to under
certain conditions, to elide locks and improve performance.
Elision behavior can be modified by setting the following tunables in
the elision namespace:
The glibc.elision.enable tunable enables lock elision if the feature is
supported by the hardware. If elision is not supported by the hardware this
tunable has no effect.
Elision tunables are supported for 64-bit Intel, IBM POWER, and z System architectures.
The glibc.elision.skip_lock_busy tunable sets how many times to use a
non-transactional lock after a transactional failure has occurred because the
lock is already acquired. Expressed in number of lock acquisition attempts.
The default value of this tunable is ‘3’.
The glibc.elision.skip_lock_internal_abort tunable sets how many times
the thread should avoid using elision if a transaction aborted for any reason
other than a different thread’s memory accesses. Expressed in number of lock
acquisition attempts.
The default value of this tunable is ‘3’.
The glibc.elision.skip_lock_after_retries tunable sets how many times
to try to elide a lock with transactions, that only failed due to a different
thread’s memory accesses, before falling back to regular lock.
Expressed in number of lock elision attempts.
This tunable is supported only on IBM POWER, and z System architectures.
The default value of this tunable is ‘3’.
The glibc.elision.tries sets how many times to retry elision if there is
chance for the transaction to finish execution e.g., it wasn’t
aborted due to the lock being already acquired. If elision is not supported
by the hardware this tunable is set to ‘0’ to avoid retries.
The default value of this tunable is ‘3’.
The glibc.elision.skip_trylock_internal_abort tunable sets how many
times the thread should avoid trying the lock if a transaction aborted due to
reasons other than a different thread’s memory accesses. Expressed in number
of try lock attempts.
The default value of this tunable is ‘3’.