[RFC PATCH 1/2] zsmalloc: drop hard limit on the number of size classes

[Sergey Senozhatsky <senozhatsky@chromium.org>]

For the reasons unknown, zsmalloc limits the number of size-classes
to 256.  On PAGE_SIZE 4K systems this works pretty fine, as those
256 classes are 4096/256 = 16 (known as size class delta) bytes apart.
However, as the PAGE_SIZE grows, e.g. 16K, the hard limit pushes the
size-class delta significantly further (e.g. 16384/256 = 64) leading
to increased internal fragmentation.  For example, on a 16K page system,
an object of size 65 bytes is rounded up to the next 64-byte boundary
(128 bytes), wasting nearly 50% of the allocated space.

Instead of calculating size-class delta based on both PAGE_SIZE and
hard limit of 256 the ZS_SIZE_CLASS_DELTA is set to constant value
of 16 bytes.  This results in a much higher than 256 number of size
classes on systems with PAGE_SIZE larger than 4K.  These extra size
classes split existing cluster into smaller ones.  For example, using
tool [1] 16K PAGE_SIZE, chain size 8:

BASE (delta 64 bytes)
=====================
Log   | Phys  | Chain | Objs/Page | TailWaste  | MergeWaste
[..]
 1072 |  1120 |  8    |  117      |  32        |  5616
 1088 |  1120 |  8    |  117      |  32        |  3744
 1104 |  1120 |  8    |  117      |  32        |  1872
 1120 |  1120 |  8    |  117      |  32        |  0
[..]

PATCHED (delta 16 bytes)
========================
[..]
 1072 |  1072 |  4    |  61       |  144       |  0
 1088 |  1088 |  1    |  15       |  64        |  0
 1104 |  1104 |  6    |  89       |  48        |  0
 1120 |  1120 |  8    |  117      |  32        |  0
[..]

In default configuration (delta 64) size classes 1072 to 1104
are merged into 1120.  Size class 1120 holds 117 objects
per-zspage, so worst case every zspage can lose 5616 bytes
(1120-1072 times 117).  With delta 16 this cluster doesn't
exist, reducing memory waste.

[1] https://github.com/sergey-senozhatsky/simulate-zsmalloc/blob/main/simulate_zsmalloc.c

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 mm/zsmalloc.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 5bf832f9c05c..5e7501d36161 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -92,7 +92,7 @@
 
 #define HUGE_BITS	1
 #define FULLNESS_BITS	4
-#define CLASS_BITS	8
+#define CLASS_BITS	12
 #define MAGIC_VAL_BITS	8
 
 #define ZS_MAX_PAGES_PER_ZSPAGE	(_AC(CONFIG_ZSMALLOC_CHAIN_SIZE, UL))
@@ -115,8 +115,13 @@
  *
  *  ZS_MIN_ALLOC_SIZE and ZS_SIZE_CLASS_DELTA must be multiple of ZS_ALIGN
  *  (reason above)
+ *
+ * We set ZS_SIZE_CLASS_DELTA to 16 bytes to maintain high granularity
+ * even on systems with large PAGE_SIZE (e.g. 16K, 64K). This prevents
+ * internal fragmentation. CLASS_BITS is increased to 12 to address the
+ * larger number of size classes on such systems (up to 4096 classes on 64K).
  */
-#define ZS_SIZE_CLASS_DELTA	(PAGE_SIZE >> CLASS_BITS)
+#define ZS_SIZE_CLASS_DELTA	16
 #define ZS_SIZE_CLASSES	(DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE, \
 				      ZS_SIZE_CLASS_DELTA) + 1)
 
-- 
2.52.0.351.gbe84eed79e-goog