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

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Date:	Sun, 16 Sep 2007 11:15:36 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jörn Engel <joern@...fs.org>
cc:	Andrea Arcangeli <andrea@...e.de>,
	Goswin von Brederlow <brederlo@...ormatik.uni-tuebingen.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Christoph Lameter <clameter@....com>,
	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 Sun, 16 Sep 2007, Jörn Engel wrote:
> 
> I have been toying with the idea of having seperate caches for pinned
> and movable dentries.  Downside of such a patch would be the number of
> memcpy() operations when moving dentries from one cache to the other.

Totally inappropriate.

I bet 99% of all "dentry_lookup()" calls involve turning the last dentry 
from having a count of zero ("movable") to having a count of 1 ("pinned").

So such an approach would fundamentally be broken. It would slow down all 
normal dentry lookups, since the *common* case for leaf dentries is that 
they have a zero count.

So it's much better to do it on a "directory/file" basis, on the 
assumption that files are *mostly* movable (or just freeable). The fact 
that they aren't always (ie while kept open etc), is likely statistically 
not all that important.

			Linus
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