topic Re: What happened with reserved pages ? in Operating System - OpenVMS

What happened with reserved pages ?

Wim Van den Wyngaert — Wed, 18 May 2005 04:19:53 GMT

In the output of sh mem/fi under 6.2 you had the column reservable. In 7+ this column is gone while the rsrvpagcnt parameter still exists.

Is reserving still used ? If yes, how can I check the number of reserved pages ?

Wim

Re: What happened with reserved pages ?

Ian Miller. — Wed, 18 May 2005 04:53:45 GMT

basically it does not do reserving any more.

Before V7.3 when a process was created it was alocated to a page file and reserved RSRVPAGCNT pages. When a process needed space in the pagefile it used the pages it has reserved and reserved more when it used them all. A process could use up to four pagefiles.

In V7.3 and later processes are not allocated to a specific page file on creation but a process (actually the modified page writer on behalf of the process) will allocate pages in a pagefile when they are actually needed.

Re: What happened with reserved pages ?

Wim Van den Wyngaert — Wed, 18 May 2005 05:00:31 GMT

Ia,

I checked the release notes of 7.3 but didn't find any notes on it. Where did you find it ? Why is the rsrvpagcnt still present ?

WIm

Re: What happened with reserved pages ?

Ian Miller. — Wed, 18 May 2005 05:39:21 GMT

I found it from internals information. As far as I can tell the rsrvpagcnt system paramter is not used.

Re: What happened with reserved pages ?

Jan van den Ende — Wed, 18 May 2005 05:55:21 GMT

Wim,

Why is the rsrvpagcnt still present ?

because they never ever get removed (upward compatibility, remember?)
It is still there, and any historic manipulations just stay valid, only they have no langer any effect.

Proost.

Have one on me.

jpe

Re: What happened with reserved pages ?

Wim Van den Wyngaert — Wed, 18 May 2005 05:58:26 GMT

Jan,

What I mean is that the help text is still not adjusted. And nothing in the release notes. Should not occur ...

Have some on me but keep it Belgian

Wim

Re: What happened with reserved pages ?

Wim Van den Wyngaert — Thu, 19 May 2005 01:31:35 GMT

10 points for he/she who has a link to a text explaining the pagefile allocation and usage in detail (7.3+) ...

Wim

Re: What happened with reserved pages ?

David B Sneddon — Thu, 19 May 2005 02:12:37 GMT

Wim,

Not an explanation of the allocation but the help appears to be
fixed in 7.3-2

tardis_FTA7> sysgen help sys_para rsrv

Sys_Parameters

RSRVPAGCNT

This parameter has been obsolete on Alpha systems since OpenVMS
Version 7.2.

On VAX systems, RSRVPAGCNT sets the number of pages that are
reserved and escrowed for the current process page file.

This special parameter is used by HP and is subject to change. Do
not change this parameter unless HP recommends that you do so.

I too have been unable to find anything in the release notes
from 7.3 through 8.2.
Doing a SYSGEN SHOW RSRVPAGCNT also does not show it as
being obsolete.

Dave

Re: What happened with reserved pages ?

comarow — Wed, 01 Jun 2005 13:56:28 GMT

In earlier versions of VMS, when a process was created, it actually allocated pages of page file quota in the page file.

Later that number became strictly theoretical, if all the pages of pagefile were used, what would the number be. It really was just theoretical since that condition had very little relationship to reality.

A lot of people were concerned and thought they had a problem when that had a negative number, but it really was simply an abstract concept and NOT a real problem.

Consequently, it was removed.

The real issue is $show mem/files
you want to be 50% free or at least a couple of hundred thousand free pages.

As you get low the system will slow down and at zero can hang.

You can, on the fly, install an additional page file, and save a hang or forced reboot.

Bob

Re: What happened with reserved pages ?

comarow — Wed, 01 Jun 2005 18:45:35 GMT

To add to this topic. What is a page file?

When a page is modified, but won't fit into the working set, it is put on to the modified page list.

If one of those pages is needed, it will soft fault back into the working set.

However, certain events, such as a memory shortage, or growth of the modified page list, will cause the system to flush pages from the modified page list to the page file.

At that point, a process has to actually read from the page file.

Now, how to examine this on your system.

$monitor page

Look for page read I/Os to how often we are reading from the page file.

A real performance hit is write I/Os, how often you are writing to the page file.

If you are writing a lot to your page file, think about adding more to working sets and adding memory.

And I can't emphasize strongly enough, never run low of free page file space!

Re: What happened with reserved pages ?

Volker Halle — Thu, 02 Jun 2005 03:22:33 GMT

Wim,

the best information (I could find) regarding the new pagefile allocation design is to be found in section 5.6 of the V7.3 New Features and Documentation Overview manual:

http://h71000.www7.hp.com/doc/73FINAL/6620/6620pro_004.html#large_pf_sec

Volker.