vmstat Command

AIX Version 4.3 Commands Reference, Volume 6

vmstat Command

Purpose

Reports virtual memory statistics.

Syntax

vmstat [ -f ] [ -i ] [ -s ] [ PhysicalVolume ... ] [ Interval [ Count ] ]

Description

The vmstat command reports statistics about kernel threads, virtual memory, disks, traps and CPU activity. Reports generated by the vmstat command can be used to balance system load activity. These system-wide statistics (among all processors) are calculated as averages for values expressed as percentages, and as sums otherwise.

If the vmstat command is invoked without flags, the report contains a summary of the virtual memory activity since system startup. If the -f flag is specified, the vmstat command reports the number of forks since system startup. The PhysicalVolume parameter specifies the name of the physical volume.

The Interval parameter specifies the amount of time in seconds between each report. The first report contains statistics for the time since system startup. Subsequent reports contain statistics collected during the interval since the previous report. If the Interval parameter is not specified, the vmstat command generates a single report and then exits. The Count parameter can only be specified with the Interval parameter. If the Count parameter is specified, its value determines the number of reports generated and the number of seconds apart. If the Interval parameter is specified without the Count parameter, reports are continuously generated. A Count parameter of 0 is not allowed.

The kernel maintains statistics for kernel threads, paging, and interrupt activity, which the vmstat command accesses through the use of the knlist subroutine and the /dev/kmem pseudo-device driver. The disk input/output statistics are maintained by device drivers. For disks, the average transfer rate is determined by using the active time and number of transfers information. The percent active time is computed from the amount of time the drive is busy during the report.

The following example of a report generated by the vmstat command contains the column headings and their description:

kthr: kernel thread state changes per second over the sampling interval.

`r`	Number of kernel threads placed in run queue.
`b`	Number of kernel threads placed in wait queue (awaiting resource, awaiting input/output).

Memory: information about the usage of virtual and real memory. Virtual pages are considered active if they have been accessed. A page is 4096 bytes.

`avm`	Active virtual pages.
`fre`	Size of the free list. Note: A large portion of real memory is utilized as a cache for file system data. It is not unusual for the size of the free list to remain small.

Page: information about page faults and paging activity. These are averaged over the interval and given in units per second.

`re`	Pager input/output list.
`pi`	Pages paged in from paging space.
`po`	Pages paged out to paging space.
`fr`	Pages freed (page replacement).
`sr`	Pages scanned by page-replacement algorithm.
`cy`	Clock cycles by page-replacement algorithm.

Faults: trap and interrupt rate averages per second over the sampling interval.

`in`	Device interrupts.
`sy`	System calls.
`cs`	Kernel thread context switches.

Cpu: breakdown of percentage usage of CPU time.

`us`	User time.
`sy`	System time.
`id`	CPU idle time.
`wa`	CPU cycles to determine that the current process is wait and there is pending disk input/output.

Disk: Provides the number of transfers per second to the specified physical volumes that occurred in the sample interval. The PhysicalVolume parameter can be used to specify one to four names. Transfer statistics are given for each specified drive in the order specified. This count represents requests to the physical device. It does not imply an amount of data that was read or written. Several logical requests can be combined into one physical request.

Flags

Note: Both the -f and -s flags can be entered on the command line, but the system will only accept the first flag specified and override the second flag.

-f	Reports the number of forks since system startup.
-i	Displays the number of interrupts taken by each device since system startup.
-s	Writes to standard output the contents of the sum structure, which contains an absolute count of paging events since system initialization. The -s option is exclusive of the other vmstat command options. These events are described as follows: address translation faults Incremented for each occurrence of an address translation page fault. I/O may or may not be required to resolve the page fault. Storage protection page faults (lock misses) are not included in this count. page ins Incremented for each page read in by the virtual memory manager. The count is incremented for page ins from page space and file space. Along with the page out statistic, this represents the total amount of real I/O initiated by the virtual memory manager. page outs Incremented for each page written out by the virtual memory manager. The count is incremented for page outs to page space and for page outs to file space. Along with the page in statistic, this represents the total amount of real I/O initiated by the virtual memory manager. paging space page ins Incremented for VMM initiated page ins from paging space only. paging space page outs Incremented for VMM initiated page outs to paging space only. total reclaims Incremented when an address translation fault can be satisfied without initiating a new I/O request. This can occur if the page has been previously requested by VMM, but the I/O has not yet completed; or if the page was pre-fetched by VMM's read-ahead algorithm, but was hidden from the faulting segment; or if the page has been put on the free list and has not yet been reused. zero-filled page faults Incremented if the page fault is to working storage and can be satisfied by assigning a frame and zero-filling it. executable-filled page faults Incremented for each instruction page fault. pages examined by the clock VMM uses a clock-algorithm to implement a pseudo least recently used (lru) page replacement scheme. Pages are aged by being examined by the clock. This count is incremented for each page examined by the clock. revolutions of the clock hand Incremented for each VMM clock revolution (that is, after each complete scan of memory). pages freed by the clock Incremented for each page the clock algorithm selects to free from real memory. backtracks Incremented for each page fault that occurs while resolving a previous page fault. (The new page fault must be resolved first and then initial page faults can be backtracked.) lock misses VMM enforces locks for concurrency by removing addressability to a page. A page fault can occur due to a lock miss, and this count is incremented for each such occurrence. free frame waits Incremented each time a process is waited by VMM while free frames are gathered. extend XPT waits Incremented each time a process is waited by VMM due to a commit in progress for the segment being accessed. pending I/O waits Incremented each time a process is waited by VMM for a page-in I/O to complete. start I/Os Incremented for each read or write I/O request initiated by VMM. This count should equal the sum of page-ins and page-outs. iodones Incremented at the completion of each VMM I/O request. CPU context switches Incremented for each CPU context switch (dispatch of a new process). device interrupts Incremented on each hardware interrupt. software interrupts Incremented on each software interrupt. A software interrupt is a machine instruction similar to a hardware interrupt that saves some state and branches to a service routine. System calls are implemented with software interrupt instructions that branch to the system call handler routine. traps Not maintained by the AIX operating system. syscalls Incremented for each system call.

Examples

To display a summary of the statistics since boot, enter:
```
vmstat
```
To display five summaries at 2-second intervals, enter:
```
vmstat 2 5
```
The first summary contains statistics for the time since boot.
To display a summary of the statistics since boot including statistics for logical disks scdisk13 and scdisk14, enter:
```
vmstat scdisk13 scdisk14
```
To display fork statistics, enter:
```
vmstat -f
```
To display the count of various events, enter:
```
vmstat -s
```

Files

/unix

Symbolic link to the kernel boot image.

/usr/bin/vmstat

Contains the vmstat command.

Related Information

The iostat command.

The knlist subroutine.

The /dev/kmem special file.

Monitoring and Tuning Memory Use in AIX Versions 3.2 and 4 Performance Tuning Guide.