LMS and real time priorities

One of the oft mentioned RAC best practices is that the priority of LMS processes which ship blocks across the interconnect be higher so that they are not competing for cpu cycles with other processes.
Starting with 10.2 LMS is supposed to run in the real time class. This is new functionality which is governed by the underscore parameter _os_sched_high_priority

However in 10.2.0.1 LMS still runs in the time-sharing TS class (SCHED_OTHER standard time-sharing) due to the absence of the oradism executable
buffalo >ls -al oradism
-r-sr-s--- 1 root dba 0 Jul 1 2005 oradism

buffalo >ps -efc|grep lms
oracle 27263 1 TS 24 2006 ? 00:12:38 ora_lms0_F8900DMO1
oracle 27273 1 TS 24 2006 ? 00:04:30 ora_lms1_F8900DMO1

When you apply the 10.2.0.3 patchset you notice that the oradism executable seems to be generated and LMS runs in the RR class (SCHED_RR round robin)

elephant-> ls -al oradism
-r-sr-s--- 1 root dba 14456 Nov 15 12:52 oradism

elephant-> ps -efc | grep lms

oracle 11554 1 RR 90 15:09 ? 00:00:01 ora_lms0_F8902PRD1
oracle 11558 1 RR 90 15:09 ? 00:00:01 ora_lms1_F8902PRD1

whereas other background processes like PMON still run in TS

elephant-> ps -efc | grep pmon | grep PRD
oracle 11544 1 TS 23 Feb14 ? 00:00:00 ora_pmon_F8902PRD1

I am not sure if this is ideal on a box with a low number of CPUs or if cache fusion traffic is not a major concern.If you want LMS to run in the same class as other processes you need to set _os_sched_high_priority back to 0 from its default value of 1 as seen from below

But doing this does not seem to change the class to TS

SQL> alter system set "_os_sched_high_priority"=0 scope=spfile;

System altered.

SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Production
With the Partitioning, Real Application Clusters, OLAP and Data Mining options
elephant-> srvctl stop database -d F8902PRD
elephant-> srvctl start database -d F8902PRD
elephant-> ps -efc | grep lms | grep PRD
oracle 17097 1 RR 90 09:54 ? 00:00:00 ora_lms0_F8902PRD1
oracle 17101 1 RR 90 09:54 ? 00:00:00 ora_lms1_F8902PRD1

1 select a.ksppinm "Parameter",
2 b.ksppstvl "Session Value",
3 c.ksppstvl "Instance Value"
4 from x$ksppi a, x$ksppcv b, x$ksppsv c
5 where a.indx = b.indx and a.indx = c.indx
6* and a.ksppinm like '%os_sched%'
SQL> /

Parameter
--------------------------------------------------------------------------------
Session Value
-------------------------------------------------------------------------------------------------------------------------------------------------
Instance Value
-------------------------------------------------------------------------------------------------------------------------------------------------
_os_sched_high_priority
0
0

From bug 5635098 it appears there is another parameter called
_high_priority_processes which needs to be set to null for this to work.

SQL> alter system set "_high_priority_processes"='' scope=spfile;
System altered.

elephant-> srvctl stop database -d F8902PRD
elephant-> srvctl start database -d F8902PRD

elephant-> ps -efc | grep lms | grep -v grep

oracle 31654 1 TS 24 00:58 ? 00:00:01 ora_lms0_F8902PRD1
oracle 31656 1 TS 24 00:58 ? 00:00:00 ora_lms1_F8902PRD1

As you can lms is now running in TS class.
On Solaris Sparc 64 bit be aware of bug 5258549 which causes boxes with low number of CPUs to freeze.