Why Is The OS Allocating Fewer Semaphores Per Set Even If SEMMSL Is Set To A Higher Value
(Doc ID 1670642.1)
Last updated on MARCH 05, 2020
Applies to:
Oracle Database - Enterprise Edition - Version 11.2.0.3 and laterOracle Database Cloud Schema Service - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Linux x86-64
Symptoms
The OS system has been noticed to be allocating only 150 semaphores per set even if SEMMSL has been set to 275.
In the example, the specific kernel on Linux system (OEL 5.9) is 2.6.18-348.1.1.0.1.el5.
There are about 25 instances running (versions 10.2, 11.2), and most of them have the PROCESSES parameter set to 1200.
The current semaphores settings are:
SEMMSL: 275
SEMMNS: 350000
SEMOPM: 100
Semaphore use by oracle processes is shown like below:
oracle$> ipcs -s ------ Semaphore Arrays -------- key semid owner perms nsems ... 0xd8b11da5 11206678 oracle 640 152 0xd8b11da6 11239447 oracle 640 152 0xbd91aad0 15302680 oracle 640 154 0x574fde04 11370521 oracle 640 150 0x574fde05 11403290 oracle 640 150 0x574fde06 11436059 oracle 640 150 0x574fde07 11468828 oracle 640 150 0x574fde08 11501597 oracle 640 150 0x574fde09 11534366 oracle 640 150 0x574fde0a 11567135 oracle 640 150 0x574fde0b 11599904 oracle 640 150 0x574fde0c 11632673 oracle 640 150 0xcb0a8a88 1736738 oracle 640 150 0xcb0a8a89 1769507 oracle 640 150 0xcb0a8a8a 1802276 oracle 640 150 0xcb0a8a8b 1835045 oracle 640 150 0xcb0a8a8c 1867814 oracle 640 150 0xcb0a8a8d 1900583 oracle 640 150 0xcb0a8a8e 1933352 oracle 640 150 0xcb0a8a8f 1966121 oracle 640 150 0xcb0a8a90 1998890 oracle 640 150 0x0328f108 2129963 oracle 640 150 ...
Semaphore usage by grid processes shows 252 like below:
------ Semaphore Arrays -------- key semid owner perms nsems 0x56565656 393227 root 666 3 0x000003d4 491534 root 644 1 0xf405fe54 622607 grid 640 252 0xf405fe55 655376 grid 640 252 0xf405fe56 688145 grid 640 252 ...
Values for SEMMNI, SEMMNS need to be adjusted.
However, it is not clear if it makes sense to increase SEMMSL if the system will keep allocating close to 150 per array anyway.
Changes
Cause
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In this Document
Symptoms |
Changes |
Cause |
Solution |
References |