Page size

Someone asked me the followind two questions, and I wanted to verify before I gave an answer:

Is page size tunable at run time, and can the page size be determined at run time.

I was under the impression that page size was a build time parameter.

Mike Madore
Software Engineer
TurboLinux, Inc.

Mike-

You are correct, page size is a compile time constant, you cannot change it at run time.

The `getpagesize' GLIBC function returns the kernel's page size.

Don Dugger
"Censeo Toto nos in Kansa esse decisse." - D. Gale
[email protected]
Ph: 303/938-9838

Mike> Someone asked me the followind two questions, and I wanted to Mike> verify before I gave an answer:

Mike> Is page size tunable at run time, and can the page size be Mike> determined at run time.

Mike> I was under the impression that page size was a build time Mike> parameter.

Don already answered the question as far as the status quo is concerned.

As for the future: I'd like to start playing with superpage support fairly soon (once I get some spare time on my hands... ;-). With superpages, physically contiguous regions of memory could be mapped with a single TLB entry, giving you most of the benefit of (very) large page sizes.

	--david

What portion of source would I need to modify to change the memory page size?

I ran across some info stating that a current optimum page size for IA64-Linux is 16KB; any validity to this?

Are there any major side effects of changing the page size?

---------------------------
We are porting and beginning to benchmark a subset of high performance applications, are there any other recommendations for optimizing the Linux kernel, or is there a better tuned kernel available anywhere?

Thanks.

***************************************************************************

Donny Cooper                     | Direct Phone : (281) 465-1506
NEC Systems, Inc.                | Houston Metro: (281) 296-0912 x 1506
High Performance Computing Center| Fax          : (281) 465-1599
4200 Research Forest Dr, #400    | E-mail       : [email protected]
The Woodlands, TX 77381          | URL          : http://www.necservers.com

***************************************************************************

-----Original Message-----
From: [email protected] [mailto:[email protected]]On Behalf Of Don Dugger
Sent: Monday, April 16, 2001 3:16 PM
To: Michael Madore
Cc: [email protected]
Subject: Re: [Linux-ia64] Page size

Mike-

You are correct, page size is a compile time constant, you cannot change it at run time.

The `getpagesize' GLIBC function returns the kernel's page size.

before

run

--
Don Dugger
"Censeo Toto nos in Kansa esse decisse." - D. Gale [email protected]
Ph: 303/938-9838

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Donny-

The kernel currently supports 4K, 8K and 16K page sizes. You can select any of these page sizes from the kernel config menu's, no change to source files required. There's a patch out that allows 64K page size but that hasn't been integrated into the kernel tree yet. (32K page size is not supported by the Itanium, in case you were wondering).

We haven't done exhaustive studies to identify what is the optimal' page size yet. The problem is that larger page sizes can result is less paging but also can result in more memory being wasted. Larger page sizes can also result in more paging work' as you page in more data than you actually need. Unfortunately, the true answer will probably depend upon the bencmark you use. My intuition right now is that 4K is definitely too small and 64K is probably too large. That's why I make all of my kernels with 16K pages.

Don Dugger
"Censeo Toto nos in Kansa esse decisse." - D. Gale
[email protected]
Ph: 303/938-9838