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Index > OS Construction > Page sizes: 4KB vs 2MB vs 4MB

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Chewy509



Joined: 19 Jun 2003
Posts: 297
Location: Bris-vegas, Australia
Chewy509
Hi guys,

Just interested if anyone is using a page size other than 4KByte? Since most apps seem to be quite large (data wise) and most PC's having at least 512MB of RAM installed, and with the limited TLB size, would it start making sense to start making more use of the larger page sizes?

Admittedly a basic app would need 3x pages (one for code (R-X), one of data (RW-) and another for stack (RW-)), which means up to 12MB allocated (or for 64bit long mode, 6MB allocated, since long mode only offers 4K or 2MB pages).

While I can see that as a significant downside (less granular memory), the upside is that there will be less strain on the TLB and make memory allocation more efficient when slab allocators are being used (since they are the most common), since you're modifying the page tables less often.

I don't see swapping pages to/from disk as being a problem, as most drives like long streams of data (2/4MB) over small chunks (eg 4K).

It just seems that most people are sticking with 4K pages due to popular convention and/or the abundance of tutorials that only reference using 4K pages?

Thought or comments appreciated?

PS.
R-X - Readonly, Execute
RW- - Read/Write, No execute.
Post 10 Apr 2007, 03:07
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
I've been wondering the same myself.

I'd probably stick to 4k pages for normal .code/.data though, since those sections tend to be _relatively_ small - well below 2mb (or even 4mb) for most apps. But for heap allocation, 4meg pages would be nice.

Haven't played at this system level for quite some years, though. Dunno if NT uses large pages anywhere?
Post 10 Apr 2007, 06:34
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Hayden



Joined: 06 Oct 2005
Posts: 132
Hayden
I think it comes down to the efficency of the MCB chain, at least in DOS.
I cant eleberate more on the executive level code though
Post 10 Apr 2007, 06:43
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Adam Kachwalla



Joined: 01 Apr 2006
Posts: 150
Adam Kachwalla
How about adjustable sizes? You can have an "Advanced [OS Name] Setup Menu" and then submenus to select page sizes of 512 bytes/1K/2K/4K/8K/16K/32K/64K/128K/256K/512K/1M/ETC.


Personally, I would use no less than 512 bytes, but no more than 1M.
Post 25 Apr 2007, 08:25
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
Adam: not doable, we're talking hardware page size here. Page sizes of less than 4kb would require way too much management overhead...
Post 25 Apr 2007, 09:55
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mkriegel



Joined: 15 Jul 2007
Posts: 19
Location: Germany
mkriegel
What I think the reason is:

Loading a large page from HDD to RAM for only one short access would take too long.
So if the pages are small you have to read unused ones from HDD very often when executing long code sequences.
If the pages are very long you have to load the whole page from HDD even if you need only one byte(as data or so).
Just try to find a good value in between of both.
Post 15 Jul 2007, 10:19
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Adam Kachwalla



Joined: 01 Apr 2006
Posts: 150
Adam Kachwalla
OK... hang on.. if it is hardware pages here, then (correct me if I'm wrong) 32-bit x86 CPUs have a page size of 4K, where as x86-64 CPUs have a page size of 8K.
Post 16 Jul 2007, 01:10
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
Adam: the 80386 introduced proper protected mode (unlike the 80286 hacky shit) with paging - with a fixed size of 4k pages.

Later on 2meg and 4meg pages were introduced, can't remember which was implemented first and where, but a good bet would be that the ppro introduced it.

Google instead of making assumptions.
Post 17 Jul 2007, 13:18
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Gizmo



Joined: 19 Jul 2007
Posts: 25
Gizmo
according to intels manual
Intel® 64 and IA-32 Architectures
Software Developer’s Manual
Volume 3A:
System Programming Guide, Part 1

Quote:

The PSE flag enables large page sizes: 4-MByte pages or 2-MByte pages (when the
PAE flag is set). When the PSE flag is clear, the more common page length of
4 KBytes is used.

(2 mb or 4 mb pages)
PSE (page size extensions) flag. Bit 4 of CR4 (introduced in the Pentium
processor).

(36 bit physical addresses to access up to 36 gb of ram)
PAE (physical address extension) flag. Bit 5 of CR4 (introduced in the
Pentium Pro processors).

The PAE flag provides a method of extending physical addresses to 36 bits. This
physical address extension can only be used when paging is enabled. It relies on an
additional page directory pointer table that is used along with page directories and
page tables to reference physical addresses above FFFFFFFFH.
Post 26 Jul 2007, 14:33
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
Thanks for looking it up, Gizmo Smile
Post 26 Jul 2007, 22:59
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Gizmo



Joined: 19 Jul 2007
Posts: 25
Gizmo
If your operating system uses the PAE flag to get 34 gb of ram and you wanted to allow greater than 4gb of ram access to your user mode aps, you would have to setup segmentation?

(It says you get 36 bit addresses in your page table, but from what I read your logical addresses are still the same.)

How would you make a c api for apps to use it, make some function call that allocates a size in any upper segment not full yet, then you can copy to and from this address using similar function calls with a handle?
(to avoid a non-standard malloc, apps simply open a handle to the upper segment address space, and they can either feed a low segment address to copy memory back and forth or use a mapping method)
(could be useful for databases that can fetch the data store it locally and then save changes made to local copy, but would be real slow for real time graphics because of the extra allocating and copying)
Post 27 Jul 2007, 10:23
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
For 32bit systems, you'd need something like the "address windows extensions" in Windows... basically you map/unmap views of the extended memory range, kinda like if you use memory mapped files to access huge files.

So >4gig ram 32bit systems would be useful for either for software that's able to handle this kind of mapping (I think MS SQL server is?), or systems where you need lots of memory but not necessarily for one single app... ie., a terminal server running lots of users with standard apps.
Post 27 Jul 2007, 10:46
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