[PATCHv3 3/4] zsmalloc: make zspage chain size configurable

From: Sergey Senozhatsky
Date: Tue Jan 17 2023 - 20:03:18 EST

Remove hard coded limit on the maximum number of physical
pages per-zspage.

This will allow tuning of zsmalloc pool as zspage chain
size changes `pages per-zspage` and `objects per-zspage`
characteristics of size classes which also affects size
classes clustering (the way size classes are merged).

Signed-off-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
---
Documentation/mm/zsmalloc.rst | 168 ++++++++++++++++++++++++++++++++++
mm/Kconfig | 19 ++++
mm/zsmalloc.c | 12 +--
3 files changed, 191 insertions(+), 8 deletions(-)

diff --git a/Documentation/mm/zsmalloc.rst b/Documentation/mm/zsmalloc.rst
index 6e79893d6132..40323c9b39d8 100644
--- a/Documentation/mm/zsmalloc.rst
+++ b/Documentation/mm/zsmalloc.rst
@@ -80,3 +80,171 @@ Similarly, we assign zspage to:
* ZS_ALMOST_FULL when n > N / f
* ZS_EMPTY when n == 0
* ZS_FULL when n == N
+
+
+Internals
+=========
+
+zsmalloc has 255 size classes, each of which can hold a number of zspages.
+Each zspage can contain up to ZSMALLOC_CHAIN_SIZE physical (0-order) pages.
+The optimal zspage chain size for each size class is calculated during the
+creation of the zsmalloc pool (see calculate_zspage_chain_size()).
+
+As an optimization, zsmalloc merges size classes that have similar
+characteristics in terms of the number of pages per zspage and the number
+of objects that each zspage can store.
+
+For instance, consider 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. This means that when we
+need to store an object of size, say, 1568 bytes, we end up using size class
+#100 instead of size class #96. Size class #100 is meant for objects of size
+1632 bytes, so each object of size 1568 bytes wastes 1632-1568=64 bytes.
+
+Size class #100 consists of zspages with 2 physical pages each, which can
+hold a total of 5 objects. If we need to store 13 objects of size 1568, we
+end up allocating three zspages, or 6 physical pages.
+
+However, if we take a closer look at size class #96 (which is meant for
+objects of size 1568 bytes) and trace `calculate_zspage_chain_size()`, we
+find that the most optimal zspage configuration for this class is a chain
+of 5 physical pages:::
+
+ pages per zspage wasted bytes used%
+ 1 960 76
+ 2 352 95
+ 3 1312 89
+ 4 704 95
+ 5 96 99
+
+This means that a class #96 configuration with 5 physical pages can store 13
+objects of size 1568 in a single zspage, using a total of 5 physical pages.
+This is more efficient than the class #100 configuration, which would use 6
+physical pages to store the same number of objects.
+
+As the zspage chain size for class #96 increases, its key characteristics
+such as pages per-zspage and objects per-zspage also change. This leads to
+dewer class mergers, resulting in a more compact grouping of classes, which
+reduces memory wastage.
+
+Let's take a closer look at the bottom of `/sys/kernel/debug/zsmalloc/zramX/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
+ ...
+
+Size class #202 stores objects of size 3264 bytes and has a maximum of 4 pages
+per zspage. Any object larger than 3264 bytes is considered huge and belongs
+to size class #254, which stores each object in its own physical page (objects
+in huge classes do not share pages).
+
+Increasing the size of the chain of zspages also results in a higher watermark
+for the huge size class and fewer huge classes overall. This allows for more
+efficient storage of large objects.
+
+For zspage chain size of 8, 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 zspage chain size of 16, 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
+ ...
+
+Overall the combined zspage chain size effect on zsmalloc pool configuration:::
+
+ pages per zspage number of size classes (clusters) huge size class watermark
+ 4 69 3264
+ 5 86 3408
+ 6 93 3504
+ 7 112 3584
+ 8 123 3632
+ 9 140 3680
+ 10 143 3712
+ 11 159 3744
+ 12 164 3776
+ 13 180 3792
+ 14 183 3808
+ 15 188 3840
+ 16 191 3840
+
+
+A synthetic test
+----------------
+
+zram as a build artifacts storage (Linux kernel compilation).
+
+* `CONFIG_ZSMALLOC_CHAIN_SIZE=4`
+
+ zsmalloc classes stats:::
+
+ class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
+ ...
+ Total 13 51 413836 412973 159955 3
+
+ zram mm_stat:::
+
+ 1691783168 628083717 655175680 0 655175680 60 0 34048 34049
+
+
+* `CONFIG_ZSMALLOC_CHAIN_SIZE=8`
+
+ zsmalloc classes stats:::
+
+ class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
+ ...
+ Total 18 87 414852 412978 156666 0
+
+ zram mm_stat:::
+
+ 1691803648 627793930 641703936 0 641703936 60 0 33591 33591
+
+Using larger zspage chains may result in using fewer physical pages, as seen
+in the example where the number of physical pages used decreased from 159955
+to 156666, at the same time maximum zsmalloc pool memory usage went down from
+655175680 to 641703936 bytes.
+
+However, this advantage may be offset by the potential for increased system
+memory pressure (as some zspages have larger chain sizes) in cases where there
+is heavy internal fragmentation and zspool compaction is unable to relocate
+objects and release zspages. In these cases, it is recommended to decrease
+the limit on the size of the zspage chains (as specified by the
+CONFIG_ZSMALLOC_CHAIN_SIZE option).
diff --git a/mm/Kconfig b/mm/Kconfig
index 4eb4afa53e6d..1cfc0ec4e35e 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -191,6 +191,25 @@ config ZSMALLOC_STAT
information to userspace via debugfs.
If unsure, say N.

+config ZSMALLOC_CHAIN_SIZE
+ int "Maximum number of physical pages per-zspage"
+ default 4
+ range 4 16
+ depends on ZSMALLOC
+ help
+ This option sets the upper limit on the number of physical pages
+ that a zmalloc page (zspage) can consist of. The optimal zspage
+ chain size is calculated for each size class during the
+ initialization of the pool.
+
+ Changing this option can alter the characteristics of size classes,
+ such as the number of pages per zspage and the number of objects
+ per zspage. This can also result in different configurations of
+ the pool, as zsmalloc merges size classes with similar
+ characteristics.
+
+ For more information, see zsmalloc documentation.
+
menu "SLAB allocator options"

choice
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index ee8431784998..1a7f68c46ccd 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -73,13 +73,6 @@
*/
#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_HANDLE_SIZE (sizeof(unsigned long))

/*
@@ -120,10 +113,13 @@
#define HUGE_BITS 1
#define FULLNESS_BITS 2
#define CLASS_BITS 8
-#define ISOLATED_BITS 3
+#define ISOLATED_BITS 5
#define MAGIC_VAL_BITS 8

#define MAX(a, b) ((a) >= (b) ? (a) : (b))
+
+#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(CONFIG_ZSMALLOC_CHAIN_SIZE, UL))
+
/* 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))
--
2.39.0.314.g84b9a713c41-goog