Very Computer

Hi there

What is the optimum size of paging space on a machine which has
10 GB RAM. In performance tuning book IBM recommends the maximum
size of paging space with a formula. But I dont think that for
this RAM size it will work.

What I usually do is to make paging space nearly twice the size
of RAM, but I dont think that I require this much amount of
paging space over here. So what the best thing to do over here?

Your comments/advice are deeply appreciated.

Regards

Salman

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From what I've read a 'sensible' metric should be applied for systems with
large amounts of physical memory.

On the basis that paging space must exceed the total physical RAM I've had
figures of RAM + 10% suggested for systems with this quantity of physical
memory, so suggest using e.g. 11 Gb of paging space.

At the end of the day it's sensible to look at how much paging space the VMM
can usefully address given a large amount of physical memory, which probably
isn't very much greater than the physical memory size anyway.

Try monitoring the size of 'avm' using vmstat - this should help.

- DT

Quote:> Hi there

> What is the optimum size of paging space on a machine which has
> 10 GB RAM. In performance tuning book IBM recommends the maximum
> size of paging space with a formula. But I dont think that for
> this RAM size it will work.

> What I usually do is to make paging space nearly twice the size
> of RAM, but I dont think that I require this much amount of
> paging space over here. So what the best thing to do over here?

> Your comments/advice are deeply appreciated.

> Regards

> Salman

> * Sent from RemarQ http://www.remarq.com The Internet's Discussion Network
*
> The fastest and easiest way to search and participate in Usenet - Free!

On Wed, 31 May 2000 11:07:58 -0700, sghani73

 The amount af paging space needed is totally dependent on what your
system is doing.  In other words, you need as much paging space as you
need.

I'd start with 2GB or 4GB and monitor usuage.  I might also define
some paging spaces that are not active on boot if there is some danger
that others would try to grab disk space or you want to be able to
issue swapon on the fly.

You MUST have at least as much paging space as real memory in an AIX system  
as the Virtual memory manager creates (as a minimum) 1 virtual memory page
on disk (paging space) for every 4KB page of real memory. Therefore you
need a minimum of 10GB paging space if your real memory is 10GB in size.

Gavin.

Hi !

Does that also apply to RAM pages whose contents is already stored
on disk ?
I think about the contents of data files and program code,
IMHO this should not need allocated paging space. Correct ?

Regards, Joerg Bruehe

--
Joerg Bruehe, SQL Datenbanksysteme GmbH, Berlin, Germany
     (speaking only for himself)

AFAIK, this is old information.  It does not apply to version 4.3.

With 4.3, the paging allocation algorithm changed so you could run
without paging space if you want to.

--Joe
--
They say never to buy a "0" release of software.
Windows 2000 has 3 of 'em.

** I know the algorithms were changed, but has anybody tested this conjecture?
Some systems will do some initial paging when booting up.  I don't know if AIX is
one of them.

I know for a fact that you can run Linux without paging space.  Did if for months
accidentally.

--
Norman Levin

:>On Wed, 31 May 2000 11:07:58 -0700, sghani73
:>
:>>What is the optimum size of paging space on a machine which has
:>>10 GB RAM. In performance tuning book IBM recommends the maximum
:>>size of paging space with a formula. But I dont think that for
:>>this RAM size it will work.
:>>
:> The amount af paging space needed is totally dependent on what your
:>system is doing.  In other words, you need as much paging space as you
:>need.
:>
:>I'd start with 2GB or 4GB and monitor usuage.  I might also define
:>some paging spaces that are not active on boot if there is some danger
:>that others would try to grab disk space or you want to be able to
:>issue swapon on the fly.
:>

: You MUST have at least as much paging space as real memory in an AIX system  
: as the Virtual memory manager creates (as a minimum) 1 virtual memory page
: on disk (paging space) for every 4KB page of real memory. Therefore you
: need a minimum of 10GB paging space if your real memory is 10GB in size.

  This is definitely not so in AIX 4.2.1, amd I would guess even before.
We have an R50 with two GB of physical memory but only one GB of page
space.  At 4.2.1, you do need enough page space so that every page AIX
might want to write can be written.  If you don't fill memory with stuff
that may need to be paged, you can get away with less page space than
physical memory.  On this R50, much of the memory turns out to be used for
file caching (the machine is currently over-configured memory-wise, but
this may not last forever).

  As of AIX 4.3.2, the system does not _allocate_ page space until it is
actually going to write.  This means that if you don't actually overcommit
memory, so that real paging is minimal, you can get away with much less
page space.  This was done as part of support for very large memory: 4.3.2
supports 32 MB; I'd hate to have to dedicate that much disk space for
paging unless the workload needed it.  I doubt that you can get away with
zero page space, but we do currently have a very idle one GB H70 which is
using one megabyte of page space.

            | 494-3332     | University Computing & Information Services
                           | Dalhousie University, Halifax, N.S., Canada

[posted and mailed]

Joerg,

The initial allocation done by the VMM is at system boot time and is
totally independent of what may be stored in memory in the future. This is
why you need to have your paging space set to at least the size of real
memory.

To answer your second query, Any real memory not utilised by active
processes will be used to "buffer" any data and program code read from disk
(this is why AIX memory utilisation always appears to be at or near 100%).
If the demand for "real" memory increases then these memory pages are the
first to be "stolen" as no "page out" is required (the origional data is
still available on disk) and can be "paged in" when required.

The thresholds that control the VMM behaviour may be altered (tuned) using
the vmtune command.

Regards Gavin.

** do not believe this is true.  If you do a new install of 4.3.3, it will greate
an hd6 of 64mb (I could be wrong here) - but it will not be the size of real memory.
I do know that AIX can run without paging (at least for a while) because there was
a bug in a version of 4.3.2 (or was it 4.3.3?) where a mirrored hd6 on reboot was
not getting swapped on because hd6 was not 'synced'.  Now why anybody would care
if hd6 was synced on a reboot beats me - but there was a problem.  AIX was up but
I have no idea if it did anything useful in multiuser mode.  Anybody have a chance
to test this?

Quote:

> To answer your second query, Any real memory not utilised by active
> processes will be used to "buffer" any data and program code read from disk
> (this is why AIX memory utilisation always appears to be at or near 100%).
> If the demand for "real" memory increases then these memory pages are the
> first to be "stolen" as no "page out" is required (the origional data is
> still available on disk) and can be "paged in" when required.

> The thresholds that control the VMM behaviour may be altered (tuned) using
> the vmtune command.

> Regards Gavin.

--
Norman Levin

I think the correct way to look at it is that you don't have to
initially configure paging space that you might not need.  That is, you
don't have to automatically configure 1xRAM paging space even though you
never expect to page.  This is because the VMM in 4.3.3 doesn't
initially go out and grab a page of VM for every page of real at boot
time (without even knowing if it will be used).

*BUT*, if it comes to a point where you *do* really need to page, you
better have some paging space defined.  The VMM will then go out and
dynamically init virtual memory pages as needed.

Sent via Deja.com http://www.deja.com/
Before you buy.

Hi Gavin [cc: by mail] !

This is not visible in program output (still AIX 4.1):
I have a machine with 128 MB RAM and two paging spaces of 128 MB each,
only one is activated automatically (so page space size == RAM size).
'lsps -a' shows it is used 16 % only (unloadad machine), so it does
not show the space to be allocated from boot.

Quote:

> To answer your second query, Any real memory not utilised by active
> processes will be used to "buffer" any data and program code read from disk
> (this is why AIX memory utilisation always appears to be at or near 100%).

Agreed.

Quote:> If the demand for "real" memory increases then these memory pages are the
> first to be "stolen" as no "page out" is required (the origional data is
> still available on disk) and can be "paged in" when required.

Agreed.
But this "page in" will AFAIK come from the file system, not paging LV.
Then why should these RAM pages have got allocated page area, if there
is no need to page them out ? (Other than LRU buffer writes to files.)

As the future use of RAM ("buffer", code, or "real program data" like
stack and heap) can not be told at boot time, IMHO there is no use in
allocating paging space for the physical RAM at boot time.
I expect AIX to do that only when processes (or kernel functions)
require address space for stuff without disk backup (= stack + heap +
SHM segments + kernel data like MSQs),
so the system should boot and work with much less paging space than RAM
if large areas serve for code segments and file buffering.

(I currently do not feel like re-configuring my machine with reduced
"hd6" and testing, but AFAIR I had such a configuration on a machine
we had on loan - it had 1 GB RAM and disks of 1 or 2 GB each, and
I think we had a small "hd6" and activated other page spaces in other
VGs only later, so it booted with less paging space than RAM.)

Regards, Joerg Bruehe

--
Joerg Bruehe, SQL Datenbanksysteme GmbH, Berlin, Germany
     (speaking only for himself)

While this was traditionally true, it is no longer true on AIX 4.3.3 (and
4.3.2, I believe). See the readme file in /usr/lpp/bos.

--
Urban A. Haas
CEO - Urban Technology, Inc.
Minneapolis, MN  USA
Phone: (952) 595-8810    Fax: (952) 595-8710

Web: http://www.urbantechnology.com

This e-mail was composed of 100% recycled bits.

Yes, I have an F50 with 2Gb real memory, and 1280 Mb paging space, running
AIX 4.3.2.  100% memory (ram) usage has been achieved.

--Joe
--
They say never to buy a "0" release of software.
Windows 2000 has 3 of 'em.

[posted and mailed]

Joerg,

Sorry a bit of confusion on my part between "required" and "recommended"
under AIX version 4.3.3

Below is an extract from the "AIX Versions 3.2 and 4 Performance Tuning
Guide" regarding paging sizes and placement.

"Placement and Sizes of Paging Spaces

The general recommendation is that the sum of the sizes of the paging
spaces should be equal to at least twice the size of the real memory of the
machine, up to a memory size of 256MB (512MB of paging space). For memories
larger than 256MB, we recommend:

   total paging space = 512MB + (memory size - 256MB) * 1.25"

This information can be located on the following web page:

http://www.rs6000.ibm.com/doc_link/en_US/a_doc_lib/aixbman/prftungd/p...
t.htm#hWQAj387wate

There is also some good information regarding "Paging space allocation
policies" on the following page:

http://www.rs6000.ibm.com/doc_link/en_US/a_doc_lib/aixbman/admnconc/p...
ce_under.htm#A8F021

Regards Gavin.