git: serialize: Optimize atomic_intr_cond_{enter,try,exit}()

[Sepherosa Ziehau]

commit 24befe94b31f6491cdca91f4f4f87bbd74787db1
Author: Sepherosa Ziehau <sephe at dragonflybsd.org>
Date:   Fri Jan 3 20:10:32 2014 +0800

    serialize: Optimize atomic_intr_cond_{enter,try,exit}()
    
    Use counter (30bits) of __atomic_intr_t as wait counter instead of request
    counter:
    - This avoids counter updates in atomic_intr_cond_try().
    - Move counter decrement from atomic_intr_cond_exit() to
      atomic_intr_cond_enter().
    - Try obtaining intr_cond first in atomic_intr_cond_enter().  If the try
      failed, counter would be incremented then.
    
    This reduces the number of locked bus cycle intructions.
    - For "try ok/exit" sequence: 4 -> 2.
    - For "try fail": 3 -> 1.
    - For uncontended "enter/exit" sequence: 3 -> 2
    
    For contended "enter/exit" sequence, this increases the number of locked
    bus cycle intructions from 3 to 4.  Compared with the sleep, this should
    be relatively cheap.
    
    Tested on 8 HT (i7-3770) box, using kq_accept_server/kq_connect_client:
    - 4/4 TX/RX rings device (BCM5719, using MSI-X), slight improvement.
    - 8/8 TX/RX rings device (Intel 82580, using MSI-X), slight improvement.
    - 1/2 TX/RX rings device (Intel 82599, using MSI), no observable
      improvement.

Summary of changes:
 sys/cpu/i386/include/atomic.h   | 15 +++++++--------
 sys/cpu/x86_64/include/atomic.h | 15 +++++++--------
 2 files changed, 14 insertions(+), 16 deletions(-)

http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/24befe94b31f6491cdca91f4f4f87bbd74787db1


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