linux-kernel - Re: [RFC][PATCH -mm] swsusp: Use rbtree for tracking allocated swap

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Date:	Mon, 9 Apr 2007 12:39:20 +0000
From:	Pavel Machek <pavel@....cz>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nigel Cunningham <ncunningham@...uxmail.org>
Subject: Re: [RFC][PATCH -mm] swsusp: Use rbtree for tracking allocated swap

Hi!

> > > Some time ago we discussed the possibility of simplifying the swsusp's approach
> > > towards tracking the swap pages allocated by it for saving the image (so that
> > > they can be freed if there's an error).
> > > 
> > > I think we can get back to it now, as it is a nice optimization that should
> > > allow us to use less memory (almost always) and improve performance a bit.
> > > 
> > 
> > Well, I do not think you can measure the difference, but...
> 
> As far as the memory usage is concerned, I can. :-)  Usually, it takes 1 extent
> (40 B on x86_64) to register all of the allocated swap pages.  If bitmaps are
> used, we need as many bits as there are swap pages available (for 1 GB swap
> and 4 KB pages that would be ~250000 bits, which gives ~8 pages, and we can
> save more than 800 extents using that much memory).

Well... obviously it works for the best case. OTOH, for the worst, it
needs 40bytes for every 2 bits. That's 16000% worse. And for that
nightmare-fragmented 1GB swap, you'll need 5000000bytes... which is
pretty bad.

OTOH 5MB RAM per 1GB swap is not _too_ bad... so we can do it...

> > >  kernel/power/power.h  |   27 +---------
> > >  kernel/power/swap.c   |   18 +-----
> > >  kernel/power/swsusp.c |  135 ++++++++++++++++++++++++++------------------------
> > >  kernel/power/user.c   |   22 +-------
> > >  4 files changed, 85 insertions(+), 117 deletions(-)

...and call it 'cleanup' not 'speedup'.


> > ....as it removes code... I think we can do that. But it is 2.6.23+
> > material.
> 
> Yes, I think so, but still we can ask Andrew to include it into -mm earlier? ;-)

Yes, that's okay with me.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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