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

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Message-Id: <200707202343.29307.rjw@sisk.pl>
Date:	Fri, 20 Jul 2007 23:43:28 +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 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?

Greetings,
Rafael


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