My original post was:
Hello all,
I think this topic has been discussed in the past, but with the advent of
systems with much larger amounts of memory, I'd like to rehash it.
I realize that swap space and var space requirements depend upon the
specific system usage, but I would like to get opinions and recommendations.
I guess my question relates specifically to general purpose systems
utilizing immediate mode of swapping rather than over-committment mode.
In immediate mode, it has typically been recommended to provide at least 2.5
to 3 times the amount of memory.
With older systems maxing out around 512mb of system memory, this is not a
problem. But what about newer systems like the Alpha 8400s that are now
capable of up to 28gb of memory, 84gb of swap seems ridiculous.
With var, I had heard that it was recommended to provide at least memory
capacity plus about 30mb. I realize that var space is also dependent upon
specific utilization of the system, but is there a general rule for a minimum?
Thank in advance.
-------------------------------------------------
In general the answer is "it depends".
Thanks to the following people.
From: alan_at_nabeth.cxo.dec.com
> In immediate mode, the amount of page/swap space you need
> is the amount of virtual memory you want to have. The
> 2-3 times recommendation comes from the observation that
> a typical interactive load use 2-3 times as much virtual
> memory as physical memory. If you user load scales up
> with the amount of memory, then you could need that much
> page/swap space, since the virtual memory needs will stay
> about the same per-user.
>
> On the other hand, if your user load will remain approximately
> constant and the extra memory will be for the occasional large
> process, larger shared memory or more UBC, then you may only
> need a little more page/swap space than physical memory. The
> key is that in immediate mode reservation of space, the virtual
> memory limit is controlled by the amount of page/swap space
> and has little to do with physical memory.
>
> What you may be able to do is examine the typical process
> load by user and see what the per-user virtual memory
> requirements are. Scale that for the number of users you
> expect and then add in the virtual memory requirements that
> don't scale per user. The output of ps(1) can help a little,
> but I don't think its virtual memory numbers take into account
> the parts that are shared. Top may pick up more detailed
> information. If all else fails, both of these work through
> the /proc interface and it is documented.
>
From: Kurt Carlson <snkac_at_java.sois.alaska.edu>
With a very large memory system, you likely have some application which
is going to wire a large chunk of memory. Once you determine that,
the rule-of-thumb for 2.5x your remaining memory may still be accurate.
For example, if you're running a 6gb system of which 2gb is dedicated
to Oracle sga, you only need to plan 10gb of paging space. That's
presuming you have enough demands on memory to be doing any paging
or swapping. If you're constrained enough to need more than 10gb
of swap space, you may very well need more memory.
-------------------------------------------
/--------------------------
| John Seel
| Senior Systems Engineer
| Total Tec Systems, Inc.
| (732) 906-6500 ext 185
| Fax (732) 744-2246
| "johns_at_totaltec.com"
\--------------------------
Received on Tue Sep 23 1997 - 15:04:57 NZST