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

[Wu Fengguang]

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)

For you reference, here is the free memory before/after
hibernate_preallocate_memory():

        # free
                     total       used       free     shared    buffers     cached
        Mem:          1933       1917         15          0          0       1845
        -/+ buffers/cache:         72       1861
        Swap:            0          0          0

        # free
                     total       used       free     shared    buffers     cached
        Mem:          1933        920       1012          0          0        356
        -/+ buffers/cache:        563       1369
        Swap:            0          0          0

It seems that the preallocated memory is not freed on -ENOMEM.

+       error = memory_bm_create(&orig_bm, GFP_IMAGE, PG_ANY);
+       if (error)
+               goto err_out;
+
+       error = memory_bm_create(&copy_bm, GFP_IMAGE, PG_ANY);
+       if (error)
+               goto err_out;

memory_bm_create() is called a number of times, each time it will
call create_mem_extents()/memory_bm_free(). Can they be optimized to
be called only once?

A side note: there are somehow duplicated *_extent_*() logics in the
filesystems, is it possible that we abstract out some of the common code?

+       for_each_populated_zone(zone) {
+               size += snapshot_additional_pages(zone);
+               count += zone_page_state(zone, NR_FREE_PAGES);
+               if (!is_highmem(zone))
+                       count -= zone->lowmem_reserve[ZONE_NORMAL];
+       }

Why [ZONE_NORMAL] instead of [zone]? ZONE_NORMAL may not always be the largest zone,
for example, My 4GB laptop has a tiny ZONE_NORMAL and a large ZONE_DMA32.

+       /* If size < max_size, preallocating enough memory may be impossible. */
+       if (count > 0 && size == max_size)
+               error = -ENOMEM;
+       if (error)
+               goto err_out;

The two if()s can be merged.

At last, I'd express my major concern about the transition to preallocate
based memory shrinking: will it lead to more random swapping IOs?

Thanks,
Fengguang

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