Configuring kernel parameters

When to change system parameters

Among the cases in which you may need to reallocate system resources are:

You install additional physical memory and thus have greater memory resources to allocate.
Persistent error messages are being displayed on the system console indicating that certain resources are used up, such as inodes or table entries.
The system response time is consistently slow, indicating that other resources are too constrained for the system to operate efficiently (as when too little physical memory is installed).
Resource usage needs to be tailored to meet the needs of a particular application.

If one of your performance goals is to reduce the size of the kernel (usually because the system is paging excessively or swapping), first concentrate on tunable parameters that control large structures. The following table lists a small subset of kernel tunable parameters and indicates the cost (or benefit) in bytes of incrementing (or decrementing) each parameter by a single unit. For example, if NCLIST set to 200, this requires 200 times 72 bytes, or approximately 14KB of memory.

Parameter Number of bytes per unit parameter

CACHEENTS 44

HASHQS 8

NBUF 1024

NCLIST 72 (64 for the buffer + 8 for the header)

NHBUF 8

NHINODE 8

NMPBUF 4096

MSGMAP 8

NSPTTYS 246

NSTREAM 80 (52 for the STREAMS header + 28 for the extended header)

MAX_INODE 76 per entry added to the dynamic in-core inode table

MAX_PROC 344 per entry added to the dynamic process table

MAX_FILE 12 per entry added to the dynamic open file table

MAX_REGION 76 per entry added to the dynamic region table

Dynamic table parameters such as MAX_PROC usually have their values set to 0. Each table grows in size as more entries are needed. The memory overhead of the grown kernel table can be found by multiplying the values shown above by the number of table entries reported by getconf(C). For example, from the Korn shell, you can find the current size of the process table by entering:

let nproc=344$(getconf KERNEL_PROC)
echo "Size of process table in bytes is $nproc"

Parameter	Number of bytes per unit parameter
CACHEENTS	44
HASHQS	8
NBUF	1024
NCLIST	72 (64 for the buffer + 8 for the header)
NHBUF	8
NHINODE	8
NMPBUF	4096
MSGMAP	8
NSPTTYS	246
NSTREAM	80 (52 for the STREAMS header + 28 for the extended header)
MAX_INODE	76 per entry added to the dynamic in-core inode table
MAX_PROC	344 per entry added to the dynamic process table
MAX_FILE	12 per entry added to the dynamic open file table
MAX_REGION	76 per entry added to the dynamic region table

Specialized applications often require the reallocation of key system resources for optimum performance. For example, users with large databases may find that they need more System V semaphores than are currently allocated.

Most of the tunable parameters discussed in this chapter are defined in /etc/conf/cf.d/mtune. This file lists the default, maximum and minimum values respectively of each of the parameters specified. To change the values of specific tunable parameters manually, use the appropriate tool as described in ``Configuration tools''.