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

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Message-ID: <20061024221950.GB5851@elf.ucw.cz>
Date:	Wed, 25 Oct 2006 00:19:50 +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!

> > > > Switch from bitmaps to using extents to record what swap is allocated;
> > > > they make more efficient use of memory, particularly where the allocated
> > > > storage is small and the swap space is large.
> > > 
> > > As I said before, I like the overall idea, but I have a bunch of
> > > comments.
> > 
> > Okay, if Rafael likes it... lets take a look.
> > 
> > First... what is the _worst case_ overhead? AFAICT extents are very
> > good at the best case, but tend to suck for the worst case...?
> 
> That's right. In using this, we're relying on the fact that the swap
> allocator tries to act sensibly. I've only seen worse case performance
> when a user had two swap devices with the same priority (striped), but
> that was a bug. :)

Ok, but if the allocator somehow manages to stripe between two swap
devices, what happens?

IIRC original code was something like .1% overhead (8bytes per 4K, or
something?), bitmaps should be even better. If it is 1% in worst case,
that's probably okay, but it would be bad if it had overhead bigger
than 10times original code (worst case).
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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