Re: [PATCH v2] zsmalloc: merge size_class to reduce fragmentation

[Dan Streetman]

On Wed, Sep 24, 2014 at 2:03 AM, Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> zsmalloc has many size_classes to reduce fragmentation and they are
> in 16 bytes unit, for example, 16, 32, 48, etc., if PAGE_SIZE is 4096.
> And, zsmalloc has constraint that each zspage has 4 pages at maximum.
>
> In this situation, we can see interesting aspect.
> Let's think about size_class for 1488, 1472, ..., 1376.
> To prevent external fragmentation, they uses 4 pages per zspage and
> so all they can contain 11 objects at maximum.
>
> 16384 (4096 * 4) = 1488 * 11 + remains
> 16384 (4096 * 4) = 1472 * 11 + remains
> 16384 (4096 * 4) = ...
> 16384 (4096 * 4) = 1376 * 11 + remains
>
> It means that they have same characteristics and classification between
> them isn't needed. If we use one size_class for them, we can reduce
> fragementation and save some memory.

Just a suggestion, but you might want to further clarify the example
by saying something like:

since both the 1488 and 1472 sized classes can only fit 11 objects
into 4 pages, and an object that's 1472 bytes can fit into an object
that's 1488 bytes, merging these classes to always use objects that
are 1488 bytes will reduce the total number of size classes.  And
reducing the total number of size classes reduces overall
fragmentation, because a wider range of compressed pages can fit into
a single size class, leaving less unused objects in each size class.


> For this purpose, this patch
> implement size_class merging. If there is size_class that have
> same pages_per_zspage and same number of objects per zspage with previous
> size_class, we don't create and use new size_class. Instead, we use
> previous, same characteristic size_class. With this way, above example
> sizes (1488, 1472, ..., 1376) use just one size_class so we can get much
> more memory utilization.
>
> Below is result of my simple test.
>
> TEST ENV: EXT4 on zram, mount with discard option
> WORKLOAD: untar kernel source code, remove directory in descending order
> in size. (drivers arch fs sound include net Documentation firmware
> kernel tools)
>
> Each line represents orig_data_size, compr_data_size, mem_used_total,
> fragmentation overhead (mem_used - compr_data_size) and overhead ratio
> (overhead to compr_data_size), respectively, after untar and remove
> operation is executed.
>
> * untar-nomerge.out
>
> orig_size compr_size used_size overhead overhead_ratio
> 525.88MB 199.16MB 210.23MB  11.08MB 5.56%
> 288.32MB  97.43MB 105.63MB   8.20MB 8.41%
> 177.32MB  61.12MB  69.40MB   8.28MB 13.55%
> 146.47MB  47.32MB  56.10MB   8.78MB 18.55%
> 124.16MB  38.85MB  48.41MB   9.55MB 24.58%
> 103.93MB  31.68MB  40.93MB   9.25MB 29.21%
>  84.34MB  22.86MB  32.72MB   9.86MB 43.13%
>  66.87MB  14.83MB  23.83MB   9.00MB 60.70%
>  60.67MB  11.11MB  18.60MB   7.49MB 67.48%
>  55.86MB   8.83MB  16.61MB   7.77MB 88.03%
>  53.32MB   8.01MB  15.32MB   7.31MB 91.24%
>
> * untar-merge.out
>
> orig_size compr_size used_size overhead overhead_ratio
> 526.23MB 199.18MB 209.81MB  10.64MB 5.34%
> 288.68MB  97.45MB 104.08MB   6.63MB 6.80%
> 177.68MB  61.14MB  66.93MB   5.79MB 9.47%
> 146.83MB  47.34MB  52.79MB   5.45MB 11.51%
> 124.52MB  38.87MB  44.30MB   5.43MB 13.96%
> 104.29MB  31.70MB  36.83MB   5.13MB 16.19%
>  84.70MB  22.88MB  27.92MB   5.04MB 22.04%
>  67.11MB  14.83MB  19.26MB   4.43MB 29.86%
>  60.82MB  11.10MB  14.90MB   3.79MB 34.17%
>  55.90MB   8.82MB  12.61MB   3.79MB 42.97%
>  53.32MB   8.01MB  11.73MB   3.73MB 46.53%
>
> As you can see above result, merged one has better utilization (overhead
> ratio, 5th column) and uses less memory (mem_used_total, 3rd column).

This patch is definitely a good idea!

>
> Changed from v1:
> - More commit description about what to do in this patch.
> - Remove nr_obj in size_class, because it isn't need after initialization.
> - Rename __size_class to size_class, size_class to merged_size_class.
> - Add code comment for merged_size_class of struct zs_pool.
> - Add code comment how merging works in zs_create_pool().
>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> ---
>  mm/zsmalloc.c |   57 ++++++++++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 48 insertions(+), 9 deletions(-)
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index c4a9157..586c19d 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -214,6 +214,11 @@ struct link_free {
>  };
>
>  struct zs_pool {
> +       /*
> +        * Each merge_size_class is pointing to one of size_class that have
> +        * same characteristics. See zs_create_pool() for more information.
> +        */
> +       struct size_class *merged_size_class[ZS_SIZE_CLASSES];
>         struct size_class size_class[ZS_SIZE_CLASSES];

Isn't this confusing and wasteful?  merged_size_class is what
everything should use, and each of those just point to one of the
size_class entries, and not all size_class entries will be used.

Instead can we just keep only size_class[], but change it to pointers,
and use kmalloc in zs_create_pool?  That wastes no memory and doesn't
have duplicate arrays with confusingly similar names :-)


>
>         gfp_t flags;    /* allocation flags used when growing pool */
> @@ -468,7 +473,7 @@ static enum fullness_group fix_fullness_group(struct zs_pool *pool,
>         if (newfg == currfg)
>                 goto out;
>
> -       class = &pool->size_class[class_idx];
> +       class = pool->merged_size_class[class_idx];
>         remove_zspage(page, class, currfg);
>         insert_zspage(page, class, newfg);
>         set_zspage_mapping(page, class_idx, newfg);
> @@ -929,6 +934,22 @@ fail:
>         return notifier_to_errno(ret);
>  }
>
> +static unsigned int objs_per_zspage(struct size_class *class)
> +{
> +       return class->pages_per_zspage * PAGE_SIZE / class->size;
> +}
> +
> +static bool can_merge(struct size_class *prev, struct size_class *curr)
> +{
> +       if (prev->pages_per_zspage != curr->pages_per_zspage)
> +               return false;
> +
> +       if (objs_per_zspage(prev) != objs_per_zspage(curr))
> +               return false;
> +
> +       return true;
> +}
> +
>  /**
>   * zs_create_pool - Creates an allocation pool to work from.
>   * @flags: allocation flags used to allocate pool metadata
> @@ -949,9 +970,14 @@ struct zs_pool *zs_create_pool(gfp_t flags)
>         if (!pool)
>                 return NULL;
>
> -       for (i = 0; i < ZS_SIZE_CLASSES; i++) {
> +       /*
> +        * Loop reversly, because, size of size_class that we want to use for
> +        * merging should be larger or equal to current size.
> +        */
> +       for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
>                 int size;
>                 struct size_class *class;
> +               struct size_class *prev_class;
>
>                 size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
>                 if (size > ZS_MAX_ALLOC_SIZE)
> @@ -963,6 +989,22 @@ struct zs_pool *zs_create_pool(gfp_t flags)
>                 spin_lock_init(&class->lock);
>                 class->pages_per_zspage = get_pages_per_zspage(size);
>
> +               pool->merged_size_class[i] = class;
> +               if (i == ZS_SIZE_CLASSES - 1)
> +                       continue;
> +
> +               /*
> +                * merged_size_class is used for normal zsmalloc operation such
> +                * as alloc/free for that size. Although it is natural that we
> +                * have one size_class for each size, there is a chance that we
> +                * can get more memory utilization if we use one size_class for
> +                * many different sizes whose size_class have same
> +                * characteristics. So, we makes merged_size_class point to
> +                * previous size_class if possible.
> +                */
> +               prev_class = pool->merged_size_class[i + 1];
> +               if (can_merge(prev_class, class))
> +                       pool->merged_size_class[i] = prev_class;
>         }
>
>         pool->flags = flags;
> @@ -1003,7 +1045,6 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
>  {
>         unsigned long obj;
>         struct link_free *link;
> -       int class_idx;
>         struct size_class *class;
>
>         struct page *first_page, *m_page;
> @@ -1012,9 +1053,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
>         if (unlikely(!size || size > ZS_MAX_ALLOC_SIZE))
>                 return 0;
>
> -       class_idx = get_size_class_index(size);
> -       class = &pool->size_class[class_idx];
> -       BUG_ON(class_idx != class->index);
> +       class = pool->merged_size_class[get_size_class_index(size)];

As this change implies, class->index will no longer always be equal to
the index used in pool->class[index], since with merged size classes
the class->index will be the highest index of the merged classes.

Most places in the code won't care about this, but the two places that
definitely do need fixing are where classes are iterated by index
number.  I believe those places are zs_destroy_pool() and
zs_get_total_size_bytes().  Probably, the for() iteration currently in
use should be replaced by a for_each_size_class() function, that
automatically skips size classes that are duplicates in a merged size
class.  Of course, the for() iteration in zs_create_pool() has to
stay, since that's where the merged classes are setup.


>
>         spin_lock(&class->lock);
>         first_page = find_get_zspage(class);
> @@ -1067,7 +1106,7 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
>         first_page = get_first_page(f_page);
>
>         get_zspage_mapping(first_page, &class_idx, &fullness);
> -       class = &pool->size_class[class_idx];
> +       class = pool->merged_size_class[class_idx];
>         f_offset = obj_idx_to_offset(f_page, f_objidx, class->size);
>
>         spin_lock(&class->lock);
> @@ -1128,7 +1167,7 @@ void *zs_map_object(struct zs_pool *pool, unsigned long handle,
>
>         obj_handle_to_location(handle, &page, &obj_idx);
>         get_zspage_mapping(get_first_page(page), &class_idx, &fg);
> -       class = &pool->size_class[class_idx];
> +       class = pool->merged_size_class[class_idx];
>         off = obj_idx_to_offset(page, obj_idx, class->size);
>
>         area = &get_cpu_var(zs_map_area);
> @@ -1162,7 +1201,7 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle)
>
>         obj_handle_to_location(handle, &page, &obj_idx);
>         get_zspage_mapping(get_first_page(page), &class_idx, &fg);
> -       class = &pool->size_class[class_idx];
> +       class = pool->merged_size_class[class_idx];
>         off = obj_idx_to_offset(page, obj_idx, class->size);
>
>         area = this_cpu_ptr(&zs_map_area);
> --
> 1.7.9.5
>
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