Question about swapping

[Daniel Schneider_4]

Hi all,
I want to explore, how I have to increase our Memory.
We have 640 MB on our HP-D380, and we have great performance problems, when
many users are using the application.
Now I think, that the Machine looses many performance by swapping and I'm
searching how much Memory I must increase.
Can I use the program top, with only adding the three Memory-fields to find out
the value?
What means Memory real, virtual and free, and the values in the brackets?

[Daniel Schneider_4]

Hi all,
I have yet another question about swap:
when I start swapinfo -t ,there is a row wich is named with 'reserve'. What
means the values in this row?
In other words, what says me the following result about my swap or about my
memory?:
# swapinfo -t
Mb Mb Mb PCT START/ Kb
TYPE AVAIL USED FREE USED LIM RES PRI NAME
dev 655 41 613 6% 0 - 1 /dev/vg00/sw1
dev 1392 42 1349 3% 0 - 1 /dev/vg01/sw2
reserve - 522 -522
total 2048 606 1441 30% - 0 -

Does it mean, that I have to increas my Memory on about 41+42 Mb??

Thanks for all response

Regards
Daniel

[Melvyn Burnard_1]

To decide how much memory you will need to add is not an exact science. It will
depend on what the system is running, and how many users. Also, are you
planning to add users/processes/applications in the near future?
As far as the swapinfo command goes, that is generally used to observe how the
swap space you have configured is being used.
To answer the second question, the reserve field is, as per the man page for
swapinfo:

reserve
Paging space on reserve. This is the amount of paging space that could be
needed by processes that are currently running, but that has not yet been
allocated from one of the above paging areas. See "Paging Allocation" below.

The main question is: Is it definitely memory bottlnecks?
The Glance product can help you to investigate this more closely.
It might be worth it to increase the memory substantially, say double it,
leaving some in reserve for future increases in workload.

[Daniel Schneider_4]

Hi Melvyn,
thank you for your reply.
We don't want to change our system. That is not the Question. I have to analyse
the situation we have at the moment.
Let me tell you a little more about the situation:

I can observe with the program 'top', that the Memory decreases in the morning,
when less users are at the system. It leaves the field 'free' and goes to the
fields 'real' and 'virtual', when more users are loggin in. The swapinfo says:
used=0.
Then at about 9:30 the machine beginns with swapping, when more than 40 users
are on the system. I see this again with swapinfo, because the 'used'-field
grows up.
And at that Moment, the users deplore themselves about the performance.
So I'm really sure, that we have a problem with memory.
Now the question is, can I add the three memory fields in the program 'top' to
determine how much Memory the machine really uses? Eg.: The machine has 640Mb
memory, the summary of the fields in 'top' is 840Mb, so I have to increase the
Memory by at least 200 Mb.

Is that right?

Regards
Daniel

[Melvyn Burnard_1]

Personally I have found top to be a bit inaccurate at times, but I would say
yes, you need to look at adding at least another 256Mb. It may be worthwhile
having some performance consulting if you are really worried, but at the price
memory is today, probably just as cheap to buy and add in extra 256-512Mb.

[Rick Taylor_5]

What version of HP-UX are you running?

[Daniel Schneider_4]

We run HP-UX 10.20.

[Rick Taylor_5]

I know this isn't answering your questions about swapinfo output but...

What is the buffer cache set to? The reason I ask is if the buffer cache is at
the default (dbc_max_pct 50) then the system may come under memory pressure
(i.e. start paging) because it has too much buffer cache.

If you haven't adjusted the buffer cache I would suggest starting there. Try
setting down to 20% and if the read cache hit rate stays high then perhaps it
can be set lower. Doing this will free up more memory for user processes.

[Fusuy Deivan]

Hi there,

To answer your question about the memory fields displayed in top output, I have
to say that the term virtual memory refers to extending memory beyond physical
(real) memory by including secondary storage areas (for example; swap areas,
memory mapped files, etc.) Using addresses from the virtual address space you
can access the contents of physical memory or secondary storage. Values in
parenthesis is the amount which is being used by active processes. My
observation is that the free memory field value is similar to the free memory
value Glance reports. Reducing the Dynamic Buffer Cache to 20%, if you are
using it at > 20%, is a good idea. You may use Glance to calculate your total
memory requirements and then decide if you need to increase memory.
DF

[Daniel Schneider_4]

The value of dbc_max_pct is set to 10. So I think that isn't the problem. Or
what do you think?

Regards
Daniel

[Rick Taylor_5]

Daniel,

You are right. With dbc_max_pct set to 10, buffer cache probably isn't part of
the problem (this assumes nbuf & bufpages aren't overriding dbc_max_pct.

I'm going to email to you a performance troubleshooting paper I got from HP a
few years ago. Maybe this will help.

Also, and this is just my curiosity, is there a particular reason you have
pseudo swap turned off?