linux-kernel - Re: [patch 00/14] Page cache cleanup in anticipation of Large Blocksize support

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Date:	Thu, 14 Jun 2007 15:22:46 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	linux-kernel@...r.kernel.org, hch@...radead.org
Subject: Re: [patch 00/14] Page cache cleanup in anticipation of Large
 Blocksize support

On Thu, 14 Jun 2007, Andrew Morton wrote:

> With 64k pagesize the amount of memory required to hold a kernel tree (say)
> will go from 270MB to 1400MB.   This is not an optimisation.

I do not think that the 100% users will do kernel compiles all day like 
we do. We likely would prefer 4k page size for our small text files.

> Several 64k pagesize people have already spent time looking at various
> tail-packing schemes to get around this serious problem.  And that's on
> _server_ class machines.  Large ones.  I don't think
> laptop/desktop/samll-server machines would want to go anywhere near this.

I never understood the point of that exercise. If you have variable page 
size then the 64k page size can be used specific to files that benefit 
from it. Typically usage scenarios are video audio streaming I/O, large 
picture files, large documents with embedded images. These are the major
usage scenarioes today and we suck the. Our DVD/CD subsystems are 
currently not capable of directly reading from these devices into the page 
cache since they do not do I/O in 4k chunks.

> > fsck times etc etc are becoming an issue for desktop 
> > systems
> 
> I don't see what fsck has to do with it.
> 
> fsck is single-threaded (hence no locking issues) and operates against the
> blockdev pagecache and does a _lot_ of small reads (indirect blocks,
> especially).  If the memory consumption for each 4k read jumps to 64k, fsck
> is likely to slow down due to performing a lot more additional IO and due
> to entering page reclaim much earlier.

Every 64k block contains more information and the number of pages managed
is reduced by a factor of 16. Less seeks , less tlb pressure , less reads, 
more cpu cache and cpu cache prefetch friendly behavior.
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