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

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Date:	Sat, 15 Sep 2007 14:14:42 +0200
From:	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Joern Engel <joern@...fs.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Christoph Lameter <clameter@....com>, andrea@...e.de,
	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)

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Tue, 11 Sep 2007 14:12:26 +0200 Jörn Engel <joern@...fs.org> wrote:
>
>> While I agree with your concern, those numbers are quite silly.  The
>> chances of 99.8% of pages being free and the remaining 0.2% being
>> perfectly spread across all 2MB large_pages are lower than those of SHA1
>> creating a collision.
>
> Actually it'd be pretty easy to craft an application which allocates seven
> pages for pagecache, then one for <something>, then seven for pagecache, then
> one for <something>, etc.
>
> I've had test apps which do that sort of thing accidentally.  The result
> wasn't pretty.

Except that the applications 7 pages are movable and the <something>
would have to be unmovable. And then they should not share the same
memory region. At least they should never be allowed to interleave in
such a pattern on a larger scale.

The only way a fragmentation catastroph can be (proovable) avoided is
by having so few unmovable objects that size + max waste << ram
size. The smaller the better. Allowing movable and unmovable objects
to mix means that max waste goes way up. In your example waste would
be 7*size. With 2MB uper order limit it would be 511*size.

I keep coming back to the fact that movable objects should be moved
out of the way for unmovable ones. Anything else just allows
fragmentation to build up.

MfG
        Goswin
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