topic Re: Semaphore contention during databases startup in Operating System - HP-UX

Semaphore contention during databases startup

Timmy Sin — Mon, 30 Sep 2002 01:09:34 GMT

Hi All,

I've encountered a problem in my HP-UX 10.20 box with four Oracle 7.3.2 databases. For easy understanding, I named them as database A, B, C and D. The following database combination can coexist on the system.
A, C, D
B, C, D
The problem is that database A and B can not coexist on the system. Whenever either A or B has been started, the startup of the other database will result in the following error:
SVRMGR> Connected to an idle instance.
SVRMGR> ORA-07279: spcre: semget error, unable to get first semaphore set.
HP-UX Error: 28: No space left on device
Additional information: 1
SVRMGR>
I've looked up the error message from the forum and most of the recommendation is to increase the kernal parameter SEMMNS due to insufficient semaphore. It seems that it is not my case.
I got further information from the output from ipcs.
IPC status from /dev/kmem as of Mon Sep 30 09:45:23 2002
T ID KEY MODE OWNER GROUP
s 1516 0x00000000 --ra-r----- oracle dba
s 717 0x00000000 --ra-r----- oracle dba
s 319 0x00000000 --ra-r----- oracle dba
Upon my testing, I found that database A, B, C and D would get the semaphore ID 16, 16, 17 and 19 respectively during each startup. Database A and B could not coexist because they would try to acquire the same semaphore ID 16 during startup.
Do anyone have some idea on my foundings?

Many thanks.

Regards,
Timmy

Re: Semaphore contention during databases startup

Lee Tae-kyung — Mon, 30 Sep 2002 01:21:12 GMT

Re: Semaphore contention during databases startup

Timmy Sin — Mon, 30 Sep 2002 02:02:49 GMT

Hi Lee,

Thanks for your information.
My problem is that the four databases can coexist in the past. As far as I can remember, no change has been imposed on the server. The current founding is that for each database startup individually, they will acquire the sempahore ID each time(A-16,B-16,C-17,D-19). Two of the databases A and B would acquire the same ID 16 and thus cannot be started if the other one has already been started.

Regards,
Timmy

Re: Semaphore contention during databases startup

Lee Tae-kyung — Mon, 30 Sep 2002 03:10:42 GMT

Hi Timmy...

Refer to this document.
********************************************
A shared memory collision has occurred. Oracle uses the ORACLE_HOME and ORACLE_SID to hash a key id. The source of this problem is that a database name can be 8 characters long but a key (for shmget()) is only an integer (4 bytes). Therefore, some hashing must occur to compress the dbname into a key. As with all hashing, there is the possibility of different data hashing to the same key. Changing the name of one of the databases/SIDs causes a different key result from the hashing algorithm.
********************************************

From korea.

Re: Semaphore contention during databases startup

Timmy Sin — Mon, 30 Sep 2002 03:27:37 GMT

Hi Lee,

Thanks for your information again. It matches with my situation. However, if there is a chance that the same key would be resulted, there should be some mechanism to allocate another key. It sounds unreasonable to change the SID or home directory to get around this kind of problem.

Regards,
Timmy

Re: Semaphore contention during databases startup

Lee Tae-kyung — Mon, 30 Sep 2002 04:02:19 GMT

Hi Timmy...

As my opinion,
I think that this problem could occur because semmns and semmni is insufficient.

It seems a good idea that semmns and semmni is increased.
...
^_^

Good luck~~

Re: Semaphore contention during databases startup

Brian Crabtree — Mon, 30 Sep 2002 18:58:48 GMT

You should check the "processes" and "sessions" init.ora parameters on both databases. Oracle will attempt to allocate the full number of processes each time the database starts up, and if both databases are attempting to load a large number of processes, it will fail trying to bring up the second instance.

The parameter to set is 'semmni'.

Thanks,

Brian

Re: Semaphore contention during databases startup

V. V. Ravi Kumar_1 — Tue, 01 Oct 2002 03:04:29 GMT

hi,
try increasing SEMMNS and SEMMNI. typical values are 8000 and 4000 respectively.

regds