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

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Date:	Tue, 11 Sep 2007 13:07:06 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Jörn Engel <joern@...fs.org>
cc:	Nick Piggin <nickpiggin@...oo.com.au>, 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)

On Tue, 11 Sep 2007, Jörn Engel 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.  I don't see anyone abandoning git or rsync, so
> your extreme example clearly is the wrong one.
> 
> Again, I agree with your concern, even though your example makes it look
> silly.

You may want to consider Mel's antifrag approaches which certainly 
decreases the chance of this occurring. Reclaim can open up the needed 
linear memory hole in a intentional way. The memory compaction approach 
can even move pages to open up these 2M holes. The more pages we make 
movable (see f.e. the targeted slab reclaim patchset that makes slab 
pages movable) the more reliable higher order allocations become.