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Thanks, Craig.How To Change The Priority Of Oracle Background Processes
Before you get in a huf, it can be done! You can change an Oracle Database background process
priority through an instance parameter! I'm not saying it's a good idea, but it can be done.
In this post I explore how to make the change, just how far you can take it and when you may want to consider changing an Oracle background process priority.
To get your adrenaline going, check out the instance parameter _high_priority_processes from one of your production Oracle system with a version of 11 or greater. Here is an example using my OSM tool, ipx.sql on my Oracle Database version 12.1.0.2.0.SQL> @ipx _highest_priority_processesDatabase: prod40 27-OCT-14 02:22pmReport: ipx.sql OSM by OraPub, Inc. Page 1 Display ALL Instance ParametersInstance Parameter and Value Description Dflt?-------------------------------------------------- -------------------- -----_highest_priority_processes = VKTM Highest Priority TRUE Process Name MaskThen at the Linux prompt, I did:
$ ps -eo pid,class,pri,nice,time,args | grep prod40 2879 TS 19 0 00:00:00 ora_pmon_prod40 2881 TS 19 0 00:00:01 ora_psp0_prod40 2883 RR 41 - 00:02:31 ora_vktm_prod40 2889 TS 19 0 00:00:01 ora_mman_prod40 2903 TS 19 0 00:00:00 ora_lgwr_prod40 2905 TS 19 0 00:00:01 ora_ckpt_prod40 2907 TS 19 0 00:00:00 ora_lg00_prod40 2911 TS 19 0 00:00:00 ora_lg01_prod40...Notice the "pri" for priority of the ora_vktm_prod40 process? It is set to 41 while all the rest of the Oracle background processes are set to the default of 19. Very cool, eh?
Surprised By What I Found
Surprised? Yes, surprised because changing Oracle process priority is a pandoras box. Just imagine if an Oracle server (i.e., foreground) process has its priority lowered just a little and then attempts to acquire a latch or a mutex? If it doesn't get the latch quickly, I might never ever get it!
From a user experience perspective, sometimes performance really quick and other times the application just hangs.
This actually happened to a customer of mine years ago when the OS started reducing a process's priority after it consumed a certain amount of CPU. I learned that when it comes to Oracle processes, they are programed to expect an even process priority playing field. If you try to "game" the situation, do so at your own risk... not Oracle's.
Then why did Oracle Corporation allow background process priority to be changed. And why did Oracle Corporation actually change a background processes priority?!
Doing A Little Exploration
It turns out there are a number of "priority" related underscore instance parameters! On my 11.2.0.1.0 system there 6 "priority" parameters. On my 12.1.0.1.0 system there are 8 "priority" parameters. On my 12.1.0.2.0 system there are 13 "priority" parameters! So clearly Oracle is making changes! In all cases, the parameter I'm focusing on, "_high_priority_processes" exists.
In this posting, I'm going to focus on my Oracle Database 12c version 12.1.0.2.0 system. While you may see something different in your environment, the theme will be the same.
While I'll be blogging about all four of the below parameters, in this posting my focus will be on the _high_priority_processes parameter. Below are the defaults on my system:
_high_priority_processes LMS*_highest_priority_processes VKTM_os_sched_high_priority 1_os_sched_highest_priority 1
Messing With The LGWR Background Processes
I'm not testing this on a RAC system, so I don't have an LMS background process. When I saw the "LMS*" I immediately thought, "regular expression." Hmmm... I wonder if I can change the LGWR background process. So I made the instance parameter change and recycled the instance. Below shows the instance parameter change:
SQL> @ipx _high_priority_processesDatabase: prod40 27-OCT-14 02:36pmReport: ipx.sql OSM by OraPub, Inc. Page 1 Display ALL Instance ParametersInstance Parameter and Value Description Dflt?-------------------------------------------------- -------------------- -----_high_priority_processes = LMS*|LGWR High Priority FALSE Process Name Mask
Below is an operating system perspective using the ps command:
ps -eo pid,class,pri,nice,time,args | grep prod40... 5521 RR 41 - 00:00:00 ora_vktm_prod40 5539 TS 19 0 00:00:00 ora_dbw0_prod40 5541 RR 41 - 00:00:00 ora_lgwr_prod40 5545 TS 19 0 00:00:00 ora_ckpt_prod40 5547 TS 19 0 00:00:00 ora_lg00_prod40 5551 TS 19 0 00:00:00 ora_lg01_prod40...
How Far Can I Take This?
At this point in my journey, my mind was a blaze! The log file sync wait event can be really difficult to deal with and especially so when there is a CPU bottleneck. Hmmm... Perhaps I can increase the priority of all the log writer background processes?
So I made the instance parameter change and recycled the instance. Below shows the instance parameter change:
SQL> @ipx _high_priority_processesDatabase: prod40 27-OCT-14 02:44pmReport: ipx.sql OSM by OraPub, Inc. Page 1 Display ALL Instance ParametersInstance Parameter and Value Description Dflt?-------------------------------------------------- -------------------- -----_high_priority_processes = LMS*|LG* High Priority FALSE Process Name Mask
Below is an operating system perspective using the ps command:
ps -eo pid,class,pri,nice,time,args | grep prod40... 5974 TS 19 0 00:00:00 ora_psp0_prod40 5976 RR 41 - 00:00:00 ora_vktm_prod40 5994 TS 19 0 00:00:00 ora_dbw0_prod40 5996 RR 41 - 00:00:00 ora_lgwr_prod40 6000 TS 19 0 00:00:00 ora_ckpt_prod40 6002 RR 41 - 00:00:00 ora_lg00_prod40 6008 RR 41 - 00:00:00 ora_lg01_prod40 6014 TS 19 0 00:00:00 ora_lreg_prod40...
So now all the log writer background processes have a high priority. My hope would be that if there is an OS CPU bottleneck and the log writer background processes wanted more CPU, I now have the power to give that to them! Another tool in my performance tuning arsenal!
Security Hole?
At this point, my exuberance began to turn into paranoia. I thought, "Perhaps I can increase the priority of an Oracle server process or perhaps any process." If so, that would be a major Oracle Database security hole.
With fingers trembling, I changed the instance parameters to match an Oracle server process and recycled the instance. Below shows the instance parameter change:
SQL> @ipx _high_priority_processesDatabase: prod40 27-OCT-14 02:52pmReport: ipx.sql OSM by OraPub, Inc. Page 1 Display ALL Instance ParametersInstance Parameter and Value Description Dflt?-------------------------------------------------- -------------------- -----_high_priority_processes = High Priority FALSELMS*|LG*|oracleprod40 Process Name Mask
Below is an operating system perspective using the ps command:
$ ps -eo pid,class,pri,nice,time,args | grep prod40... 6360 TS 19 0 00:00:00 ora_psp0_prod40 6362 RR 41 - 00:00:00 ora_vktm_prod40 6366 TS 19 0 00:00:00 ora_gen0_prod40 6382 RR 41 - 00:00:00 ora_lgwr_prod40 6386 TS 19 0 00:00:00 ora_ckpt_prod40 6388 RR 41 - 00:00:00 ora_lg00_prod40 6394 RR 41 - 00:00:00 ora_lg01_prod40 6398 TS 19 0 00:00:00 ora_reco_prod40... 6644 TS 19 0 00:00:00 oracleprod40......
OK, that didn't work so how about this?
SQL> @ipx _high_priority_processesDatabase: prod40 27-OCT-14 02:55pmReport: ipx.sql OSM by OraPub, Inc. Page 1 Display ALL Instance ParametersInstance Parameter and Value Description Dflt?-------------------------------------------------- -------------------- -----_high_priority_processes = High Priority FALSELMS*|LG*|*oracle* Process Name Mask
Let's see what happened at the OS.
$ ps -eo pid,class,pri,nice,time,args | grep prod40... 6701 RR 41 - 00:00:00 ora_vktm_prod40 6705 RR 41 - 00:00:00 ora_gen0_prod40 6709 RR 41 - 00:00:00 ora_mman_prod40 6717 RR 41 - 00:00:00 ora_diag_prod40 6721 RR 41 - 00:00:00 ora_dbrm_prod40 6725 RR 41 - 00:00:00 ora_vkrm_prod40 6729 RR 41 - 00:00:00 ora_dia0_prod40 6733 RR 41 - 00:00:00 ora_dbw0_prod40... 6927 RR 41 - 00:00:00 ora_p00m_prod40 6931 RR 41 - 00:00:00 ora_p00n_prod40 7122 TS 19 0 00:00:00 oracleprod40 ... 7124 RR 41 - 00:00:00 ora_qm02_prod40 7128 RR 41 - 00:00:00 ora_qm03_prod40
Oh Oh... That's not good! Now EVERY Oracle background process has a higher priority and my Oracle server process does not.
So my "*" wildcard caused all the Oracle processes to be included. If all the processes a high prioirty, then the log writer processes have no advantage over the others. And to make matters even worse, my goal of increasing the server process priority did not occur.
However, this is actually very good news because it appears this is not an Oracle Database security hole! To me, it looks like the priority parameter is applied during the instance startup for just the background processes. Since my server process was started after the instance was started and for sure not included in the list of background processes, its priority was not affected. Good news for security, not as good of news for a performance optimizing fanatic such as myself.
Should I Ever Increase A Background Process Priority?
Now that we know how to increase an Oracle Database background process priority, when would we ever want to do this? The short answer is probably never. But the long answer is the classic, "it depends."
Let me give you an example. Suppose there is an OS CPU bottleneck and the log writer background processes are consuming lots of CPU while handling all the associated memory management when server process issues a commit. In this situation, performance may benefit by making it easier for the log writer processes to get CPU cycles, therefore improving performance. But don't even think about doing this unless there is a CPU bottleneck. And even then, be very very careful.
In my next block posting, I'll detail an experiment where I changed the log writer background processes priority.
Thanks for reading!
Craig.
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