vmstat interpretation !!!

[Chris Fung]

Hi there,

Can anyone tell me the meaning for "avm" and "free" column under the "memory" main column in vmstat output?

From my understanding, each page frame is 4k in size, however, I have no idea on what avm - active virtual memory is composed of!! Is it talking about the paging space or the real memory or both ?

How the avm and free column related to the system memory and paging spaces?

Appreciated for your clarifications and inputs.

Many thanks,

Chris,

[Jordan Bean]

This is what I understand of VM:

Virtual memory is just that: "virtual". All processes reference virtual memory. The kernel maps virtual memory to real memory (physical and swap).

To be considered active, such pages must belong to processes that are running or have run in the last 20 seconds (during which time some pages may have been paged out).

The number of free pages tells how much physical memory is available for running processes.

[fg_1]

Chris

The AVM columm means ACTIVE VIRTUAL MEMORY, it takes the sum of text, data, and stack pages in use for all processes, not including shared library or shared memory pages.

FREE is the number of free memory pages.

BTW the most important columns on VMSTAT are:

r - the # of processes in the run queue
b - the # of processes blocked
w - the # of processes swapped out
pi/po - the # of pages paged in or out.

Hope this helps.

GL.

FG.

[Dietmar Konermann]

Active virtual pages are virtual pages owned by a non-system process with at least one thread running during the last 20s.

Not only private data is included:

/*
* For shared segments, calculating "our share" of the
* pages involves dividing by the number of people using
* that shared segment.
*/

Regards...
Dietmar.

[Chris Fung]

Hi there,

Thanks for your inputs, however, I still have difficulties in understand the "avm" and "free" column especially when correlate them with the no. of real memory and no. of paging space.

Can anyone give me some examples on how they are correlated??

E.g. I have 2GB real memory and 4GB swap space defined. If the size is 1xxxx and free is 1xx. What is the significant for the "avm" value and "free" value in the vmstat?

Thank in advance for your further input and advice.

Cheers,

Chris Fung,

[Dietmar Konermann · Accepted solution]

Hi, Chris!

Try to let me explain that by comparing a kememinfo output with vmstat...

vmstat:
# vmstat -n
VM
memory page faults
avm free re at pi po fr de sr in sy cs
123292 315155 24 3 0 0 0 0 0 1075 6750 566

kmeminfo:
Physical memory usage summary (in pages):

Physmem = 522240 Available physical memory:
Freemem = 314563 Free physical memory
Used = 207677 Used physical memory:
System = 46507 by kernel:
Static = 14815 for text and static data
Dynamic = 0 for dynamic data
Bufcache = 31670 for file-system buffer cache
Eqmem = 22 for equiv. mapped page pool
SCmem = 0 for system critical page pool
User = 85690 by user processes
Uarea = 3968 for thread uareas
Disowned = 13080 disowned pages


In the kmeminfo output you see how your PHYSICAL memory (2GB) is currently used. The "Freemem" value matches (nearly) the vmstat "free" metric, meaning this is the amount of physical memory being currently unused and available. This has nothing to do with swap... we are only talking about physical pages here.

The "avm" metric talks about "virtual memory", see my comment before. There is no direct relationship! Of course there is a chance that "active pages" are still mapped... but see the difference in the examples above ("User" vs. "avm").

If you are really interested in more details, you should have a look at the memory management white paper.

http://docs.hp.com/cgi-bin/otsearch/getfile?id=/hpux/onlinedocs/os/11i/mem_mgt.html&searchterms=HP-UX%7cMemory%7cManagement%7cWhite%7cPaper&queryid=20021014-024430

Regards...
Dietmar.

[Dietmar Konermann]

You should run kmeminfo yourself... it's really unreadable without indents.

[Chris Fung]

Hi Dietmar,

Thanks for your detail explanation with illustration.

Cheers,

Chris,