linux-kernel - Re: RFT: updatedb "morning after" problem [was: Re: -mm merge plans for 2.6.23]

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Message-ID: <46ACB40C.2040908@gmail.com>
Date:	Sun, 29 Jul 2007 17:36:44 +0200
From:	Rene Herman <rene.herman@...il.com>
To:	Ray Lee <ray-lk@...rabbit.org>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>, david@...g.hm,
	Daniel Hazelton <dhazelton@...er.net>,
	Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frank Kingswood <frank@...gswood-consulting.co.uk>,
	Andi Kleen <andi@...stfloor.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Jesper Juhl <jesper.juhl@...il.com>,
	ck list <ck@....kolivas.org>, Paul Jackson <pj@....com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: RFT: updatedb "morning after" problem [was: Re: -mm merge plans
 for 2.6.23]

On 07/29/2007 05:20 PM, Ray Lee wrote:

>> I understand what log structure is generally, but how does it help swapin?
> 
> Look at the swap out case first.
> 
> Right now, when swapping out the kernel places whatever it can
> wherever it can inside the swap space. The closer you are to filling
> your swap space, the more likely that those swapped out blocks will be
> all over place, rather than in one nice chunk. Contrast that with a
> log structured scheme, where the writeout happens to sequential spaces
> on the drive instead of scattered about.

This seems to be now fixing the different problem of swap-space filling up. 
I'm quite willing to for now assume I've got plenty free.

> So, at some point when the system needs to fault those blocks that
> back in, it now has a linear span of sectors to read instead of asking
> the drive to bounce over twenty tracks for a hundred blocks.

Moreover though -- what I know about log structure is that generally it 
optimises for write (swapout) and might make read (swapin) worse due to 
fragmentation that wouldn't happen with a regular fs structure.

I guess that cleaner that Alan mentioned might be involved there -- I don't 
know how/what it would be doing.

> So, it eliminates the seeks. My laptop drive can read (huh, how odd,
> it got slower, need to retest in single user mode), hmm, let's go with
> about 25 MB/s. If we ask for a single block from each track, though,
> that'll drop to 4k * (1 second / seek time) which is about a megabyte
> a second if we're lucky enough to read from consecutive tracks. Even
> worse if it's not.
> 
> Seeks are the enemy any time you need to hit the drive for anything,
> be it swapping or optimizing a database.

I am very aware of the costs of seeks (on current magnetic media).

Rene.

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