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

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Message-Id: <200701282114.02539.rjw@sisk.pl>
Date:	Sun, 28 Jan 2007 21:14:02 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	nigel@...el.suspend2.net
Cc:	Xavier Maillard <zedek@....org>, linux-kernel@...r.kernel.org
Subject: Re: [Q] Prefered suspend to ram or disk method ?

Hi,

On Sunday, 28 January 2007 20:57, Nigel Cunningham wrote:
> 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.

Okay, I stand corrected.

> 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.

Do I understand correctly that you:
- save the LRU,
- copy data into them,
- compute MD5 checksums of their contents,
- save the image,
- suspend,
- resume,
- load the image,
- compute MD5 checksums of the loaded data,
and the sums computed before saving the image and after loading it may differ
for up to 20 pages?

Rafael


-- 
If you don't have the time to read,
you don't have the time or the tools to write.
		- Stephen King
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