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Another question about Global Pages

 
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Jack Trachtman
Super Advisor

Another question about Global Pages

VMS V7.3-2 (w/most patches)
GS1280

Fact #1:
========

$ install list/glo/sum

Galaxy Group Global Sections


1298 Global Sections Used, 36254432/255685040 Global Pagelets Used/Unused

Fact #2:
========

sho mem/phy
System Memory Resources on 19-AUG-2005 09:31:31.96

Physical Memory Usage (pages): Total Free In Use Modified
Main Memory (64.00GB) 8388608 2402302 5649259 337047

What gives?
===========
From Fact #1:
- 36254432 pagelets used = about 18GB mem
(very reasonable)

- 255685040 pagelets free = about 128GB mem,
yet Fact #2 shows we only have 64GB total!

Can someone explain this to me? Thanks.
6 REPLIES
Volker Halle
Honored Contributor

Re: Another question about Global Pages

Jack,

the 'V' in VMS stands for 'Virtual' Memory System.

Global Pages can be paged and don't need to occupy physical pages.

Volker.
Antoniov.
Honored Contributor

Re: Another question about Global Pages

Jack,
use sh mem command as follow
$ SHOW MEM/UNIT=BYTE/PHYS !Physical pages
$ SHOW MEM/UNIT=BYTE/FILES !Virtual mem
I guess you can understand why you virtual memory is greater than physical memory.

Antonio Vigliotti
Antonio Maria Vigliotti
Volker Halle
Honored Contributor
Solution

Re: Another question about Global Pages

Jack,

global pages are an OpenVMS memory management construct to allow several processes to share parts of their virtual address space.

If a process references a global page, which is not yet in physical memory, it has to be loaded into memory from it's backing store (global page fault). If another process then accesses the same global page, it just has to be made valid in his workingset (global valid fault). Using global pages saves physical memory and pagefault IOs.

Note that there could also be global pages physically locked into memory (memory resident global sections - see the SYSMAN RESERVED_MEMORY commands or the $CRMPSC_GDZRO_64 system service).

Volker.
Robert Gezelter
Honored Contributor

Re: Another question about Global Pages

Jack,

Process private and global pages are essentially differ in scope, not concept. Process private pages are private to an individual process, global pages are available to multiple processes. These differing classes of pages appear in different areas of your process' address space (see the "OpenVMS Programming Concepts Manual", available in PDF and HTML from the OpenVMS www site at http://www.hp.com/go/openvms; there is also a good description of the issues surrounding global sections in various chapters of the Internals and Data Structures books -- the general concepts have not changed extensively in a VERY long time).

With either process-private or global pages, you can (and I can argue, should) have more virtual memory than real memory in most cases. This is the reason for the advent of virtual memory about 40 years ago. In short, much of the program and data used by a program is not used often, if ever. Before the advent of virtual memory, and on machines not supporting virtual memory, program controlled overlaying (of code and data) was used to fit applications into machines. It was a grueling process. Virtual memory removes that laborious process to the operating system. When information is needed, it is paged in. When information has not been used for an extended time, the "page" containing the information (if it is writeable) is written to the temporary backing store (aka, the page file) and the memory is recycled. Generally speaking, this is an efficient process, and not a problem. When paging becomes excessive, this leads to a performance problem referred to as "thrashing". There is a wealth of pre-VMS literature on the issues surrounding paging, in journals such as those published by the ACM (www.acm.org) and IEEE (www.ieee.org). There are also some classic books covering the origins of paging, including that by Bell and Sieworiek (my apologies to Dan Sieworiek if I have mis-spelled his name, I don't have the book handy).

The amount of virtual memory to real memory is referred to (appropriately enough) the virtual to real ratio. The suitability of a specific virtual to real ratio is workload dependant. 2:1 is not a particularly high ratio for a timesharing workload.

My apologies for the long post, I hope that the background is useful.

- Bob Gezelter, http://www.rlgsc.com
Wim Van den Wyngaert
Honored Contributor

Re: Another question about Global Pages

Jack,

You are not the first one that is supprised.
Check http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=855313

Wim
Wim
Jack Trachtman
Super Advisor

Re: Another question about Global Pages

Thanks for the clarification.

Years back I just presumed that the output from the INSTALL LIST cmd was refering to physical pages. Now I know better!