From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Pavel Machek <pavel@suse.cz>
Subject: Re: Which is simpler? (Was Re: [Suspend2-devel] Re: [ 00/10] [Suspend2] Modules support.)
Date: Fri, 24 Feb 2006 11:58:07 +0100
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Cc: Nigel Cunningham <ncunningham@cyclades.com>,
       Dmitry Torokhov <dtor_core@ameritech.net>,
       Andreas Happe <andreashappe@snikt.net>, linux-kernel@vger.kernel.org,
       Suspend2 Devel List <suspend2-devel@lists.suspend2.net>
References: <20060201113710.6320.68289.stgit@localhost.localdomain> <200602240927.51338.ncunningham@cyclades.com> <20060223234403.GF1662@elf.ucw.cz>
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Hi,

On Friday 24 February 2006 00:44, Pavel Machek wrote:
> > > > > [Because pagecache is freeable, anyway, so it will be freed. Now... I
> > > > > have seen some problems where free_some_memory did not free enough,
> > > > > and schedule()/retry helped a bit... that probably should be fixed.]
> > > >
> > > > It seems I need to understand correctly what the difference between what
> > > > we do and what Nigel does is.  I thought the Nigel's approach was to save
> > > > some cache pages to disk first and use the memory occupied by them to
> > > > store the image data.  If so, is the page cache involved in that or
> > > > something else?
> > >
> > > I believe Nigel only saves pages that could have been freed anyway
> > > during phase1. Nigel, correct me here... suspend2 should work on same
> > > class of machines swsusp can, but will be able to save caches on
> > > machines where swsusp can not save any.
> > 
> > I'm not used to thinking in these terms :). It would be normally be right, 
> > except that there will be some LRU pages that will never be freed. These 
> > would allow suspend2 to work in some (not many) cases where swsusp can't. 
> > It's been ages since I did the intensive testing on the image preparation 
> > code, but I think that if we free as much memory as we can, we will always 
> > still have at least a few hundred LRU pages left. That's not much, but on 
> > machines with less ram, it might make the difference in a greater percentage 
> > of cases (compared to machines with more ram)?
> 
> Well, pages in LRU should be user pages, and therefore freeable,
> AFAICT. It is possible that there's something wrong with freeing in
> swsusp1...

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.

Greetings,
Rafael
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