vmstats interpreting

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# 1  
Old 02-21-2002
Data vmstats interpreting

We are having performance issues on an alpha4100 server.
I can't paste a snapshot of my vmstat in here, but...

We have 4gb of memory. The actual memory stays consistant around 306k. Free is dropping into the 120 area. Wire is around 206k consistantly. consistantly.
My manual says that unix should be intelligent enough not to run out of memory.
We have 0% idle cpu. some page out.
Our online transacations seem to be "not too bad" but many of the things running at the command level are poor.
If the free memory is dropping this low and we have 4gb of memory, shouldnt the used be going up proportionally?
would this be indicative of a memory leak?
# 2  
Old 02-22-2002
There is not much to go on here since you are keeping the vmstat output secret, but I can make a few comments.

I'm not familiar with alphas, but few versions of unix work very well when main memory is exhausted. The only risc based unix system that I ever seen keep its head above water under this condition is HP-UX. And HP-UX does it by "deactivating" some processes rather than attempting a swap. A deactivated process cannot run at all.

It is literally true that you cannot allocate every page of memory, but performance suffers badly when you don't have enough just the same.

If you are getting "some" pageouts, then you will see "some" benefit from more memory.

0% idle time is also very bad news and if this lasts for long periods, it may be your real problem.

Can you add another cpu? And more memory? Talk to your salesperson and see if can arrange to borrow these items. Then you can see how well they address your problem and decide if they are worth the cost.
# 3  
Old 02-22-2002

We are maxed out on cpu and memory. It would be nice to have room to grow.

I'm not keeping the vmstat a secret.. I could not get it in a format that would paste into the message area on this posting and be readable. I would have had to re-type it.

What I'm not sure on is if I "SHOULD" see the active memory go up when the free memory goes down. It sounds logical that I should, but the tuning manual says that free memory plus wire should total "around" the total memory capacity. But it doesn't.
I'm not sure if overhead memory doesn't show, because we are short approx 50k pages in the memory reporting.
When we took the apps down, there was still 53k pages in use. After booting, there were 11k in use.
It never shows me the full 4gb in use even when the free goes as low as 20 pages...
# 4  
Old 02-22-2002
What do you mean by "wire"? I haven't heard that term in relation to vmstat before.

Again I don't know your system, but active memory is normally memory which has been used in the past 20 seconds (and perhaps a few other pages that waiting for disk operation to finish that somehow has exceeded 20 seconds). "free memory" is on the free list and can be reused at will. In between you have some stuff that is not free but hasn't been used recently either. Run this command line:
sleep 100000
and the process you create will be active for 20 seconds. After that, no it isn't free, nor is it active. I've seen inactive telnet sessions that last for months waiting the next command. When memory like this is freed, it tends to stay freed for a long time. Active memory is a real problem...you page it out and you will probably need to page it back in very quickly.
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