Reverse the logic for ADAPTIVE_SX option and enable it by default.

Introduce for this operation the reverse NO_ADAPTIVE_SX option.
The flag SX_ADAPTIVESPIN to be passed to sx_init_flags(9) gets suppressed
and the new flag, offering the reversed logic, SX_NOADAPTIVE is added.

Additively implements adaptive spininning for sx held in shared mode.
The spinning limit can be handled through sysctls in order to be tuned
while the code doesn't reach the release, after which time they should
be dropped probabilly.

This change has made been necessary by recent benchmarks where it does
improve concurrency of workloads in presence of high contention
(ie. ZFS).

KPI breakage is documented by __FreeBSD_version bumping, manpage and
UPDATING updates.

Requested by:	jeff, kmacy
Reviewed by:	jeff
Tested by:	pho

1 files changed, 8 insertions, 0 deletions

diff --git a/UPDATING b/UPDATING
index f60c804..a036380 100644
--- a/UPDATING
+++ b/UPDATING
@@ -22,6 +22,14 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 8.x IS SLOW:
 	to maximize performance.  (To disable malloc debugging, run
 	ln -s aj /etc/malloc.conf.)
 
+20090528:
+	The compiling option ADAPTIVE_SX has been retired while it has been
+	introduced the option NO_ADAPTIVE_SX which handles the reversed logic.
+	The KPI for sx_init_flags() changes as accepting flags:
+	SX_ADAPTIVESPIN flag has been retired while the SX_NOADAPTIVE flag
+	has been introduced in order to handle the reversed logic.
+	Bump __FreeBSD_version to 800092.
+
 20090527:
 	Add support for hierarchical jails.  Remove global securelevel.
 	Bump __FreeBSD_version to 800091.