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    SubjectRe: [PATCH 5/5] PM/Hibernate: Do not release preallocated memory unnecessarily (rev. 2)
    On Thu, May 07, 2009 at 04:54:09AM +0800, Rafael J. Wysocki wrote:
    > 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@sisk.pl>
    > > > > >
    > > > > > 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.

    Ah yes - I didn't notice that count was subtracted here:

    for (count -= size; count > 0; count--) {

    Make "count -= size" a standalone line to make that more obvious?

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

    Right, that would be a less problem.


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