mbox

[v4,0/3] Optimize CAS [BZ #28537]

Message ID 20211110001614.2087610-1-hjl.tools@gmail.com
Headers

Message

H.J. Lu Nov. 10, 2021, 12:16 a.m. UTC 
  CAS instruction is expensive.  From the x86 CPU's point of view, getting
a cache line for writing is more expensive than reading.  See Appendix
A.2 Spinlock in:

https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/xeon-lock-scaling-analysis-paper.pdf

The full compare and swap will grab the cache line exclusive and cause
excessive cache line bouncing.

Optimize CAS in low level locks and pthread_mutex_lock.c:

1. Do an atomic load and skip CAS if compare may fail to reduce cache
line bouncing on contended locks.
2. Replace atomic_compare_and_exchange_bool_acq with
atomic_compare_and_exchange_val_acq to avoid the extra load.
3. Drop __glibc_unlikely in __lll_trylock and lll_cond_trylock since we
don't know if it's actually rare; in the contended case it is clearly not
rare.

This is the first patch set to optimize CAS.  I will investigate the rest
CAS usages in glibc after this patch set has been accepted.

H.J. Lu (3):
  Reduce CAS in low level locks [BZ #28537]
  Reduce CAS in __pthread_mutex_lock_full [BZ #28537]
  Optimize CAS in __pthread_mutex_lock_full [BZ #28537]

 nptl/lowlevellock.c         | 12 ++++-----
 nptl/pthread_mutex_lock.c   | 53 ++++++++++++++++++++++++++++---------
 sysdeps/nptl/lowlevellock.h | 29 +++++++++++++-------
 3 files changed, 67 insertions(+), 27 deletions(-)