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

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Message-ID: <Pine.LNX.4.64.0707200835380.19248@asgard.lang.hm>
Date:	Fri, 20 Jul 2007 08:36:54 -0700 (PDT)
From:	david@...g.hm
To:	Jim Crilly <jim@....dont.jablowme.net>
cc:	"Rafael J. Wysocki" <rjw@...k.pl>, 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 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

David Lang
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