16.5 AIX System Parameters

On AIX systems, you never need to rebuild a kernel because system parameters may be changed on a running system. The current values of AIX system parameters may be displayed with the lsattr command:

# lsattr -EHl sys0                      Final option letter is a lowercase L.
attribute    value    description                    user_settable
  
keylock      normal   State of system keylock at boot time   False
maxbuf       20       Maximum pages in block I/O BUFFER CACHE True
maxmbuf      2048     Maximum KB real memory allowed for MBUFSTrue
maxuproc     400      Maximum # PROCESSES allowed per user    True
autorestart  false    Automatically REBOOT after a crash      True
iostat       false    Continuously maintain DISK I/O history  True
realmem      65536    Amount of usable physical memory (KB)  False
conslogin    enable   System Console Login                   False
fwversion    IBM,SPH01184   Firmware version,revision levels False
maxpout      0        HIGH water mark pending write I/Os/file True
minpout      0        LOW water mark pending write I/Os/file  True
fullcore     false    Enable full CORE dump                   True
pre430core   false    Use pre-430 style CORE dump             True
ncargs       6        ARG/ENV list size in 4KB blocks         True
rtasversion  1        Open Firmware RTAS version             False
modelname    IBM,7044-270   Machine name                     False
systemid     IBM,011000189  Hardware system identifier       False
boottype     disk     N/A                                    False
SW_dist_intr false    Enable SW distribution of interrupts    True
cpuguard     disable  CPU Guard                               True
frequency    93750000 System Bus Frequency                   False

The list includes parameters that can be modified and ones that can't. Being able to see, for example, the amount of physical memory present on the system and the current setting of the front panel key (keylock) can be useful and convenient. The latter item is especially useful when the CPU unit is positioned so that the physical key position not readily visible.

The chdev command may be used to change many of these parameters. For example, the following command raises the maximum number of simultaneous processes that a user may run to 500:

# chdev -l sys0 -a maxuproc=500

You can also use the smit chgsys command to modify several settings at the same time.

Table 16-2 lists the most important AIX system parameters, along with their associated attribute names.

Table 16-2. AIX system parameters

Parameter	Meaning
Maximum user processes (maxuproc)	Maximum number of processes that any user can have at one time (the default is 200). Does not apply to root. Increasing maxuproc takes immediate effect, but decreases wait until the system is rebooted.
Block I/O buffer cache size (maxbuf)	Size of the buffer cache for reads/writes to block special files. Normal file I/O doesn't use the buffer cache (nearly all physical memory is used as an I/O cache under AIX), so leave this one at its default of 20 4K-pages. (I/O to a raw device—in other words, to a logical volume without a filesystem—does use the buffer cache.)
Maximum memory used for MBUFS (maxmbuf)	Maximum amount of memory to be used for MBUFS (TCP/IP and NFS in-memory data structures). This parameter is the same as the wall attribute tunable with the no command (the default is 2048 KB).
Automatic reboot status (autorestart)	Whether or not to reboot the system automatically after a crash (the default is false).
Disk I/O history (iostat)	Whether or not to keep records of the I/O activity to the various disks on the system since boot time (if available, this information is displayed as the first report from the iostat command). This generally useless data is no loss, so turning off this parameter does no harm, but it has little effect on system performance either way (default is on).
Disk I/O pacing parameters (minpout and maxpout)	Pending I/O operation watermarks (the I/O pacing facility is discussed in $PERF). The defaults are both 0, which disables the facility.
Full core dump (fullcore)	Whether to include all of memory in a crash dump (by default, some types of data are excluded).
Use pre-430 style CORE dump (pre430core)	Whether to use the core file format used by AIX version 4.2 and earlier (the default is not to).
ARG/ENV list size in 4 KB blocks (ncargs)	Maximum size of the argument list and environment variables for executables, in units of 4 KB. The default is 6 and the maximum is 128. Increase this value only if commands/applications have failed because of the default limitations.