memory 100% used

[bcpsadmin]

Hi All:

I am running AIX 4.3.3 on an H80 with 3070mb of ram and 2848mb of paging space.

I use IBM's nmon to monitor system performance and general health. The m (memory) option of nmon shows the following:


Memory Use Physical Virtual Paging pages/sec In Out
% Used 99.9% 41.1% to Paging Space 0.0 36.0
% Free 0.1% 58.9% to File System 203.4 .0
MB Used 3070.4MB 1169.1MB
MB Free 1.5MB 1678.9MB minfree 120
maxfree 128
Total(MB) 3072.0MB 2848.0MB


I removed some of the other vm columns due to lack of space.

To date there are no performance problems. Some paging occurs but the system is not thrasing.

Question:

Does 100% memory used indicate that I am out of physical ram and need to add more or does it mean that ram pages have been occupied by files and data from previous processes but those pages will be used on demand by new jobs etc?

Thanks for you help.

bcpsadmin

 

[MoshiachNow]

Hi,

Normally the real memory usage is close to 100%.

As long as your paging steel available - your system is still alive.

This behaviour is normal on AIX. "Long live king Moshiach !"
h

http://www.noahide.com/movement.htm

 

[bcpsadmin]

Thanks for your quick reply.
is 2848mb of paging space enough for 3070mb of real ram?

 

[nwardez]

Hi,

It's a good thing to have, at least, paging space = physical memory.

Programs often allocate (not use) memory in paging space. Even if they do not need it at this time, it's just in case ..., so that pages can be quickly swapped when it's necessary, without having to allocate space first.

Having paging space < physical memory, sometimes, you may still have physical memory free but can't start any process, just because all paging space has been allocated (and maybe empty !). Too bad ...

 

[nwardez]

Sorry, it's too late, I feel feverish ...

I wanted to say that programs PRE-allocate pages in paging space, (and in fact, they let the Virtual Memory Manager do the job), it's silly to say that they &quot;often allocate memory in ps&quot; ...

 

[bcpsadmin]

nmon is giving me a warning about paging space < real memory. I did not want to follow the 2 * memory rule, usually suggested for paging space, because that is not necessary for systems with 1gb or more of ram. In addition, I read that too much paging space can cause premature paging.

I will take your suggestion and make paging space about equal to ram.

Thanks.

 

[aixat]

Hello,

All the above updates are WRONG.
nmon is not IBM software.

If you change the memory model and set memory allocation for paging early then it is allocated early - if that is not changed then.

Look at vmstat output -
if fre column fall below 120 then paging takes place and page from real memory that has not been used is moved to paging space to make room for the program that is running in cpu and needs memory pages.

If you look at lsps -a or lsps -s and paging is not being used to much like less then 25% then i will not increase paging space.

install bos.adt.samples fileset then look at vmtune command then go to techsupport.services.ibm.com then find document on vmm - it explain how Virtual memory manager work.

 

[bcpsadmin]

Thanks for your input.
nmon is a very comprehensive performance monitoring program which was used by IBM's performance analysis team.
In fact it was written by Nigel Griffiths, a member of that team.

The program has been made available to the public but it is not supported by IBM. The nmon analyser is also available to the public. The analyzer produces fantastic graphs and charts in either Excel or Lotus.

nmon and information on nmon can be found at

http://www.ibm.com/servers/esdd/articles/analyze_aix/index.html

As far as allocation of paging space goes, I read that AIX 4.3.2 and later uses the defered allocation policy. Paging space is allocated only when a page needs to be paged out.

vmtune confirms that this is the case on 4.3.3 unless it was changed.
I will continue to monitor paging space and incrase if I need to do so.

Thanks all.

 

[nwardez]

Just wanted to add that swap space is reserved for a shared memory segment (used by Oracle processes, for example) at the time it is allocated, even if it has not been touched yet ...

 

[Yegolev]

I have machines with 2GB of RAM that have 8GB of paging... I am not a perf expert but I believe you should not see any problems. In general we have 4xRAM in paging, even on the larger machines, but I have one with 10GB or RAM and 16GB of paging, so this is not a hard rule.

Low fre is caused by file cache which is normal and good. IBM Certified -- AIX 4.3 Obfuscation

 

[bcpsadmin]

Do you see any paging activity on your machine with 8gb?
I encounter some paging but it is mainly persistent files being written back to disk.