Date: Tue, 11 Sep 2007 17:04:41 -0700 (PDT)
From: Christoph Lameter <clameter@sgi.com>
To: Andrea Arcangeli <andrea@suse.de>
cc: =?iso-8859-1?Q?J=F6rn?= Engel <joern@logfs.org>,
       Nick Piggin <nickpiggin@yahoo.com.au>, torvalds@linux-foundation.org,
       linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
       Christoph Hellwig <hch@lst.de>, Mel Gorman <mel@skynet.ie>,
       William Lee Irwin III <wli@holomorphy.com>, David Chinner <dgc@sgi.com>,
       Jens Axboe <jens.axboe@oracle.com>,
       Badari Pulavarty <pbadari@gmail.com>,
       Maxim Levitsky <maximlevitsky@gmail.com>,
       Fengguang Wu <fengguang.wu@gmail.com>, swin wang <wangswin@gmail.com>,
       totty.lu@gmail.com, hugh@veritas.com
Subject: Re: [00/41] Large Blocksize Support V7 (adds memmap support)
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On Wed, 12 Sep 2007, Andrea Arcangeli wrote:

> On Tue, Sep 11, 2007 at 01:41:08PM -0700, Christoph Lameter wrote:
> > The advantages of this approach over Andreas is basically that the 4k 
> > filesystems still can be used as is. 4k is useful for binaries and for 
> 
> If you mean that with my approach you can't use a 4k filesystem as is,
> that's not correct. I even run the (admittedly premature but
> promising) benchmarks on my patch on a 4k blocksized
> filesystem... Guess what, you can even still mount a 1k fs on a 2.6
> kernel.

Right you can use a 4k filesystem. The 4k blocks are buffers in a larger 
page then.
 
> The main advantage I can see in your patch is that distributions won't
> need to ship a 64k PAGE_SIZE kernel rpm (but your single rpm will be
> slower).

I would think that your approach would be slower since you always have to 
populate 1 << N ptes when mmapping a file? Plus there is a lot of wastage 
of memory because even a file with one character needs an order N page? So 
there are less pages available for the same workload.

Then you are breaking mmap assumptions of applications becaused the order 
N kernel will no longer be able to map 4k pages.  You likely need a new 
binary format that has pages correctly aligned. I know that we would need 
one on IA64 if we go beyond the established page sizes.
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