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

["Paul A. Clarke" <pc@us.ibm.com>]

On Wed, Nov 10, 2021 at 10:41:50AM -0800, H.J. Lu wrote:
> 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

I read that as about 2% slower with your changes. Is that the desired result?

PC