linux-kernel - Re: [Q] Prefered suspend to ram or disk method ?

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Date:	Mon, 29 Jan 2007 06:57:38 +1100
From:	Nigel Cunningham <nigel@...el.suspend2.net>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Xavier Maillard <zedek@....org>, linux-kernel@...r.kernel.org
Subject: Re: [Q] Prefered suspend to ram or disk method ?

Hi.

On Sun, 2007-01-28 at 20:47 +0100, Rafael J. Wysocki wrote:
> > How does suspend compare to ususpend/suspend2 ?
> 
> swsusp and ususpend use the same low-level kernel code, so if one of them
> works for you, the other should as well.  The difference is mainly that with
> swsusp the kernel saves the image into a swap, while with usupend the image
> is saved (and loaded on resume) by a userland process.  Additionally, ususpend
> can compress and/or encrypt the image, supports suspend-to-disk-and-RAM,
> splash-based progress meters and some such.  Gerenally, it adds the features
> that, in the opinion of its authors, are better implemented in the userland.
> Plus IMHO ususpend's resume is a bit more convenient for calling from within
> initrd images.
> 
> suspend2 uses some different low-level code, but as far as the stopping of
> tasks and handling devices are concerned, it should be equivalent to swsusp
> and ususpend.  It generally allows you to create bigger suspend images (the
> images created by swsusp and ususpend are at most as large as 50% of RAM),
> but it makes some strong assumptions regarding memory management which are
> not proven to be always satisfied, although there's no evidence showing
> otherwise.  It also implements approximately the same set of additional
> features as ususpend, but in the kernel.

That's not true anymore. Suspend2 uses exactly the same lowlevel code.
It just modifies the C that runs around the lowlevel code slightly so
that suspend2 instead of swsusp code is executed before and afterwards.

Regarding the assumptions (about LRU pages not changing), I have that in
progress. The content of the LRU list definitely doesn't change, but by
calculating MD5 checksums of the changes before and after saving those
pages, we've seen some (up to 20) pages change on a few computers. I
need (obviously) to put time into finding the cause of those changes.

Regards,

Nigel

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