topic Re: Reason for page outs in Operating System - HP-UX

Reason for page outs

Ed Hon — Wed, 05 Jun 2002 13:33:12 GMT

Glance shows I'm using 90% of available memory. Yet, when I view the memory report, it shows I'm still doing occasional pageouts. Why is that?

Re: Reason for page outs

A. Clay Stephenson — Wed, 05 Jun 2002 13:38:46 GMT

1) You may actually be paging out during peak times.
2) Memory-mapped files. You will always see a bit of this even with little memory pressure.

Re: Reason for page outs

Duncan Edmonstone — Wed, 05 Jun 2002 13:41:07 GMT

Ed,

There's no point in closing the stable door after the horse has bolted! If HPUX waited until memory utilisation reached 100% before starting to page out then the whole system would grind to a halt very quickly! HPUX has a threshold which once reached starts to activate paging... looks like at 90% you've reached that threshold.

HTH

Duncan

Re: Reason for page outs

S.K. Chan — Wed, 05 Jun 2002 13:45:50 GMT

I call this the "hidden" pageout which we don't know about. Yes it makes sense for pageouts to happen if you're low on memory but you wonder since you got 10% left why pageout still happened. The other reason for the occasional pageout (the "hidden" reason) is that your application may be doing memory mapping file and calls to those functions can be the reason why you're getting the occasional pageouts. In glance take a look at funtions like mmap and msync (in the SYSCALLS screen) ..
# glance -Y
If they exist .. this then is the reason why you still get pageouts.

Re: Reason for page outs

ashish nanjiani — Wed, 05 Jun 2002 14:21:25 GMT

I have faced a similiar problem where I was seeing paging activity eventhough I had 15 % physical RAM left. In my case it was related to some processes which ran in the night that were utilizing more memory causing the system to reach 100% of mem and hence the system started to page out.

Re: Reason for page outs

Duncan Edmonstone — Wed, 05 Jun 2002 14:36:47 GMT

Ed, there's an excellent document on memory management installed on every HPUX system, take a look at the file /usr/share/doc/mem_mgt.txt - particularly the section on paginh thresholds and the kernel parameters minfree, desfree, lotsfree and gpgslim. You can see what these values are set to on your system using these command:

echo "minfree/D" | adb /stand/vmunix /dev/mem
echo "desfree/D" | adb /stand/vmunix /dev/mem
echo "lotsfree/D" | adb /stand/vmunix /dev/mem
echo "gpgslim/D" | adb /stand/vmunix /dev/mem

This shows the values in pages (4K blocks).

HTH

Duncan

Re: Reason for page outs

Ed Hon — Wed, 05 Jun 2002 14:38:56 GMT

I opened two Glance sessions, one with the memory report and one with the syscalls report. Whenever there was mmap activity, there was corresponding pageout activity. Otherwise, both were zero. (I never saw the msync call.) Thanks.