linux-kernel - Re: [linux-pm] Re: Hibernation considerations

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Message-Id: <200707210022.03205.rjw@sisk.pl>
Date:	Sat, 21 Jul 2007 00:22:02 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	david@...g.hm
Cc:	Jim Crilly <jim@....dont.jablowme.net>,
	Milton Miller <miltonm@....com>,
	linux-pm <linux-pm@...ts.linuxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	"Huang, Ying" <ying.huang@...el.com>,
	Jeremy Maitin-Shepard <jbms@....edu>
Subject: Re: [linux-pm] Re: Hibernation considerations

On Friday, 20 July 2007 23:39, david@...g.hm wrote:
> On Fri, 20 Jul 2007, Rafael J. Wysocki wrote:
> 
> > On Friday, 20 July 2007 17:36, david@...g.hm wrote:
> >> On Fri, 20 Jul 2007, Jim Crilly wrote:
> >>
> >>>>> has
> >>>>> requested the image to be not greater than 50% of RAM.  In that case you
> >>>>> have
> >>>>> to free some memory _before_ identifying memory to save and you must not
> >>>>> race with applications that attempt to allocate memory while you're doing
> >>>>> it.
> >>>>
> >>>> I disagree a little bit.
> >>>>
> >>>> first off, only the suspending kernel can know what can be freed and what
> >>>> is needed to do so (remember this is kernel internals, it can change from
> >>>> patch to patch, let alone version to version)
> >>>>
> >>>> second, if you have a lot of memory to free, and you can't just throw away
> >>>> caches to do so, you don't know what is going to be involved in freeing
> >>>> the memory, it's very possilbe that it is going to involve userspace, so
> >>>> you can't freeze any significant portion of the system, so you can't
> >>>> eliminate all chance of races
> >>>>
> >>>> what you can do is
> >>>>
> >>>> 1. try to free stuff
> >>>> 2. stop the system and account for memory, is enough free
> >>>> if not goto 1
> >>>>
> >>>> if userspace is dirtying memory fast enough, or is just useing enough
> >>>> memory that you can't meet your limit you just won't be able to suspend.
> >>>>
> >>>> but under any other conditions you will eventually get enough memory free.
> >>>>
> >>>> so try several times and if you still fail tell the user they have too
> >>>> much stuff running and they need to kill something.
> >>>
> >>> Which would be a pretty big regression from what we have now. With the
> >>> current implementation I can hibernate under virtually any workload because
> >>> the freezer stops everything and there's no competition for resources.
> >>
> >> as long as what you are trying to save is <=50% of ram (at least with some
> >> implementations). if you are trying to save more then 50% of ram with some
> >> current implmenetations you just can't
> >
> > With some, you can't, with the others, you can. :-)
> >
> > The argument given was about the freezer and IMO it was valid.
> >
> > Why didn't you address it directly?
> 
> I thought it had been covered in other messages (with as big as this 
> thread is I'm trying to avoid repeating the same thing more then a couple 
> times a day :-)
> 
> there was another message talking about ways that you could reduce the 
> image size without it being racy (allocate pinned memory until the 
> remainder is small enough, then don't backup the pinned memory)
> 
> that's a much cleaner answer then what I was thinking, so I'll go with it 
> instead ;-)

Wouldn't that cause the OOM killer to act, in some cases?

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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