From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Nigel Cunningham <ncunningham@cyclades.com>
Subject: Re: Which is simpler? (Was Re: [Suspend2-devel] Re: [ 00/10] [Suspend2] Modules support.)
Date: Sat, 25 Feb 2006 01:56:48 +0100
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Cc: Pavel Machek <pavel@ucw.cz>, Dmitry Torokhov <dtor_core@ameritech.net>,
       Andreas Happe <andreashappe@snikt.net>, linux-kernel@vger.kernel.org,
       Suspend2 Devel List <suspend2-devel@lists.suspend2.net>
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Hi,

On Saturday 25 February 2006 01:26, Nigel Cunningham wrote:
> On Saturday 25 February 2006 10:20, Rafael J. Wysocki wrote:
> > On Saturday 25 February 2006 00:11, Nigel Cunningham wrote:
> > > On Saturday 25 February 2006 06:22, Rafael J. Wysocki wrote:
> > > > On Friday 24 February 2006 14:12, Pavel Machek wrote:
> > > > > On Pá 24-02-06 11:58:07, Rafael J. Wysocki wrote:
> >
> > }-- snip --{
> >
> > > > > > Well, if all of the pages that Nigel saves before snapshot are
> > > > > > freeable in theory, there evidently is something wrong with freeing
> > > > > > in swsusp, as we have a testcase in which the user was unable to
> > > > > > suspend with swsusp due to the lack of memory and could suspend
> > > > > > with suspend2.
> > > > > >
> > > > > > However, the only thing in swsusp_shrink_memory() that may be wrong
> > > > > > is we return -ENOMEM as soon as shrink_all_memory() returns 0.
> > > > > > Namely, if shrink_all_memory() can return 0 prematurely (ie. "there
> > > > > > still are some freeable pages, but they could not be freed in
> > > > > > _this_ call"), we should continue until it returns 0 twice in a row
> > > > > > (or something like that).  If this doesn't help, we'll have to fix
> > > > > > shrink_all_memory() I'm afraid.
> > > > >
> > > > > I did try shrink_all_memory() five times, with .5 second delay
> > > > > between them, and it freed more memory at later tries.
> > > >
> > > > I wonder if the delays are essential or if so, whether they may be
> > > > shorter than .5 sec.
> > > >
> > > > > Sometimes it even freed 0 pages at the first try.
> > > > >
> > > > > I did not push the patch because
> > > > >
> > > > > 1) it was way too ugly
> > > >
> > > > I think I can do something like that in swsusp_shrink_memory() and it
> > > > won't be very ugly.
> > > >
> > > > > 2) shrink_all_memory() should be fixed. It should not really return
> > > > > if there are more pages freeable.
> > > >
> > > > Well, that would be a long-run solution.  However, until it's fixed we
> > > > can use a workaround IMHO. ;-)
> > >
> > > Isn't trying to free as much memory as you can the wrong solution anyway?
> >
> > No, it isn't.  There are situations in which we would like to suspend
> > "whatever it takes" and then we should be able to free as much memory as
> > _really_ possible.
> >
> > Besides, it is supposed to work, and it doesn't, so it needs fixing.
> >
> > > I mean, that only means that the poor system has more pages to fault back
> > > in at resume time, before the user can even begin to think about doing
> > > anything useful.
> >
> > Well, that's not the only possibility.  After we fix the memory freeing
> > issue we can use the observation that page cache pages need not be saved to
> > disk during suspend, because they already are in a storage.  We only need
> > to create a map of these pages during suspend with the information on where
> > to get them from and prefetch them into memory during resume independently
> > of the page fault mechanism.
> >
> > This way we won't have to actually save anything before we snapshot the
> > system and the system should be reasonably responsive after resume.
> 
> But this is going to be much more complicated than simply saving the pages in 
> the first place. You'll need some mechanism for figuring out what pages to 
> get, how to fault them in, etc.

Yes, this is going to be quite difficult indeed.  Still there's a price for saving the
pages and I'd like to avoid paying it. ;-)

> In addition, it will be much slower than simply reading them back from (ideally)
> contiguous storage.

Not necessarily.  These pages may come from contiguous storage areas as well.

Greetings,
Rafael
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