XP memory/pagefile limitations

[mfbernstein]

I understand that Windows XP (32-bit version) is limited to 3GB of physical memory. Likewise, the Windows XP pagefile is limited to 4095MB. Does this mean that a 32-bit XP machine can theoretically handle 3+4=7GB of physical+virtual memory? Or is everything beyond 3GB effectively left unusued? In other words, is there ever a situation where the pagefile can be disabled because XP won't ever be able to use it? Thanks,

MFBernstein

[Stargate199]

I think 32 bit operating systems can recognize up to 4 GB of internal memory (RAM). The limitation comes down to the BIOS. I think the T60 could have 4GB or ram installed, but would only recognize 3GB. Virtual memory can be set to any amount. The ram limitation is related to the BIOS of 32 bit computers.

[mfbernstein]

No, the limitation is not of 32-bit OSes. 32-bit Linux can address upwards of 64GB if need be. The Thinkpad X61/T61 have no chipset limitation, and so can address up to 4GB.

What I want to know is whether the pagefile for Windows XP is ever wholly redundant: if you have 3 (or 4) GB of real memory, does that mean the pagefile will never be touched, or no?

Thanks.

[RealBlackStuff]

If you have the program Photoshop, you MUST have a pagefile.
Similarly, other apps may (or may not) need that pagefile.
You can try and run your PC without one, and try programs until it fails. Then you know if you need it or not.
There is no rule written in stone about a pagefile or not.

[Stargate199]

There is a registry hack (I posted it here a while back, do a search) that will make Windows use all of the physical memory until it runs out or a program needs VM. It will make your machine a lot faster, especially if you have more than a gigabyte of memory.

[Marin85]

@Stargate199: I´m quite interested in such fixes increasing performance I did my search here but didn´t find your post about that registry fix. Could you post a link here, thanks in advance!

Marin

[ashleys]

You may wish to read this for a basic summary of how virtual storage works,

http://support.microsoft.com/kb/555223

[Stargate199]

Here is what you need to do. Always back up your registry before performing any modifications.. Go to Start > run and type regedit. click file > export to backup your registry to a safe location.

Next Navigate to HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Session Manager\Memory Management.

Select the DisablePagingExecutive value to '1'

Your done! The more ram you have, the better benefit this edit works.

also from that screen, you can change CleanPageFileAtShutdown to 1 and that will clear the page file when you turn off the computer. This can be useful if you use the pagefile alot.

[ashleys]

That registry key only affects user-mode and kernel-mode drivers and kernel-mode system code.
It makes them ineligible to be paged out on a page steal, thus leaving them in RAM.

[mfbernstein]

If you have the program Photoshop, you MUST have a pagefile.
Similarly, other apps may (or may not) need that pagefile.
You can try and run your PC without one, and try programs until it fails. Then you know if you need it or not.
There is no rule written in stone about a pagefile or not.

Thanks, but that doesn't answer my question. At any rate, Photoshop is a special case since it has its own 'virtual memory-like' system, independent from the OS.

The question is simply how much real+virtual memory can Windows XP (32-bit) handle. So far, it looks like 3GB real + 4GB virtual, but I've not seen that confirmed anywhere. Obviously, no matter how much memory you have, you will eventually run out if your run too many programs.

[ashleys]

Architecturally a 32-bit OS can support addressing of 4GB, both real and virtual.
XP supports 4GB of real storage (RAM).

The problem in the windows world is that Microsoft are using the wrong terminology for the pagefile. They call this "virtual memory". It is not.
Virtual memory, or to give it it's original title, virtual storage is a concept. It's the ability of the hardware, with appropriate OS support, to map (virtual) pages to (real) frames. This allows many address spaces to be active at the same time and have their active pages (aka Working set) resident in real storage when executing but moved out of real storage when inactive.

To understand how this works you need to understand the following terminology,

Frame - 4K of real storage
Page - 4K virtual storage
Slot or Block - 4K of auxiliary storage (aka page file).

When an address space is created, two sets of page tables are built to map the pages of the address space. As the pages are used, these tables are updated to indicate whether the page is resident in real storage or not. When your program is executing and a storage reference is made to an address that is valid within your address space but is currently page out (therefore on auxiliary storage), the hardware raises a page fault. The OS sees the page fault and then using the page tables locates the slot on auxiliary storage that contains your page. The page is then brought into real storage and your program is re-dispatched.
The OS also undertakes tasks like working set trimming, page steals and block paging to attempt to maximise the use of RAM without programs suffering any penalty to performance.

It should also be noted here that there is a significant difference between the ideas of paging and swapping. People appear to use them interchangable but shouldn't. Paging is the movement of a single page at a time, whereas swaaping is the movement of a working set (ie many pages in one operation). Windows like many other OS's implements a concept of block paging (called cluster paging in Windows). This is where, when a page fault is raised, the adjacent pages (normally up to 5) are also brought in as you're likely to reference those as well.


In theory, an OS can support any size auxiliary storage as it's not tied to the addressing limit. However, in practical terms there is usually a limit set as the supplementary control block structures for the auxiliary storage manager could get too large.

I'll try and find out what limit Windows has set on the pagefile size.

[ashleys]

It appears that by default, Windows has implemented a 4GB limit on the the pagefile by how the page mapping is done.

However, this limit can be overcome by using the following method,

http://support.microsoft.com/default.as ... -us;237740

[davidspalding]

No, the limitation is not of 32-bit OSes. 32-bit Linux can address upwards of 64GB if need be. The Thinkpad X61/T61 have no chipset limitation, and so can address up to 4GB.

What I want to know is whether the pagefile for Windows XP is ever wholly redundant: if you have 3 (or 4) GB of real memory, does that mean the pagefile will never be touched, or no?

Thanks.

I've run with little or no pagefile on my system with 2 GB, it will work, but Windows performance was slower in some areas. The pagefile DOES serve a purpose. Photoshop CS2 ran fine, but earlier versions needed the pagefile.

If you don't want much pagefile, set it to 1024 MB and see how your system performs for a few days. Also look for SysInternals PAGEDFRG utility to consolidate the pagefile(s).

[mfbernstein]

It appears that by default, Windows has implemented a 4GB limit on the the pagefile by how the page mapping is done.

However, this limit can be overcome by using the following method,

http://support.microsoft.com/default.as ... -us;237740

Ah, good find. Thanks for the comments. Looks like the pagefile is something better left enabled.