[PATCH 0/2] rwsem: performance enhancements for systems with manycores

[Tim Chen]

In this patchset, we introduce two optimizations to read write semaphore.
The first one reduces cache bouncing of the sem->count field
by doing a pre-read of the sem->count and avoid cmpxchg if possible.
The second patch introduces similar optimistic spining logic in
the mutex code for the writer lock acquisition of rw-sem.

Combining the two patches, in testing by Davidlohr Bueso on aim7 workloads
on 8 socket 80 cores system, he saw improvements of
alltests (+14.5%), custom (+17%), disk (+11%), high_systime
(+5%), shared (+15%) and short (+4%), most of them after around 500
users when i_mmap was implemented as rwsem.

Feedbacks on the effectiveness of these tweaks on other workloads
will be appreciated.


Alex Shi (1):
  rwsem: check the lock before cpmxchg in down_write_trylock and    
    rwsem_do_wake

Tim Chen (1):
  rwsem: do optimistic spinning for writer lock acquisition

 Makefile                    |    2 +-
 include/asm-generic/rwsem.h |    8 +-
 include/linux/rwsem.h       |    3 +
 init/Kconfig                |    9 +++
 kernel/rwsem.c              |   29 +++++++-
 lib/rwsem.c                 |  169 ++++++++++++++++++++++++++++++++++++++-----
 6 files changed, 195 insertions(+), 25 deletions(-)

-- 
1.7.4.4


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