Performance Management Guide

Kernel Tunable Parameters

Following are kernel parameters, grouped into the following sections:

Scheduler and Memory Load Control Tunable Parameters

Most of the scheduler and memory load control tunable parameters are fully described in the schedo man page. The following are a few other related parameters:

maxuproc

Purpose:	Specifies the maximum number of processes per user ID.
Values:	Default: 40; Range: 1 to 131072
Display:	lsattr -E -l sys0 -a maxuproc
Change:	chdev -l sys0 -a maxuproc=NewValue Change takes effect immediately and is preserved over boot. If value is reduced, then it goes into effect only after a system boot.
Diagnosis:	Users cannot fork any additional processes.
Tuning:	This is a safeguard to prevent users from creating too many processes.
Refer to: Responding to PDT Report Messages

ncargs

Purpose:	Specifies the maximum allowable size of the ARG/ENV list (in 4KB blocks) when running exec() subroutines.
Values:	Default: 6; Range: 6 to 128
Display:	lsattr -E -l sys0 -a ncargs
Change:	chdev -l sys0 -a ncargs=NewValue Change takes effect immediately and is preserved over boot.
Diagnosis:	Users cannot execute any additional processes because the argument list passed to the exec() system call is too long.
Tuning:	This is a mechanism to prevent the exec() subroutines from failing if the argument list is too long. Please note that tuning to a higher ncargs value puts additional constraints on system memory resources.

Virtual Memory Manager Tunable Parameters

The complete listing of the virtual memory manager tunable parameters is located in the vmo man page.

Synchronous I/O Tunable Parameters

Most of the synchronous I/O tunable parameters are fully described in the ioo man page. The following are a few other related parameters:

maxbuf

Purpose:	Number of (4 KB) pages in the block-I/O buffer cache.
Values:	Default: 20; Range: 20 to 1000
Display:	lsattr -E -l sys0 -a maxbuf
Change:	chdev -l sys0 -a maxbuf=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis:	If the sar --b command shows `breads` or `bwrites` with `%rcache` and `%wcache` being low, you might want to tune this parameter.
Tuning:	This parameter normally has little performance effect on systems, where ordinary I/O does not use the block-I/O buffer cache.
Refer to: Tuning Asynchronous Disk I/O

maxpout

Purpose:	Specifies the maximum number of pending I/Os to a file.
Values:	Default: 0 (no checking); Range: 0 to n (n should be a multiple of 4, plus 1)
Display:	lsattr -E -l sys0 -a maxpout
Change:	chdev -l sys0 -a maxpout=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis:	If the foreground response time sometimes deteriorates when programs with large amounts of sequential disk output are running, sequential output may need to be paced.
Tuning:	Set maxpout to 33 and minpout to 16. If sequential performance deteriorates unacceptably, increase one or both. If foreground performance is still unacceptable, decrease both.

minpout

Purpose:	Specifies the point at which programs that have reached maxpout can resume writing to the file.
Values:	Default: 0 (no checking); Range: 0 to n (n should be a multiple of 4, plus 1)
Display:	lsattr -E -l sys0 -a minpout
Change:	chdev -l sys0 -a minpout=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis:	If the foreground response time sometimes deteriorates when programs with large amounts of sequential disk output are running, sequential output may need to be paced.
Tuning:	Set maxpout to 33 and minpout to 16. If sequential performance deteriorates unacceptably, increase one or both. If foreground performance is still unacceptable, decrease both.

mount -o nointegrity

Purpose:

A new mount option (nointegrity) may enhance local file system performance for certain write-intensive applications. This optimization basically eliminates writes to the JFS log. Note that the enhanced performance is achieved at the expense of metadata integrity. Therefore, use this option with extreme caution because a system crash can make a file system mounted with this option unrecoverable. Nevertheless, certain classes of applications do not require file data to remain consistent after a system crash, and these may benefit from using the nointegrity option. Two examples in which a nointegrity file system may be beneficial is for compiler temporary files, and for doing a nonmigration or mksysb installation.

Paging Space Size

Purpose:	The amount of disk space required to hold pages of working storage.
Values:	Default: configuration-dependent; Range: 32 MB to n MB for hd6, 16 MB to n MB for non-hd6
Display:	lsps -a mkps or chps or smitty pgsp
Change:	Change is effective immediately and is permanent. Paging space is not necessarily put into use immediately, however.
Diagnosis:	Run: lsps -a. If processes have been killed for lack of paging space, monitor the situation with the psdanger() subroutine.
Tuning:	If it appears that there is not enough paging space to handle the normal workload, add a new paging space on another physical volume or make the existing paging spaces larger.

syncd Interval

Purpose:	The time between sync() calls by syncd.
Values:	Default: 60; Range: 1 to any positive integer
Display:	grep syncd /sbin/rc.boot vi /sbin/rc.boot or
Change:	Change is effective at next boot and is permanent. An alternate method is to use the kill command to terminate the syncd daemon and restart it from the command line with the command /usr/sbin/syncd interval.
Diagnosis:	I/O to a file is blocked when syncd is running.
Tuning:	At its default level, this parameter has little performance cost. No change is recommended. Significant reductions in the syncd interval in the interests of data integrity (as for HACMP) could have adverse performance consequences.

Asynchronous I/O Tunable Parameters

maxreqs

Purpose:	Specifies the maximum number of asynchronous I/O requests that can be outstanding at any one time.
Values:	Default: 4096; Range: 1 to AIO_MAX (/usr/include/sys/limits.h)
Display:	lsattr -E -l aio0 -a maxreqs
Change:	chdev -l aio0 -a maxreqs=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis:	N/A
Tuning:	This includes requests that are in progress, as well as those that are waiting to be started. The maximum number of asynchronous I/O requests cannot be less than the value of AIO_MAX, as defined in the /usr/include/sys/limits.h file, but can be greater. It would be appropriate for a system with a high volume of asynchronous I/O to have a maximum number of asynchronous I/O requests larger than AIO_MAX.
Refer to: Tuning Asynchronous Disk I/O

maxservers

Purpose:	Specifies the maximum number of AIO kprocs that will be created.
Values:	Default: 10
Display:	lsattr -E -l aio0 -a maxservers
Change:	chdev -l aio0 -a maxservers=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis:	N/A
Tuning:	This value limits the number of concurrent asynchronous I/O requests. The value should be about the same as the expected number of concurrent AIO requests. This tunable parameter only affects AIO on JFS file systems (or Virtual Shared Disks (VSD) before AIX 4.3.2).
Refer to: Tuning Asynchronous Disk I/O

minservers

Purpose:	Specifies the number of AIO kprocs that will be created when the AIO kernel extension is loaded.
Values:	Default: 1
Display:	lsattr -E -l aio0 -a maxservers
Change:	chdev -l aio0 -a minservers=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis:	N/A
Tuning:	Making this a large number is not recommended, because each process takes up some memory. Leaving this number small is acceptable in most cases because AIO will create additional kprocs up to maxservers as needed. This tunable is only effective for AIO on JFS file systems (or VSDs before AIX 4.3.2).
Refer to: Tuning Asynchronous Disk I/O

Disk and Disk Adapter Tunable Parameters

Disk Adapter Outstanding-Requests Limit

Purpose:	Maximum number of requests that can be outstanding on a SCSI bus. (Applies only to the SCSI-2 Fast/Wide Adapter.)
Values:	Default: 40; Range: 40 to 128
Display:	lsattr -E -l scsin -a num_cmd_elems
Change:	chdev -l scsin -a num_cmd_elems=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis:	Applications performing large writes to striped raw logical volumes are not obtaining the desired throughput rate.
Tuning:	Value should equal the number of physical drives (including those in disk arrays) on the SCSI bus, times the queue depth of the individual drives.

Disk Drive Queue Depth

Purpose:	Maximum number of requests the disk device can hold in its queue.
Values:	Default: IBM disks=3; Non-IBM disks=0; Range: specified by manufacturer
Display:	lsattr -E -l hdiskn
Change:	chdev -l hdiskn -a q_type=simple -a queue_depth=NewValue Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
Diagnosis:	N/A
Tuning:	If the non-IBM disk drive is capable of request-queuing, make this change to ensure that the operating system takes advantage of the capability.
Refer to: Setting SCSI-Adapter and Disk-Device Queue Limits

Interprocess Communication Tunable Parameters

msgmax

Purpose:	Specifies maximum message size.
Values:	Dynamic with maximum value of 4 MB
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

msgmnb

Purpose:	Specifies maximum number of bytes on queue.
Values:	Dynamic with maximum value of 4 MB
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

msgmni

Purpose:	Specifies maximum number of message queue IDs.
Values:	Dynamic with maximum value of 131072
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

msgmnm

Purpose:	Specifies maximum number of messages per queue.
Values:	Dynamic with maximum value of 524288
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

semaem

Purpose:	Specifies maximum value for adjustment on exit.
Values:	Dynamic with maximum value of 16384
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

semmni

Purpose:	Specifies maximum number of semaphore IDs.
Values:	Dynamic with maximum value of 131072
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

semmsl

Purpose:	Specifies maximum number of semaphores per ID.
Values:	Dynamic with maximum value of 65535
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

semopm

Purpose:	Specifies maximum number of operations per semop() call.
Values:	Dynamic with maximum value of 1024
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

semume

Purpose:	Specifies maximum number of undo entries per process.
Values:	Dynamic with maximum value of 1024
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

semvmx

Purpose:	Specifies maximum value of a semaphore.
Values:	Dynamic with maximum value of 32767
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

shmmax

Purpose:	Specifies maximum shared memory segment size.
Values:	Dynamic with maximum value of 256 MB for 32-bit processes and 0x80000000u for 64-bit
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

shmmin

Purpose:	Specifies minimum shared-memory-segment size.
Values:	Dynamic with minimum value of 1
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.

shmmni

Purpose:	Specifies maximum number of shared memory IDs.
Values:	Dynamic with maximum value of 131072
Display:	N/A
Change:	N/A
Diagnosis:	N/A
Tuning:	Does not require tuning because it is dynamically adjusted as needed by the kernel.