linux-kernel - Re: [PATCH 5/6] PM/Hibernate: Do not release preallocated memory unnecessarily (rev. 2)

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Message-Id: <200905151652.17418.rjw@sisk.pl>
Date:	Fri, 15 May 2009 16:52:16 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nigel Cunningham <nigel@...onice.net>,
	David Rientjes <rientjes@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 5/6] PM/Hibernate: Do not release preallocated memory unnecessarily (rev. 2)

On Friday 15 May 2009, Pavel Machek wrote:
> On Thu 2009-05-14 19:52:20, Rafael J. Wysocki wrote:
> > On Thursday 14 May 2009, Pavel Machek wrote:
> > > Hi!
> > > 
> > > > Since the hibernation code is now going to use allocations of memory
> > > > to make enough room for the image, it can also use the page frames
> > > > allocated at this stage as image page frames.  The low-level
> > > > hibernation code needs to be rearranged for this purpose, but it
> > > > allows us to avoid freeing a great number of pages and allocating
> > > > these same pages once again later, so it generally is worth doing.
> > > > 
> > > > [rev. 2: Take highmem into account correctly.]
> > > 
> > > I don't get it. What is advantage of this patch? It makes the code
> > > more complex... Is it supposed to be faster?
> > 
> > Yes, in some test cases it is reported to be faster (along with [4/6],
> > actually).
> > 
> > Besides, we'd like to get rid of shrink_all_memory() eventually and it is a
> > step in this direction.
> 
> Ok, but maybe we should wait with applying this until we have patches
> that actually get us rid of shrink_all_memory?

Well, the $subject patch is only an optimization of top of [4/6] that you've
just acked. ;-)

In fact [4/6] changes the approach to the memory shrinking and the $subject
one is only to avoid freeing all of the memory we've allocated and allocating
it once again later.

> Maybe it will not be feasible for speed reasons after all, or something...

At least it allows us to drop shrink_all_memory() easily for the sake of
experimentation (it's sufficient to comment out just one line of code for this
purpose).

Besides, after this patchset shrink_all_memory() is _only_ needed for
performance, so it should be possible to get rid of it relatively quckly.

Thanks,
Rafael
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