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

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Date:	Tue, 10 Apr 2007 23:00:28 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
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

On Monday, 9 April 2007 14:39, Pavel Machek wrote:
> 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...

In real-life scenarios you always need to keep free swap space enough for
suspending all the time, which IMO effectively prevents the "totally fragmented
1 GB swap" situation from happening.

Greetings,
Rafael
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