[PATCHv4 2/9] zsmalloc: turn zspage order into runtime variable

Sergey Senozhatsky
2022-10-31T05:41:27Z

zsmalloc has 255 size classes. Size classes contain a number of zspages,
which store objects of the same size. zspage can consist of up to four
physical pages. The exact (most optimal) zspage size is calculated for
each size class during zsmalloc pool creation.

As a reasonable optimization, zsmalloc merges size classes that have
similar characteristics: number of pages per zspage and number of
objects zspage can store.

For example, let's look at the following size classes:

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
..
94 1536 0 0 0 0 0 3 0
100 1632 0 0 0 0 0 2 0
..

Size classes #95-99 are merged with size class #100. That is, each time
we store an object of size, say, 1568 bytes instead of using class #96
we end up storing it in size class #100. Class #100 is for objects of
1632 bytes in size, hence every 1568 bytes object wastes 1632-1568 bytes.
Class #100 zspages consist of 2 physical pages and can hold 5 objects.
When we need to store, say, 13 objects of size 1568 we end up allocating
three zspages; in other words, 6 physical pages.

However, if we'll look closer at size class #96 (which should hold objects
of size 1568 bytes) and trace get_pages_per_zspage():

pages per zspage wasted bytes used%
1 960 76
2 352 95
3 1312 89
4 704 95
5 96 99

We'd notice that the most optimal zspage configuration for this class is
when it consists of 5 physical pages, but currently we never let zspages
to consists of more than 4 pages. A 5 page class #96 configuration would
store 13 objects of size 1568 in a single zspage, allocating 5 physical
pages, as opposed to 6 physical pages that class #100 will allocate.

A higher order zspage for class #96 also changes its key characteristics:
pages per-zspage and objects per-zspage. As a result classes #96 and #100
are not merged anymore, which gives us more compact zsmalloc.

Of course the described effect does not apply only to size classes #96 and
We still merge classes, but less often so. In other words classes are grouped
in a more compact way, which decreases memory wastage:

zspage order # unique size classes
2 69
3 123
4 191

Let's take a closer look at the bottom of /sys/kernel/debug/zsmalloc/zram0/classes:

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
...
202 3264 0 0 0 0 0 4 0
254 4096 0 0 0 0 0 1 0
...

For exactly same reason - maximum 4 pages per zspage - the last non-huge
size class is #202, which stores objects of size 3264 bytes. Any object
larger than 3264 bytes, hence, is considered to be huge and lands in size
class #254, which uses a whole physical page to store every object. To put
it slightly differently - objects in huge classes don't share physical pages.

3264 bytes is too low of a watermark and we have too many huge classes:
classes from #203 to #254. Similarly to class size #96 above, higher order
zspages change key characteristics for some of those huge size classes and
thus those classes become normal classes, where stored objects share physical
pages.

Hence yet another consequence of higher order zspages: we move the huge
size class watermark with higher order zspages, have less huge classes and
store large objects in a more compact way.

For order 3, huge class watermark becomes 3632 bytes:

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
...
202 3264 0 0 0 0 0 4 0
211 3408 0 0 0 0 0 5 0
217 3504 0 0 0 0 0 6 0
222 3584 0 0 0 0 0 7 0
225 3632 0 0 0 0 0 8 0
254 4096 0 0 0 0 0 1 0
...

For order 4, huge class watermark becomes 3840 bytes:

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
...
202 3264 0 0 0 0 0 4 0
206 3328 0 0 0 0 0 13 0
207 3344 0 0 0 0 0 9 0
208 3360 0 0 0 0 0 14 0
211 3408 0 0 0 0 0 5 0
212 3424 0 0 0 0 0 16 0
214 3456 0 0 0 0 0 11 0
217 3504 0 0 0 0 0 6 0
219 3536 0 0 0 0 0 13 0
222 3584 0 0 0 0 0 7 0
223 3600 0 0 0 0 0 15 0
225 3632 0 0 0 0 0 8 0
228 3680 0 0 0 0 0 9 0
230 3712 0 0 0 0 0 10 0
232 3744 0 0 0 0 0 11 0
234 3776 0 0 0 0 0 12 0
235 3792 0 0 0 0 0 13 0
236 3808 0 0 0 0 0 14 0
238 3840 0 0 0 0 0 15 0
254 4096 0 0 0 0 0 1 0
...

TESTS
=====

Test untars linux-6.0.tar.xz and compiles the kernel.

zram is configured as a block device with ext4 file system, lzo-rle
compression algorithm. We captured /sys/block/zram0/mm_stat after
every test and rebooted the VM.

orig_data_size mem_used_total mem_used_max pages_compacted
compr_data_size mem_limit same_pages huge_pages

ORDER 2 (BASE) zspage

1691791360 628086729 655171584 0 655171584 60 0 34043
1691787264 628089196 655175680 0 655175680 60 0 34046
1691803648 628098840 655187968 0 655187968 59 0 34047
1691795456 628091503 655183872 0 655183872 60 0 34044
1691799552 628086877 655183872 0 655183872 60 0 34047

ORDER 3 zspage

1691803648 627792993 641794048 0 641794048 60 0 33591
1691787264 627779342 641708032 0 641708032 59 0 33591
1691811840 627786616 641769472 0 641769472 60 0 33591
1691803648 627794468 641818624 0 641818624 59 0 33592
1691783168 627780882 641794048 0 641794048 61 0 33591

ORDER 4 zspage

1691803648 627726635 639655936 0 639655936 60 0 33435
1691811840 627733348 639643648 0 639643648 61 0 33434
1691795456 627726290 639614976 0 639614976 60 0 33435
1691803648 627730458 639688704 0 639688704 60 0 33434
1691811840 627727771 639688704 0 639688704 60 0 33434

Order 3 and order 4 show statistically significant improvement in
`mem_used_max` metrics.

T-test for order 3:

x order-2-maxmem
+ order-3-maxmem
N Min Max Median Avg Stddev
x 5 6.5517158e+08 6.5518797e+08 6.5518387e+08 6.551806e+08 6730.4157
+ 5 6.4170803e+08 6.4181862e+08 6.4179405e+08 6.4177684e+08 42210.666
Difference at 95.0% confidence
-1.34038e+07 +/- 44080.7
-2.04581% +/- 0.00672802%
(Student's t, pooled s = 30224.5)

T-test for order 4:

x order-2-maxmem
+ order-4-maxmem
N Min Max Median Avg Stddev
x 5 6.5517158e+08 6.5518797e+08 6.5518387e+08 6.551806e+08 6730.4157
+ 5 6.3961498e+08 6.396887e+08 6.3965594e+08 6.3965839e+08 31408.602
Difference at 95.0% confidence
-1.55222e+07 +/- 33126.2
-2.36915% +/- 0.00505604%
(Student's t, pooled s = 22713.4)

This test tends to benefit more from order 4 zspages, due to test's data
patterns.

zsmalloc object distribution analysis
=============================================================================

Order 2 (4 pages per zspage) tends to put many objects in size class 2048,
which is merged with size classes #112-#125:

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
...
71 1168 0 0 6146 6146 1756 2 0
74 1216 0 1 4560 4552 1368 3 0
76 1248 0 1 2938 2934 904 4 0
83 1360 0 0 10971 10971 3657 1 0
91 1488 0 0 16126 16126 5864 4 0
94 1536 0 1 5912 5908 2217 3 0
100 1632 0 0 11990 11990 4796 2 0
107 1744 0 1 15771 15768 6759 3 0
111 1808 0 1 10386 10380 4616 4 0
126 2048 0 0 45444 45444 22722 1 0
144 2336 0 0 47446 47446 27112 4 0
151 2448 1 0 10760 10759 6456 3 0
168 2720 0 0 10173 10173 6782 2 0
190 3072 0 1 1700 1697 1275 3 0
202 3264 0 1 290 286 232 4 0
254 4096 0 0 34051 34051 34051 1 0

Order 3 (8 pages per zspage) changed pool characteristics and unmerged
some of the size classes, which resulted in less objects being put into
size class 2048, because there are lower size classes are now available
for more compact object storage:

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
...
71 1168 0 1 2996 2994 856 2 0
72 1184 0 1 1632 1609 476 7 0
73 1200 1 0 1445 1442 425 5 0
74 1216 0 0 1510 1510 453 3 0
75 1232 0 1 1495 1479 455 7 0
76 1248 0 1 1456 1451 448 4 0
78 1280 0 1 3040 3033 950 5 0
79 1296 0 1 1584 1571 504 7 0
83 1360 0 0 6375 6375 2125 1 0
84 1376 0 1 1817 1796 632 8 0
87 1424 0 1 6020 6006 2107 7 0
88 1440 0 1 2108 2101 744 6 0
89 1456 0 1 2072 2064 740 5 0
91 1488 0 1 4169 4159 1516 4 0
92 1504 0 1 2014 2007 742 7 0
94 1536 0 1 3904 3900 1464 3 0
95 1552 0 1 1890 1873 720 8 0
96 1568 0 1 1963 1958 755 5 0
97 1584 0 1 1980 1974 770 7 0
100 1632 0 1 6190 6187 2476 2 0
103 1680 0 0 6477 6477 2667 7 0
104 1696 0 1 2256 2253 940 5 0
105 1712 0 1 2356 2340 992 8 0
107 1744 1 0 4697 4696 2013 3 0
110 1792 0 1 7744 7734 3388 7 0
111 1808 0 1 2655 2649 1180 4 0
114 1856 0 1 8371 8365 3805 5 0
116 1888 1 0 5863 5862 2706 6 0
117 1904 0 1 2955 2942 1379 7 0
118 1920 0 1 3009 2997 1416 8 0
126 2048 0 0 25276 25276 12638 1 0
128 2080 0 1 6060 6052 3232 8 0
129 2096 1 0 3081 3080 1659 7 0
134 2176 0 1 14835 14830 7912 8 0
135 2192 0 1 2769 2758 1491 7 0
137 2224 0 1 5082 5077 2772 6 0
140 2272 0 1 7236 7232 4020 5 0
144 2336 0 1 8428 8423 4816 4 0
147 2384 0 1 5316 5313 3101 7 0
151 2448 0 1 5445 5443 3267 3 0
155 2512 0 0 4121 4121 2536 8 0
158 2560 0 1 2208 2205 1380 5 0
160 2592 0 0 1133 1133 721 7 0
168 2720 0 0 2712 2712 1808 2 0
177 2864 1 0 1100 1098 770 7 0
180 2912 0 1 189 183 135 5 0
184 2976 0 1 176 166 128 8 0
190 3072 0 0 252 252 189 3 0
197 3184 0 1 198 192 154 7 0
202 3264 0 1 100 96 80 4 0
211 3408 0 1 210 208 175 5 0
217 3504 0 1 98 94 84 6 0
222 3584 0 0 104 104 91 7 0
225 3632 0 1 54 50 48 8 0
254 4096 0 0 33591 33591 33591 1 0

Note, the huge size watermark is above 3632 and there are a number of new
normal classes available that previously were merged with the huge class.
For instance, size class #211 holds 210 objects of size 3408 and uses 175
physical pages, while previously for those objects we would have used 210
physical pages.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
include/linux/zsmalloc.h | 12 +++++++
mm/zsmalloc.c | 73 +++++++++++++++++++++++-----------------
2 files changed, 54 insertions(+), 31 deletions(-)

diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
index a48cd0ffe57d..6cd1d95b928a 100644
--- a/include/linux/zsmalloc.h
+++ b/include/linux/zsmalloc.h
@@ -33,6 +33,18 @@ enum zs_mapmode {
*/
};

+#define ZS_PAGE_ORDER_2 2
+#define ZS_PAGE_ORDER_4 4
+
+/*
+ * A single 'zspage' is composed of up to 2^N discontiguous 0-order (single)
+ * pages. ZS_MAX_PAGE_ORDER defines upper limit on N, ZS_MIN_PAGE_ORDER
+ * defines lower limit on N. ZS_DEFAULT_PAGE_ORDER is recommended value.
+ */
+#define ZS_MIN_PAGE_ORDER ZS_PAGE_ORDER_2
+#define ZS_MAX_PAGE_ORDER ZS_PAGE_ORDER_4
+#define ZS_DEFAULT_PAGE_ORDER ZS_PAGE_ORDER_2
+
struct zs_pool_stats {
/* How many pages were migrated (freed) */
atomic_long_t pages_compacted;
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 065744b7e9d8..a9773566f85b 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -74,12 +74,7 @@
*/
#define ZS_ALIGN 8

-/*
- * A single 'zspage' is composed of up to 2^N discontiguous 0-order (single)
- * pages. ZS_MAX_ZSPAGE_ORDER defines upper limit on N.
- */
-#define ZS_MAX_ZSPAGE_ORDER 2
-#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)
+#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_PAGE_ORDER)

#define ZS_HANDLE_SIZE (sizeof(unsigned long))

@@ -124,10 +119,8 @@
#define ISOLATED_BITS 3
#define MAGIC_VAL_BITS 8

-#define MAX(a, b) ((a) >= (b) ? (a) : (b))
-/* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
-#define ZS_MIN_ALLOC_SIZE \
- MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
+#define ZS_MIN_ALLOC_SIZE 32U
+
/* each chunk includes extra space to keep handle */
#define ZS_MAX_ALLOC_SIZE PAGE_SIZE

@@ -141,12 +134,10 @@
* determined). NOTE: all those class sizes must be set as multiple of
* ZS_ALIGN to make sure link_free itself never has to span 2 pages.
*
- * ZS_MIN_ALLOC_SIZE and ZS_SIZE_CLASS_DELTA must be multiple of ZS_ALIGN
- * (reason above)
+ * pool->min_alloc_size (ZS_MIN_ALLOC_SIZE) and ZS_SIZE_CLASS_DELTA must
+ * be multiple of ZS_ALIGN (reason above)
*/
#define ZS_SIZE_CLASS_DELTA (PAGE_SIZE >> CLASS_BITS)
-#define ZS_SIZE_CLASSES (DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE, \
- ZS_SIZE_CLASS_DELTA) + 1)

enum fullness_group {
ZS_EMPTY,
@@ -230,12 +221,15 @@ struct link_free {
struct zs_pool {
const char *name;

- struct size_class *size_class[ZS_SIZE_CLASSES];
+ struct size_class **size_class;
struct kmem_cache *handle_cachep;
struct kmem_cache *zspage_cachep;

atomic_long_t pages_allocated;

+ u32 num_size_classes;
+ u32 min_alloc_size;
+
struct zs_pool_stats stats;

/* Compact classes */
@@ -523,15 +517,15 @@ static void set_zspage_mapping(struct zspage *zspage,
* classes depending on its size. This function returns index of the
* size class which has chunk size big enough to hold the given size.
*/
-static int get_size_class_index(int size)
+static int get_size_class_index(struct zs_pool *pool, int size)
{
int idx = 0;

- if (likely(size > ZS_MIN_ALLOC_SIZE))
- idx = DIV_ROUND_UP(size - ZS_MIN_ALLOC_SIZE,
+ if (likely(size > pool->min_alloc_size))
+ idx = DIV_ROUND_UP(size - pool->min_alloc_size,
ZS_SIZE_CLASS_DELTA);

- return min_t(int, ZS_SIZE_CLASSES - 1, idx);
+ return min_t(int, pool->num_size_classes - 1, idx);
}

/* type can be of enum type class_stat_type or fullness_group */
@@ -591,7 +585,7 @@ static int zs_stats_size_show(struct seq_file *s, void *v)
"obj_allocated", "obj_used", "pages_used",
"pages_per_zspage", "freeable");

- for (i = 0; i < ZS_SIZE_CLASSES; i++) {
+ for (i = 0; i < pool->num_size_classes; i++) {
class = pool->size_class[i];

if (class->index != i)
@@ -777,13 +771,13 @@ static enum fullness_group fix_fullness_group(struct size_class *class,
* link together 3 PAGE_SIZE sized pages to form a zspage
* since then we can perfectly fit in 8 such objects.
*/
-static int get_pages_per_zspage(int class_size)
+static int get_pages_per_zspage(u32 class_size, u32 num_pages)
{
int i, max_usedpc = 0;
/* zspage order which gives maximum used size per KB */
int max_usedpc_order = 1;

- for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
+ for (i = 1; i <= num_pages; i++) {
int zspage_size;
int waste, usedpc;

@@ -1220,7 +1214,7 @@ unsigned int zs_lookup_class_index(struct zs_pool *pool, unsigned int size)
{
struct size_class *class;

- class = pool->size_class[get_size_class_index(size)];
+ class = pool->size_class[get_size_class_index(pool, size)];

return class->index;
}
@@ -1431,7 +1425,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)

/* extra space in chunk to keep the handle */
size += ZS_HANDLE_SIZE;
- class = pool->size_class[get_size_class_index(size)];
+ class = pool->size_class[get_size_class_index(pool, size)];

/* class->lock effectively protects the zpage migration */
spin_lock(&class->lock);
@@ -1980,7 +1974,7 @@ static void async_free_zspage(struct work_struct *work)
struct zs_pool *pool = container_of(work, struct zs_pool,
free_work);

- for (i = 0; i < ZS_SIZE_CLASSES; i++) {
+ for (i = 0; i < pool->num_size_classes; i++) {
class = pool->size_class[i];
if (class->index != i)
continue;
@@ -2129,7 +2123,7 @@ unsigned long zs_compact(struct zs_pool *pool)
struct size_class *class;
unsigned long pages_freed = 0;

- for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
+ for (i = pool->num_size_classes - 1; i >= 0; i--) {
class = pool->size_class[i];
if (class->index != i)
continue;
@@ -2173,7 +2167,7 @@ static unsigned long zs_shrinker_count(struct shrinker *shrinker,
struct zs_pool *pool = container_of(shrinker, struct zs_pool,
shrinker);

- for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
+ for (i = pool->num_size_classes - 1; i >= 0; i--) {
class = pool->size_class[i];
if (class->index != i)
continue;
@@ -2215,11 +2209,27 @@ struct zs_pool *zs_create_pool(const char *name)
int i;
struct zs_pool *pool;
struct size_class *prev_class = NULL;
+ unsigned long num_pages;

pool = kzalloc(sizeof(*pool), GFP_KERNEL);
if (!pool)
return NULL;

+ num_pages = 1UL << ZS_DEFAULT_PAGE_ORDER;
+ /* min_alloc_size must be multiple of ZS_ALIGN */
+ pool->min_alloc_size = num_pages << PAGE_SHIFT >> OBJ_INDEX_BITS;
+ pool->min_alloc_size = max(pool->min_alloc_size, ZS_MIN_ALLOC_SIZE);
+
+ pool->num_size_classes =
+ DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - pool->min_alloc_size,
+ ZS_SIZE_CLASS_DELTA) + 1;
+
+ pool->size_class = kmalloc_array(pool->num_size_classes,
+ sizeof(struct size_class *),
+ GFP_KERNEL | __GFP_ZERO);
+ if (!pool->size_class)
+ goto err;
+
init_deferred_free(pool);
rwlock_init(&pool->migrate_lock);

@@ -2234,17 +2244,17 @@ struct zs_pool *zs_create_pool(const char *name)
* Iterate reversely, 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--) {
+ for (i = pool->num_size_classes - 1; i >= 0; i--) {
int size;
int pages_per_zspage;
int objs_per_zspage;
struct size_class *class;
int fullness = 0;

- size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
+ size = pool->min_alloc_size + i * ZS_SIZE_CLASS_DELTA;
if (size > ZS_MAX_ALLOC_SIZE)
size = ZS_MAX_ALLOC_SIZE;
- pages_per_zspage = get_pages_per_zspage(size);
+ pages_per_zspage = get_pages_per_zspage(size, num_pages);
objs_per_zspage = pages_per_zspage * PAGE_SIZE / size;

/*
@@ -2328,7 +2338,7 @@ void zs_destroy_pool(struct zs_pool *pool)
zs_flush_migration(pool);
zs_pool_stat_destroy(pool);

- for (i = 0; i < ZS_SIZE_CLASSES; i++) {
+ for (i = 0; i < pool->num_size_classes; i++) {
int fg;
struct size_class *class = pool->size_class[i];

@@ -2348,6 +2358,7 @@ void zs_destroy_pool(struct zs_pool *pool)
}

destroy_cache(pool);
+ kfree(pool->size_class);
kfree(pool->name);
kfree(pool);
}
--
2.38.1.273.g43a17bfeac-goog