linux-kernel - Re: [PATCH] Use extents for recording what swap is allocated.

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Message-ID: <20061025084226.GN5851@elf.ucw.cz>
Date:	Wed, 25 Oct 2006 10:42:26 +0200
From:	Pavel Machek <pavel@....cz>
To:	Nigel Cunningham <ncunningham@...uxmail.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, Andrew Morton <akpm@...l.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Use extents for recording what swap is allocated.

Hi!

> > > With the code I have in Suspend2 (which is what I'm working towards),
> > > the value includes the swap_type, so there's no overlap. Assuming the
> > > swap allocator does it's normal thing and swap allocated is contiguous,
> > > you'll probably end up with two extents: one containing the swap
> > > allocated on the first device, and the other containing the swap
> > > allocated on the second device. So (with the current version), striping
> > > would use 6 * sizeof(unsigned long) instead of 3 * sizeof(unsigned
> > > long).
> > 
> > And now, can you do same computation assuming the swap allocator goes
> > completely crazy, and free space is in 1-page chunks?
> 
> The worst case is 3 * sizeof(unsigned long) *
> number_of_swap_extents_allocated bytes.

Okay, so if we got 4GB of swap space, thats 1MB swap pages, worst case
is you have one extent per page, on x86-64 that's 24MB. +kmalloc
overhead, I assume?

And you do linear walks over those extents, leading to O(n^2)
algorithm, no? That has bitten us before...
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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