zsmalloc

This allocator is designed for use with zram. Thus, the allocator issupposed to work well under low memory conditions. In particular, itnever attempts higher order page allocation which is very likely tofail under memory pressure. On the other hand, if we just use single(0-order) pages, it would suffer from very high fragmentation --any object of size PAGE_SIZE/2 or larger would occupy an entire page.This was one of the major issues with its predecessor (xvmalloc).

To overcome these issues, zsmalloc allocates a bunch of 0-order pagesand links them together using various ‘structpage’ fields. These linkedpages act as a single higher-order page i.e. an object can span 0-orderpage boundaries. The code refers to these linked pages as a single entitycalled zspage.

For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZEsince this satisfies the requirements of all its current users (in theworst case, page is incompressible and is thus stored “as-is” i.e. inuncompressed form). For allocation requests larger than this size, failureis returned (see zs_malloc).

Additionally,zs_malloc() does not return a dereferenceable pointer.Instead, it returns an opaque handle (unsigned long) which encodes actuallocation of the allocated object. The reason for this indirection is thatzsmalloc does not keep zspages permanently mapped since that would causeissues on 32-bit systems where the VA region for kernel space mappingsis very small. So, using the allocated memory should be done through theproper handle-based APIs.

stat

With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via/sys/kernel/debug/zsmalloc/<username>. Here is a sample of stat output:

# cat /sys/kernel/debug/zsmalloc/zram0/classesclass  size       10%       20%       30%       40%       50%       60%       70%       80%       90%       99%      100% obj_allocated   obj_used pages_used pages_per_zspage freeable   ...   ...   30   512         0        12         4         1         0         1         0         0         1         0       414          3464       3346        433                1       14   31   528         2         7         2         2         1         0         1         0         0         2       117          4154       3793        536                4       44   32   544         6         3         4         1         2         1         0         0         0         1       260          4170       3965        556                2       26   ...   ...
class

index

size

object size zspage stores

10%

the number of zspages with usage ratio less than 10% (see below)

20%

the number of zspages with usage ratio between 10% and 20%

30%

the number of zspages with usage ratio between 20% and 30%

40%

the number of zspages with usage ratio between 30% and 40%

50%

the number of zspages with usage ratio between 40% and 50%

60%

the number of zspages with usage ratio between 50% and 60%

70%

the number of zspages with usage ratio between 60% and 70%

80%

the number of zspages with usage ratio between 70% and 80%

90%

the number of zspages with usage ratio between 80% and 90%

99%

the number of zspages with usage ratio between 90% and 99%

100%

the number of zspages with usage ratio 100%

obj_allocated

the number of objects allocated

obj_used

the number of objects allocated to the user

pages_used

the number of pages allocated for the class

pages_per_zspage

the number of 0-order pages to make a zspage

freeable

the approximate number of pages class compaction can free

Each zspage maintains inuse counter which keeps track of the number ofobjects stored in the zspage. The inuse counter determines the zspage’s“fullness group” which is calculated as the ratio of the “inuse” objects tothe total number of objects the zspage can hold (objs_per_zspage). Thecloser the inuse counter is to objs_per_zspage, the better.

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 thecreation of the zsmalloc pool (seecalculate_zspage_chain_size()).

As an optimization, zsmalloc merges size classes that have similarcharacteristics in terms of the number of pages per zspage and the numberof objects that each zspage can store.

For instance, consider the following size classes::

class  size       10%   ....    100% 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 weneed 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 size1632 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 canhold a total of 5 objects. If we need to store 13 objects of size 1568, weend up allocating three zspages, or 6 physical pages.

However, if we take a closer look at size class #96 (which is meant forobjects of size 1568 bytes) and tracecalculate_zspage_chain_size(), wefind that the most optimal zspage configuration for this class is a chainof 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 13objects 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 6physical pages to store the same number of objects.

As the zspage chain size for class #96 increases, its key characteristicssuch as pages per-zspage and objects per-zspage also change. This leads todewer class mergers, resulting in a more compact grouping of classes, whichreduces memory wastage.

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

class  size       10%   ....    100% 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 pagesper zspage. Any object larger than 3264 bytes is considered huge and belongsto size class #254, which stores each object in its own physical page (objectsin huge classes do not share pages).

Increasing the size of the chain of zspages also results in a higher watermarkfor the huge size class and fewer huge classes overall. This allows for moreefficient storage of large objects.

For zspage chain size of 8, huge class watermark becomes 3632 bytes::

class  size       10%   ....    100% 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       10%   ....    100% 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       10%   ....    100% 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       10%   ....    100% 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 seenin the example where the number of physical pages used decreased from 159955to 156666, at the same time maximum zsmalloc pool memory usage went down from655175680 to 641703936 bytes.

However, this advantage may be offset by the potential for increased systemmemory pressure (as some zspages have larger chain sizes) in cases where thereis heavy internal fragmentation and zspool compaction is unable to relocateobjects and release zspages. In these cases, it is recommended to decreasethe limit on the size of the zspage chains (as specified by theCONFIG_ZSMALLOC_CHAIN_SIZE option).

Functions

voidobj_to_location(unsignedlongobj,structzpdesc**zpdesc,unsignedint*obj_idx)

get (<zpdesc>, <obj_idx>) from encoded object value

Parameters

unsignedlongobj

the encoded object value

structzpdesc**zpdesc

zpdesc object resides in zspage

unsignedint*obj_idx

object index

unsignedlonglocation_to_obj(structzpdesc*zpdesc,unsignedintobj_idx)

get obj value encoded from (<zpdesc>, <obj_idx>)

Parameters

structzpdesc*zpdesc

zpdesc object resides in zspage

unsignedintobj_idx

object index

unsignedintzs_lookup_class_index(structzs_pool*pool,unsignedintsize)

Returns index of the zsmallocsize_class that hold objects of the provided size.

Parameters

structzs_pool*pool

zsmalloc pool to use

unsignedintsize

object size

Context

Any context.

Return

the index of the zsmallocsize_class that hold objects of theprovided size.

size_tzs_huge_class_size(structzs_pool*pool)

Returns the size (in bytes) of the first huge zsmallocsize_class.

Parameters

structzs_pool*pool

zsmalloc pool to use

Description

The function returns the size of the first huge class - any object of equalor bigger size will be stored in zspage consisting of a single physicalpage.

Context

Any context.

Return

the size (in bytes) of the first huge zsmallocsize_class.

unsignedlongzs_malloc(structzs_pool*pool,size_tsize,gfp_tgfp,constintnid)

Allocate block of given size from pool.

Parameters

structzs_pool*pool

pool to allocate from

size_tsize

size of block to allocate

gfp_tgfp

gfp flags when allocating object

constintnid

The preferred node id to allocate new zspage (if needed)

Description

On success, handle to the allocated object is returned,otherwise anERR_PTR().Allocation requests with size > ZS_MAX_ALLOC_SIZE will fail.

structzs_pool*zs_create_pool(constchar*name)

Creates an allocation pool to work from.

Parameters

constchar*name

pool name to be created

Description

This function must be called before anything when usingthe zsmalloc allocator.

On success, a pointer to the newly created pool is returned,otherwise NULL.