Mea Culpa (it's corrections time).

So why does NT require such a wonking great big paging file on my machine?

Larry Osterman's WebLog — Wed, 05 May 2004 20:30:00 GMT

re: Mea Culpa (it's corrections time).

/C — Wed, 05 May 2004 18:10:00 GMT

This is a good and useful blog, and I think writing about something so intrinsically part of OS as paging is difficult because I am sure it has gone though several generation of changes even though the underlying scheme of how it works has been the same. I had a couple of questions I am hoping someone can shed more light on:

- How do I set up 16 paging files? I thought each paging file is mapped to a physical disk and hence you would need to partition your drives into 16 disks to get 16 paging files.

- Isn't the size of paging file suggested by NT/2000/XP also 1.5 times the physical memory because of something to do with hibernation?

- What is PSS and PAE?

/C

re: Mea Culpa (it's corrections time).

Larry Osterman — Wed, 05 May 2004 18:22:00 GMT

Hmm. First, the easy ones:
PSS - Product Support Services = Microsoft Product Support
PAE - Physical Address Extensions = A set of extensions to the x86 processor that allows addressing more than 4G of RAM.

You may be right /C about hibernation, the MM guys didn't mention that one, so I just don't know :).

Setting up more than 16 paging files. The easiest way is to have 16 physical disks :)

Realistically, there's no reason to ever have more than one paging file on a given spindle - you get no benefit and potential difficulty (excessive head motion, inability to defragment the disk, etc).

re: Mea Culpa (it's corrections time).

Greg — Wed, 05 May 2004 18:41:00 GMT

I want 256TB of page file(s)...

I think I'll ask our SAN guys if I can borrow some space...

:|

re: Mea Culpa (it's corrections time).

Jerry Pisk — Wed, 05 May 2004 19:39:00 GMT

How can I create two page files on a single partition? Or do I need 16 partitions to have 16 page files? And is there a way to create a page file on a partition that is mounted to a folder istead of having a drive letter?

re: Mea Culpa (it's corrections time).

tim — Wed, 05 May 2004 20:55:00 GMT

if ihave 2gb ram but never use morethan 800mb peak, would i suffer any performnce hit setting the pagefile to 2mb or switching it off, then it would never page, which must be faster, right ?

re: Mea Culpa (it's corrections time).

Larry Osterman — Wed, 05 May 2004 21:35:00 GMT

Jerry/Greg: I don't know. You'd have to ask someone in the memory management team.

re: Mea Culpa (it's corrections time).

Mike Dimmick — Wed, 05 May 2004 22:49:00 GMT

Hibernation doesn't use the page file. It uses a separate hidden file called hiberfil.sys, which is the same size as your physical memory.

When actually hibernating, Windows compresses the contents of memory when writing to disk and decompresses when resuming from hibernation. It doesn't do this to save space, it does it to speed up the disk transfers (less I/O = less time).

My 512MB Windows XP (desktop) system hibernates in under ten seconds and resumes in under five (P4 2.8GHz, 10k rpm HDD). It's still just about worth doing, since a cold boot takes about 20.

re: Mea Culpa (it's corrections time).

Pavel Lebedinsky — Wed, 05 May 2004 22:59:00 GMT

>setting the pagefile to 2mb or switching it off, then it would never page, which must be faster, right ?

Wrong. First of all, this will not eliminate all paging. Binaries (EXEs and DLLs) and memory mapped files that are not backed up by pagefile will still page in and out as necessary.

So if the system really wants to free some RAM it will always find something to page out. Running without pagefile only restricts what it can choose from.

Which brings us to the second point. A lot of stuff that is normally paged out to and rarely if ever paged in will now sit in memory all the time, reducing the amount of RAM that's avaliable for other purposes.

re: Mea Culpa (it's corrections time).

Larry Osterman — Wed, 05 May 2004 23:00:00 GMT

Thanks Mike and Pavel, I always like learning new stuff.

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Larry Osterman s WebLog Mea Culpa it s corrections time | work from home

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