| ======== |
| zsmalloc |
| ======== |
| |
| This allocator is designed for use with zram. Thus, the allocator is |
| supposed to work well under low memory conditions. In particular, it |
| never attempts higher order page allocation which is very likely to |
| fail 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 pages |
| and links them together using various 'struct page' fields. These linked |
| pages act as a single higher-order page i.e. an object can span 0-order |
| page boundaries. The code refers to these linked pages as a single entity |
| called zspage. |
| |
| For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE |
| since this satisfies the requirements of all its current users (in the |
| worst case, page is incompressible and is thus stored "as-is" i.e. in |
| uncompressed form). For allocation requests larger than this size, failure |
| is returned (see zs_malloc). |
| |
| Additionally, zs_malloc() does not return a dereferenceable pointer. |
| Instead, it returns an opaque handle (unsigned long) which encodes actual |
| location of the allocated object. The reason for this indirection is that |
| zsmalloc does not keep zspages permanently mapped since that would cause |
| issues on 32-bit systems where the VA region for kernel space mappings |
| is very small. So, before using the allocating memory, the object has to |
| be mapped using zs_map_object() to get a usable pointer and subsequently |
| unmapped using zs_unmap_object(). |
| |
| stat |
| ==== |
| |
| With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via |
| ``/sys/kernel/debug/zsmalloc/<user name>``. Here is a sample of stat output:: |
| |
| # cat /sys/kernel/debug/zsmalloc/zram0/classes |
| |
| class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage |
| ... |
| ... |
| 9 176 0 1 186 129 8 4 |
| 10 192 1 0 2880 2872 135 3 |
| 11 208 0 1 819 795 42 2 |
| 12 224 0 1 219 159 12 4 |
| ... |
| ... |
| |
| |
| class |
| index |
| size |
| object size zspage stores |
| almost_empty |
| the number of ZS_ALMOST_EMPTY zspages(see below) |
| almost_full |
| the number of ZS_ALMOST_FULL zspages(see below) |
| 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 |
| |
| We assign a zspage to ZS_ALMOST_EMPTY fullness group when n <= N / f, where |
| |
| * n = number of allocated objects |
| * N = total number of objects zspage can store |
| * f = fullness_threshold_frac(ie, 4 at the moment) |
| |
| 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). |
