Tuning/tuneable kernel parameters

Note: Oracle 8.1.x seems to work fine with Linux 2.4.x kernels with no modifications

Note 2: If you see anything wrong, out of date, or something that should be changed for the better, please let me know!

The linux kernel has many parameters that may be tuned, and are not well documented.
Shared memory has been a big issue with the advent of Oracle on linux, and so has large
memory support ( > 1Gb).

/proc/sys/kernel/shmmax

cat /proc/sys/kernel/shmmax
1999999999
 

shmmax controls the maximum amount of memory to be allocated for shared memory,
1,999,999,999 in this instance.

include/asm-i386/shmparam.h
 

shmparam.h contains definitions for the shared memory structures.

#define _SHM_ID_BITS 7
#define _SHM_IDX_BITS 16
 

_SHM_ID_BITS + _SHM_IDX_BITS must be <= 24 on the i386

#define SHMMAX 0xA000000 /* max shared seg size (bytes) */
#define SHMMIN 1 /* min seg size */
#define SHMMNI 200 /* max # segments system-wide */
#define SHMSEG 200 /* max shared segments/process */
 

In this case, the maximum shared segment is 167,772,160 bytes, with a total maximum segments of 200, and a maximum of 200 segments / process.

sem.h defines how shared memory is to be used:  The number of semaphore identifiers, number of semaphores per ID, max total semaphores, max number of operations per semop call,  etc.  Oracle is concerned primarily with the numer of semaphores per ID.  In order to achieve the results that follow:

include/asm-i386/page.h

DO NOT CHANGE:

IF YOUR KERNEL IS > 2.2.10

page.h contains many memory structure definitions, one of them being the maximum amount of physical ram that is supported.

#define __PAGE_OFFSET (0x80000000)

This allows up to 2Gb of ram. The stock value is 0xC0000000 For a maximum of 1Gb of ram. As documented in page.h, this change requires a complimentary change to the kernel linker script, vmlinux.lds:

This will link the kernel with support for up to 2Gb of ram. The stock value is again 0xC0000000, as these values must compliment each other. With the above modifications and a subsequent recompilation of the linux kernel, the system is ready to support an application that needs an insane amount of memory space.

Oracle will be the test subject in this case. Normal output of Oracle installed on an un-modified system with 512Mb of ram is:

With the proper kernel modifications (and Oracle modifications detailed below), the parameters expand as follows:
 

SVRMGR> startup
ORACLE instance started.
Total System Global Area	469019920 bytes
Fixed Size	48400 bytes
Variable Size	261804032 bytes
Database Buffers	204800000 bytes
Redo Buffers	2367488 bytes

Clearly, Oracle is now ready for some real data space. However, the application itself must be modified to essentially lower it's entry point into the system's memory. Since it's algorithm is a bit odd, this allows it to utilize more physical ram.

1. cd $ORACLE_HOME/rdbms/lib
2. rm ../../lib/libclntsh.so.1.0
3. make clean
4. make ksms.s
5. Edit ksms.s and change the offset: .

set sgabeg,0X20000000 --->

.set sgabeg,0X0A000000

536,870,912 > 167,772,160
Oracle is linked at install time with the default value of 0x20000000.

This will allow Oracle to better utilize the shared memory segments. The upper limits appear to be 470Mb for one instance of Oracle at this time. Oracle's shared memory usage can be seen with ipcs(8):

[oracle@db3 lib]$ ipcs

------ Shared Memory Segments ------
key	shmid	owner	perms	bytes	nattch
0xa3f82e08	800	oracle	640	469377024	7
------ Semaphore Arrays --------
key	semid	owner	perms	bytes	nsems
0x00000000	2048	oracle	640	512
0x00000000	2177	oracle	640	512
0x00000000	2178	oracle	640	26
------ Message Queues --------
key	msqid	owner	perms	used-bytes	messages