Server OS not using Swap space

[Jerry Tilsley]

I would like to throw this out to everyone for some opinions, our clover leaf  server does not appear to be using the swap space, and therefore is consuming a lot of our physical memory.  We have 16GB of physical memory and float around 470 to 40MB of free physical memory, but we have like 15.2GB of free cache.  But none of the the swap is ever used.  This is a Red Had Enterprise AS 3.0 OS.

                 total       used       free     shared    buffers     cached

Mem:      16415528   15944424     471104          0     169060   14577844

-/+ buffers/cache:    1197520   15218008

Swap:      1959888          0    1959888

So are we about out of physical memory, or can someone point us in the correct direction to fix this issue?

Thanks!

Jerry

Topic

[John Mercogliano]

Jerry,

John Mercogliano
Sentara Healthcare
Hampton Roads, VA

[Jerry Tilsley]

Our main concern was if we were running out of memory since the free memory field shows only a little of the 16GB left.  But from what I have been able to find so far is that the memory is pretty much moved to cache? so as long as i have plenty of cache (which I have 15.2GB free) then I should be alright.  Is this a correct understanding?

[Robert Milfajt]

The command you displayed show you have 450+ MB, whatever 471104/1024 is, free memory.  So to answer your first question, you have plenty of memory left.

Swap space is not used until memory is used up completely.  Swap space is an extension of memory.  From this you look fine.

I am not sure if these are AIX only commands, but in AIX, lsps -a shows your complete swap space configuration, and lsps -s shows how much swap space you are using.  vmstat 2 10 will take 10 samples and give you more information to work with.

Hope this helps,

Robert Milfajt
Northwestern Medicine
Chicago, IL

[John Mercogliano]

Yes, I believe you understand correctly, the os will only move memory to cache if it needs to.

John Mercogliano
Sentara Healthcare
Hampton Roads, VA

[Jerry Tilsley]

Quote:

Yes, I believe you understand correctly, the os will only move memory to cache if it needs to.

What I am seeing is most of memory is already allocated to cache, but most of what is allocated to cache is just sitting there as free cache.  Will the server move it back to physical memory if it needs it?  I know the example I gave showed almost 500MB of memory, but this seems to float between 500MB and 40MB of physical memory.

[John Mercogliano]

Sorry Jerry, when you said cache I thought you were talking about swap space still.  But you still don’t have anything to worry about.  Linix and there clones don’t like to have free memory around so it creates a cache available for file data to speed up file access, but if frees it up as programs need the space.  So the fact that you have no or minimal swap usage and lots of cache memory available means you should have a screaming box.  So give a big smile  😀

John Mercogliano
Sentara Healthcare
Hampton Roads, VA

[Jerry Tilsley]

Thanks John!  This really helps me understand what is going on! And yes, I am quite pleased with the performance of our box!

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