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

[Rafael J. Wysocki]

On Wednesday 06 May 2009, Wu Fengguang wrote:
> 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.

No.  It's going to allocate (total RAM - anticipated image size) and then free
up (allocated-image_size) pages.

If we consider maximum image sizes, that means allocating slightly more than
50% of RAM, so it really shouldn't regress that much IMO.

Thanks,
Rafael
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