[PATCH v5 0/3] Optimize CAS [BZ #28537]

["H.J. Lu" <hjl.tools@gmail.com>]

Changes in v5:

1. Put back __glibc_unlikely in  __lll_trylock and lll_cond_trylock.
2. Remove an atomic load in a CAS usage which has been already optimized.
3. Add an empty statement with a semicolon to a goto label for older
compiler versions.
4. Simplify CAS optimization.

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.

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

With all CAS optimizations applied, on a machine with 112 cores,
"make check -j28" under heavy load took

3093.18user 1644.12system 22:26.05elapsed 351%CPU

vs without CAS optimizations

3746.07user 1614.93system 22:02.91elapsed 405%CPU

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   | 49 +++++++++++++++++++++++--------------
 sysdeps/nptl/lowlevellock.h | 33 +++++++++++++++++--------
 3 files changed, 60 insertions(+), 34 deletions(-)

-- 
2.33.1