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@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];
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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