linux-kernel - Re: [00/41] Large Blocksize Support V7 (adds memmap support)

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Message-ID: <Pine.LNX.4.64.0709111700410.29413@schroedinger.engr.sgi.com>
Date:	Tue, 11 Sep 2007 17:04:41 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andrea Arcangeli <andrea@...e.de>
cc:	Jörn Engel <joern@...fs.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
	Mel Gorman <mel@...net.ie>,
	William Lee Irwin III <wli@...omorphy.com>,
	David Chinner <dgc@....com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Badari Pulavarty <pbadari@...il.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Fengguang Wu <fengguang.wu@...il.com>,
	swin wang <wangswin@...il.com>, totty.lu@...il.com,
	hugh@...itas.com
Subject: Re: [00/41] Large Blocksize Support V7 (adds memmap support)

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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