linux-kernel - Re: RFC: CONFIG_PAGE_SHIFT (aka software PAGE

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Date:	Sat, 7 Jul 2007 12:25:02 +0200
From:	Andrea Arcangeli <andrea@...e.de>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: RFC: CONFIG_PAGE_SHIFT (aka software PAGE_SIZE)

On Sat, Jul 07, 2007 at 05:01:57PM +1000, Paul Mackerras wrote:
> Andrea Arcangeli writes:
> 
> > So my whole idea is to once and for all to decuple the size of the
> > pte-entry (4k on x86/amd64) with the page allocator granularity. The
> > HARD_PAGE_SHIFT will be 4k still, the common code PAGE_SIZE will be
> > variable and configurable at compile time with CONFIG_PAGE_SHIFT.
> 
> How does the page cache work with your scheme?  For example if I have
> 1000 1kB files cached in the page cache, and 16k PAGE_SIZE, does that
> use up 4M, or 16M?

It uses 16M of course. Like I said before:

   This whole issue is really a pure tradeoff between memory consumption
   and I/O and CPU performance (and for the dvd-ram and xfs also a way to

The CONFIG_PAGE_SHIFT allows you to ship a "monster" kernel for db
usage with hundred gigs of ram, with 64k page size and 64k blocksize,
getting the whole advantages. We of course must make sure that
CONFIG_PAGE_SHIFT=12 doesn't provide any slowdown.

Then us mere mortals will enjoy running with 8k page size too, with
our 2-4G of ram. I used 8k page size with an alpha workstation back in
2000 and I didn't feel any substantial ram waste, I had about 2G of
ram. Ok, now the kernel is larger, but even git learnt using packs ;)
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