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

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Message-ID: <20061025081135.GM5851@elf.ucw.cz>
Date:	Wed, 25 Oct 2006 10:11:35 +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!

> > > 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).
> 
> 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?

In particular, how much swap space can we have before we run out of
low memory? What is the overhead compared to bitmaps?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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