What happened with reserved pages ?

[Wim Van den Wyngaert]

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

[Ian Miller. · Accepted solution]

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.

[Wim Van den Wyngaert]

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

[Ian Miller.]

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

[Jan van den Ende]

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

[Wim Van den Wyngaert]

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

[Wim Van den Wyngaert]

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

Wim

[David B Sneddon]

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

[comarow]

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

[comarow]

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!