zsmalloc: make zspage chain size configurable (4ff93b29) · Commits · EulixOS / Software / Kernel

Documentation/mm/zsmalloc.rst

+168 −0

Original line number	Diff line number	Diff line
		@@ -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).

mm/Kconfig

+19 −0

Original line number	Diff line number	Diff line
		@@ -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

mm/zsmalloc.c

+4 −8

Original line number	Diff line number	Diff line
		@@ -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))

		/*
		@@ -136,10 +129,13 @@
		#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))