Date:	Wed, 24 Sep 2014 15:03:46 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>
Cc:	Nitin Gupta <ngupta@...are.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Jerome Marchand <jmarchan@...hat.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Dan Streetman <ddstreet@...e.org>,
	Luigi Semenzato <semenzato@...gle.com>, juno.choi@....com,
	"seungho1.park" <seungho1.park@....com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: [PATCH v2] zsmalloc: merge size_class to reduce fragmentation

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).

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@....com>
---
 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];
 
 	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)];
 
 	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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