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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). |