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HPUX Free Memory

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Ted Ellis_2
Honored Contributor

HPUX Free Memory

We have HPUX 11.00 and Sybase 12.0. Currently have 5 GB of RAM and 6 instances of Sybase running. All servers are running, but with the large number of instances they have varying amounts of memory assigned (based on how database is used). Does anyone have information that indicates how much memory on a system like mine should remain free to support new processes (system or user)? We had a case where available free memory dipped to abuot 12 MB (viewed with Glance). This server runs with about 190 MB of free memory during normal operations. I am hoping to justify purchasing some additional memory. The sybase allocations vary from 150 MB to 750 MB.

Ted
4 REPLIES
Jeff Schussele
Honored Contributor

Re: HPUX Free Memory

Hi Ted,

It's not so much how *much* memory is free. It's about the % of free memory pages.
vhand & sched are the processes that watch the free page % & initiates actions based on that value.

Here's a good doc on how & why vhand & sched do what they do.

http://www.docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/5965-4641/5965-4641_top.html&con=/hpux/onlinedocs/5965-4641/00/00/11-con.html&toc=/hpux/onlinedocs/5965-4641/00/00/11-toc.html&searchterms=vhand&queryid=20030417-145550

HTH,
Jeff
PERSEVERANCE -- Remember, whatever does not kill you only makes you stronger!
James Murtagh
Honored Contributor
Solution

Re: HPUX Free Memory

Hi Ted,

There is only one limit you can take in this scenario, especially if you have to justify the purchase of new memory - the kernel counter desfree. The is the desired amount of free memory the OS needs to prevent paging to the swap devices, which will have a negative impact on performance. To find this value use:

# echo desfree/D|adb -k /stand/vmunix /dev/kmem

This will report the number in 4k pages. I believe it defaults to 1/64 (or 1/32??)of physical memory on a system of your memory capacity. Also use:

# swapinfo -tam

To check for any usage of the swap devices at peak usage. This in itself may be justification of the need to add more memory if performance is critical on your server. This should also give you a guide on how much memory to add.

Regards,

James.
Ted Ellis_2
Honored Contributor

Re: HPUX Free Memory

thanks gents.. that gives me some additional data points to reference.
Bill Hassell
Honored Contributor

Re: HPUX Free Memory

As mentioned, free memory isn't that important. HP-UX is a virtual memory system so with 5Gb of RAM and perhaps 5Gb of swap space, you really have more than 8Gb of memory for processes. So the key is not how much free memory there is (the swapper will take care of low memory), it is how many pages are being swapped out per unit of time. Use vmstat (ignore free memory) and look at po. If po is zero to 20, all is well and you can pile in more processes.

On the other hand, if po is 100 or higher for long periods of time, then you are indeed short of RAM and may need to increase to 8Gb. The reason you can pile so many processes beyond the limits of RAM (and not see a slowdown) is that a lot of the processes are idle while they wait for some keyboard responses. These interactive tools are perfect for low memory systems since they can be paged out to swap and no one will notice.

On the other hand, if the 6 instances of Sybase significantly exceed the available RAM, then page-out rates will soar and overall performance will seriously degrade. The fix is easy though: add a lot more RAM. You DBAs will love the additional RAM as it will improve response time as they ask for more...


Bill Hassell, sysadmin