Re: [PATCH 5/5] PM/Hibernate: Do not release preallocated memoryunnecessarily (rev. 2)

[Wu Fengguang]

On Wed, May 06, 2009 at 07:05:09AM +0800, Rafael J. Wysocki wrote:
> On Tuesday 05 May 2009, Wu Fengguang wrote:
> > On Mon, May 04, 2009 at 08:22:38AM +0800, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rjw@xxxxxxx>
> > > 
> > > Since the hibernation code is now going to use allocations of memory
> > > to create 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: Change the strategy of preallocating memory to allocate as
> > >  many pages as needed to get the right image size in one shot (the
> > >  excessive allocated pages are released afterwards).]
> > 
> > Rafael, I tried out your patches and found doubled memory shrink speed!
> >
> > [  579.641781] PM: Preallocating image memory ... done (allocated 383900 pages, 128000 image pages kept)
> > [  583.087875] PM: Allocated 1535600 kbytes in 3.43 seconds (447.69 MB/s)
> 
> Unfortunately, I'm observing a regression and a huge one.
> 
> On my Atom-based test box with 1 GB of RAM after a fresh boot and starting X
> with KDE 4 there are ~256 MB free.  To create an image we need to free ~300 MB
> and that takes ~2 s with the old code and ~15 s with the new one.
> 
> It helps to call shrink_all_memory() once with a sufficiently large argument
> before the preallocation.
[snip]
> > At last, I'd express my major concern about the transition to preallocate
> > based memory shrinking: will it lead to more random swapping IOs?
> 
> Hmm.  I don't see immediately why would it.  Maybe the regression I'm seeing
> is related to that ...

So you do have swap file enabled? hibernate_preallocate_memory() will
firstly try to allocate as much pages as possible(savable+free), and
then to free up (allocated-image_size) pages. That means *all*
swappable pages will be swapped out in the process - that's a major
performance regression!  And the zones are likely to be *over scanned*
and go to *all unreclaimable* state! (Hopefully they may be already
small at the time.)
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