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

From: Minchan Kim
Date: Wed Sep 24 2014 - 04:11:59 EST

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Hi Joonsoo,

On Wed, Sep 24, 2014 at 03:03:46PM +0900, Joonsoo Kim 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. 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).

What a nice patch!

Below is minor but want to fix.

>
> 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];

I don't like the naming.
My preference is to keep size_class as it is. Instead, please use
__size_class or raw_size_class for internal size class.

>
> 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)

It seems to be the number of objects in the class but it doesn't.
Please use get_maxobj_per_zspage for consistent with get_pages_per_zspage.

> +{
> + 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)];
>
> 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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--
Kind regards,
Minchan Kim
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Next message: Greg KH: "Re: [char-misc-next 1/6] mei: push all standard settings into mei_device_init"
Previous message: Greg KH: "Re: [char-misc-next 01/10] mei: enable adding more IOCTL handlers"
In reply to: Joonsoo Kim: "[PATCH v2] zsmalloc: merge size_class to reduce fragmentation"
Next in thread: Joonsoo Kim: "Re: [PATCH v2] zsmalloc: merge size_class to reduce fragmentation"
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