Commands Reference, Volume 6

When -r is used in combination without a new value, the nextboot value for tunable is displayed. When -p is used in combination without a new value, a value is displayed only if the current and next boot values for tunable are the same. Otherwise NONE is displayed as the value.

When used with -a or -o without specifying a new value, values are displayed only if the current and next boot values for a parameter are the same. Otherwise NONE is displayed as the value.

When used with -a or -o without specifying a new value, next boot values for tunables are displayed instead of current values.

Name   Current  Default  Reboot   Minimum  Maximum  Unit  Type  Dependencies
       value    value    value    value    value
---------------------------------------------------------------------------

param1 5        2        4        1        10       MB/s     I  param2
                                                                param3

Parameters can be of the following types:

• D for Dynamic if tunable can be changed at any time
• S for Static if tunable can never be changed
• R for Reboot if tunable can only be changed during reboot sequence.
• B for Bosboot if tunable can only be changed by running bosboot and rebooting machine
• M for Mount if tunable is only effective for future filesystems or directory mountings
• I for Incremental if tunable can only be incremented, except at boot time

Any change (with -o, -d or -D) to a parameter of type Mount will result in amessage being displayed to warn the user that the change is only effective for future mountings.

Any attempt to change (with -o, -d or -D) a parameter of type Bosboot or Reboot without -r, will result in an error message.

Any attempt to change (with -o, -d or -D but without -r) the current value of a parameter of type Incremental with a new value smaller than the current value, will result in an error message.

Purpose:

Turns on/off Deferred Page Space Allocation (DPSA) policy.

Values:

Default: 1

Range: 0 or 1;

Value of 1 means DPSA is on

Type: Dynamic

Diagnosis:

N/A

Tuning

May be useful to turn off DPSA policy if you are concerned about page-space overcommitment. Having the value on reduces paging space requirements.

Refer To:

Choosing between LPSA and DPSA with the vmtune Command

Purpose:

Specifies the number of real memory page sets per memory pool

Values:

• Default: 2
• Range: 1 to 10
• Type: Bosboot

Diagnosis:

N/A

Tuning

N/A

Purpose:

Specifies the number of large pages to reserve for implementing with the shmget() system call with the SHM_LGPAGE flag.

Values:

• Default: 0
• Range: 0 - number of pages.
• Type: Bosboot

Diagnosis:

N/A

Tuning

lpgpg_size must also be used in addition to this option. The application has to be modified to specify the SHM_LGPAGE flag when calling shmget(). This will improve performance in the case where there are many TLB misses and large amounts of memory is being accessed.

Refer To:

Miscellaneous I/O Tuning Parameters

Purpose:

Specifies the size in bytes of the hardware-supported large pages used for the implementation for the shmget() system call with the SHM_LGPAGE flag.

Values:

• Default: 0
• Range: 0 or 268435456 (on non-POWER4), or 0, 16777216 or 268435456 (on POWER4).
• Type: Bosboot

Diagnosis:

N/A

Tuning

lgpg_region must of be set to a non-zero value in addition to this parameter. The application has to be modified to specify the SHM_LGPAGE flag when calling shmget(). This will improve performance in the case where there are many TLB misses and large amounts of memory is being accessed.

Refer To:

Miscellaneous I/O Tuning Parameters

Purpose:

Specifies the number of memory frames per bucket. The page-replacement algorithm divides real memory into buckets of frames. On systems with multiple memory pools, the lrubucket parameter is per memory pool.

Values:

• Default: 131072 frames
• Range: 65536 to total number of memory frames
• Type: Dynamic

Diagnosis:

N/A

Tuning

Tuning this parameter is not recommended on most systems. Instead of scanning every frame in the system looking for a free frame, the page replacement algorithm scans through the contents of a bucket and scans the same bucket for the second pass before going on to the next bucket.

Refer To:

Tuning lrubucket to Reduce Memory Scanning Overhead

Purpose:

Specifies maximum percentage of RAM that can be used for caching client pages. Similar to maxperm% but cannot be bigger than maxperm%.

Values:

• Default: 80
• Range: 1 to 100%.
• Type: Dynamic

Diagnosis:

If J2 file pages or NFS pages are causing working storage pages to get paged out, maxclient can be reduced.

Tuning

Decrease the value of maxclient if paging out to paging space is occurring due to too many J2 client pages or NFS client pages in memory. Increasing the value can allow more J2 or NFS client pages to be in memory before page replacement starts.

Refer To:

Miscellaneous I/O Tuning Parameters

Purpose:

Specifies the number of frames on the free list at which page-stealing is to stop.

Values:

• Default: 128
• Range: 16 to 204800
• Type: Dynamic

Diagnosis:

Observe free-list-size changes with vmstat n.

Tuning

If vmstat n shows free-list size frequently driven below minfree by application demands, increase maxfree to reduce calls to replenish the free list. Generally, keep maxfree - minfree equal to or less than 100. Setting the value too high causes page replacement to run for a longer period of time. Value must be at least 8 greater than minfree

Refer To:

Tuning VMM Page Replacement with the vmtune Command

Purpose:

Specifies the point above which the page-stealing algorithm steals only file pages.

Values:

• Default: total number of memory frames * 0.8
• Range: 1 to 100
• Type: Dynamic

Diagnosis:

Monitor disk I/O with iostat n.

Tuning

This value is expressed as a percentage of the total real-memory page frames in the system. Reducing this value may reduce or eliminate page replacement of working storage pages caused by high number of file page accesses. Increasing this value may help NFS servers that are mostly read-only. For example, if some files are known to be read repetitively, and I/O rates do not decrease with time from startup, maxperm may be too low.

Refer To:

Tuning VMM Page Replacement with the vmtune Command

Purpose:

Specifies the maximum percentage of real memory that can be pinned.

Values:

• Default: 80 percent
• Range: 1 to 99
• Type: Dynamic

Diagnosis:

Cannot pin memory, although free memory is available.

Tuning

If this value is changed, the new value should ensure that at least 4 MB of real memory will be left unpinned for use by the kernel. The maxpin values must be greater than one and less than 100. Change this parameter only in extreme situations, such as maximum-load benchmarking.

Purpose:

On pSeries 690 systems this parameter can be used to instruct VMM to allocate memory frames in the same MCM that the executing thread is running in, if possible. This parameter only enables memory affinity, which can then be turned on for a given process by setting its MEMORY_AFFINITY environment variable to MCM.

Values:

• Default: 0 (disabled)
• Range: 0 or 1 (enabled)
• Type: Bosboot

Diagnosis:

N/A

Tuning

If controlled, repeatable benchmarks are showing unexpected variability in execution time, enabling memory_affinity may be beneficial for performance.

Refer to:

Using AIX Memory Affinity Support

Purpose:

Changes the number of memory pools that will be configured at system boot time.

Values:

• Default: number of CPUs/8
• Range: 1 to number of CPUs
• Type: Bosboot

Diagnosis:

N/A

Tuning

A value of 0 restores the default number of memory pools. Changes are not allowed on UP kernel, and are not recommended on other kernels.

Refer To:

Tuning Memory Pools

Purpose:

Specifies the minimum number of frames on the free list at which the VMM starts to steal pages to replenish the free list.

Values:

• Default: maxfree - 8
• Range: 8 to 204800
• Type: Dynamic

Diagnosis:

vmstat n

Tuning

Page replacement occurs when the number of free frames reaches minfree. If processes are being delayed by page stealing, increase minfree to improve response time. The difference between minfree and maxfree should always be equal to or greater than maxpgahead.

Refer To:

Tuning VMM Page Replacement with the vmtune Command

Purpose:

Specifies the point below (in percentage of total number of memory frames) which the page-stealer will steal file or computational pages regardless of repaging rates.

Values:

• Default: 20 percent
• Range: 1 to 100.
• Type: Dynamic

Diagnosis:

Monitor disk I/O with iostat n.

Tuning

Can be useful to decrease this parameter if large number of file pages in memory is causing working storage pages to be replaced. Iif some files are known to be read repetitively, and I/O rates do not decrease with time from startup, minperm may be too low.

Refer To:

Tuning VMM Page Replacement with the vmtune Command

Purpose:

User IDs lower than this value are exempt from getting killed due to low page-space conditions.

Values:

• Default: 0 (off)
• Range: Any positive integer.
• Type: Dynamic

Diagnosis:

N/A

Tuning

System out of paging space and system administrator's processes are getting killed. Set to 1 in order to protect specific user ID processes from getting killed due to low page space or ensure there is sufficient paging space available.

Refer To:

Choosing npswarn and npskill Settings

Purpose:

Specifies the number of free paging-space pages at which the operating system begins killing processes.

Values:

• Default: MAX (64, number of paging space pages/128).
• Range: 0 to total number of paging space pages on the system
• Type: Dynamic

Diagnosis:

N/A

Tuning

Increase the value if you experience processes being killed because of low paging space.

Refer To:

Choosing npswarn and npskill Settings

Purpose:

Specifies the number of free paging-space pages at which the operating system begins sending the SIGDANGER signal to processes.

Values:

• Default: MAX (512, 4*npskill)
• Range: 0 to total number of paging space pages on the system.
• Type: Dynamic

Diagnosis:

N/A

Tuning

Increase the value if you experience processes being killed because of low paging space.

Refer To:

Choosing npswarn and npskill Settings

Purpose:

Reserve special data segment IDs for use by processes executed with the environment variable DATA_SEG_SPECIAL=Y. These data segments are assigned so that the hardware page table entries for pages within these segments are better distributed in the cache to reduce cache collisions. As many are reserved as possible up to the requested number. Running vmo -a after reboot displays the actual number reserved.

Values:

• Default: 0
• Range: 0 or a positive number
• Type: Bosboot

Diagnosis:

N/A

Tuning

The correct number to reserve depends on the number of processes run simultaneously with DATA_SEG_SPECIAL=Y and the number of data segments used by each of these processes.

Purpose:

Turns on or off page coloring in the VMM.

Values:

• Default: 0 (off)
• Range: 0 or 1.
• Type: Bosboot

Diagnosis:

N/A

Tuning

This parameter is useful for some applications that run on machines that have a direct mapped cache.

Purpose:

Specifies the point at which a new pta segment will be allocated. This parameter does not exists in the 64-bit kernel.

Values:

• Default: pta segment size * 0.5
• Range: 1 to 99.
• Type: Dynamic

Diagnosis:

System would crash from a dsi (abend code 300) with a stack similar to the following:

  findsrval64()
  shmforkws64()
  shmforkws()
  procdup()
  kforkx()
  kfork()

Dump investigation would show that the the pta segment is full for the page which generated the page fault.

Tuning

Tuning the pta balancing threshold lower will cause new pta segments to be allocated earlier, thereby reducing the chance that a pta segment will fill up and crash the system. If possible, a better solution would be to move to the 64-bit kernel which does not have this potential problem.

Purpose:

Modify the interval between the special data segment IDs reserved with num_spec_dataseg.

Values:

• Default: 512
• Range: 1 to any positive integer
• Type: Bosboot

Diagnosis:

N/A

Tuning

Generally, for processes executed with DATA_SEG_SPECIAL=Y, the more pages of the data segment they all access, the higher this value should be to optimize performance. Values that are too high, however, limit the number of special segment IDs that can be reserved. The performance impact is highly dependent on the hardware architecture as well as the application behavior and different values may be optimal for different architectures and different applications.

Purpose:

If set to 1, the maxperm value will be a hard limit on how much of RAM can be used as a persistent file cache.

Values:

• Default: 0 (off)
• Range: 0 or 1.
• Type: Dynamic

Diagnosis:

Excessive page outs to page space caused by too many file pages in RAM.

Tuning

Set to 1 in order to make the maxperm value a hard limit (use in conjunction with the tuning of the maxperm parameter).

Refer To:

Placing a Hard Limit on Persistent File Cache with strict_maxperm

Purpose:

If set to 1, will allow pinning of shared memory segments.

Values:

• Default: 0 (off)
• Range: 0 or 1.
• Type: Dynamic

Diagnosis:

Change when there is too much overhead in pinning or unpinning of AIO buffers from shared memory segments.

Tuning

Useful only if application also sets SHM_PIN flag when doing a shmget call and if doing async I/O from shared memory segments.

Refer To:

Miscellaneous I/O Tuning Parameters