266 lines
13 KiB
ReStructuredText
266 lines
13 KiB
ReStructuredText
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========
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zsmalloc
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========
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This allocator is designed for use with zram. Thus, the allocator is
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supposed to work well under low memory conditions. In particular, it
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never attempts higher order page allocation which is very likely to
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fail under memory pressure. On the other hand, if we just use single
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(0-order) pages, it would suffer from very high fragmentation --
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any object of size PAGE_SIZE/2 or larger would occupy an entire page.
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This was one of the major issues with its predecessor (xvmalloc).
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To overcome these issues, zsmalloc allocates a bunch of 0-order pages
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and links them together using various 'struct page' fields. These linked
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pages act as a single higher-order page i.e. an object can span 0-order
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page boundaries. The code refers to these linked pages as a single entity
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called zspage.
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For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
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since this satisfies the requirements of all its current users (in the
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worst case, page is incompressible and is thus stored "as-is" i.e. in
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uncompressed form). For allocation requests larger than this size, failure
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is returned (see zs_malloc).
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Additionally, zs_malloc() does not return a dereferenceable pointer.
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Instead, it returns an opaque handle (unsigned long) which encodes actual
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location of the allocated object. The reason for this indirection is that
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zsmalloc does not keep zspages permanently mapped since that would cause
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issues on 32-bit systems where the VA region for kernel space mappings
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is very small. So, before using the allocating memory, the object has to
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be mapped using zs_map_object() to get a usable pointer and subsequently
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unmapped using zs_unmap_object().
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stat
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====
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With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via
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``/sys/kernel/debug/zsmalloc/<user name>``. Here is a sample of stat output::
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# cat /sys/kernel/debug/zsmalloc/zram0/classes
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class size 10% 20% 30% 40% 50% 60% 70% 80% 90% 99% 100% obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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...
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30 512 0 12 4 1 0 1 0 0 1 0 414 3464 3346 433 1 14
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31 528 2 7 2 2 1 0 1 0 0 2 117 4154 3793 536 4 44
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32 544 6 3 4 1 2 1 0 0 0 1 260 4170 3965 556 2 26
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...
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...
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class
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index
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size
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object size zspage stores
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10%
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the number of zspages with usage ratio less than 10% (see below)
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20%
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the number of zspages with usage ratio between 10% and 20%
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30%
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the number of zspages with usage ratio between 20% and 30%
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40%
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the number of zspages with usage ratio between 30% and 40%
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50%
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the number of zspages with usage ratio between 40% and 50%
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60%
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the number of zspages with usage ratio between 50% and 60%
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70%
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the number of zspages with usage ratio between 60% and 70%
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80%
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the number of zspages with usage ratio between 70% and 80%
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90%
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the number of zspages with usage ratio between 80% and 90%
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99%
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the number of zspages with usage ratio between 90% and 99%
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100%
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the number of zspages with usage ratio 100%
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obj_allocated
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the number of objects allocated
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obj_used
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the number of objects allocated to the user
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pages_used
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the number of pages allocated for the class
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pages_per_zspage
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the number of 0-order pages to make a zspage
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freeable
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the approximate number of pages class compaction can free
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Each zspage maintains inuse counter which keeps track of the number of
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objects stored in the zspage. The inuse counter determines the zspage's
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"fullness group" which is calculated as the ratio of the "inuse" objects to
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the total number of objects the zspage can hold (objs_per_zspage). The
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closer the inuse counter is to objs_per_zspage, the better.
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Internals
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=========
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zsmalloc has 255 size classes, each of which can hold a number of zspages.
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Each zspage can contain up to ZSMALLOC_CHAIN_SIZE physical (0-order) pages.
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The optimal zspage chain size for each size class is calculated during the
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creation of the zsmalloc pool (see calculate_zspage_chain_size()).
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As an optimization, zsmalloc merges size classes that have similar
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characteristics in terms of the number of pages per zspage and the number
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of objects that each zspage can store.
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For instance, consider the following size classes:::
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class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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94 1536 0 .... 0 0 0 0 3 0
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100 1632 0 .... 0 0 0 0 2 0
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...
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Size classes #95-99 are merged with size class #100. This means that when we
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need to store an object of size, say, 1568 bytes, we end up using size class
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#100 instead of size class #96. Size class #100 is meant for objects of size
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1632 bytes, so each object of size 1568 bytes wastes 1632-1568=64 bytes.
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Size class #100 consists of zspages with 2 physical pages each, which can
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hold a total of 5 objects. If we need to store 13 objects of size 1568, we
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end up allocating three zspages, or 6 physical pages.
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However, if we take a closer look at size class #96 (which is meant for
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objects of size 1568 bytes) and trace `calculate_zspage_chain_size()`, we
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find that the most optimal zspage configuration for this class is a chain
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of 5 physical pages:::
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pages per zspage wasted bytes used%
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1 960 76
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2 352 95
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3 1312 89
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4 704 95
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5 96 99
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This means that a class #96 configuration with 5 physical pages can store 13
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objects of size 1568 in a single zspage, using a total of 5 physical pages.
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This is more efficient than the class #100 configuration, which would use 6
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physical pages to store the same number of objects.
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As the zspage chain size for class #96 increases, its key characteristics
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such as pages per-zspage and objects per-zspage also change. This leads to
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dewer class mergers, resulting in a more compact grouping of classes, which
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reduces memory wastage.
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Let's take a closer look at the bottom of `/sys/kernel/debug/zsmalloc/zramX/classes`:::
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class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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202 3264 0 .. 0 0 0 0 4 0
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254 4096 0 .. 0 0 0 0 1 0
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...
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Size class #202 stores objects of size 3264 bytes and has a maximum of 4 pages
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per zspage. Any object larger than 3264 bytes is considered huge and belongs
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to size class #254, which stores each object in its own physical page (objects
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in huge classes do not share pages).
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Increasing the size of the chain of zspages also results in a higher watermark
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for the huge size class and fewer huge classes overall. This allows for more
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efficient storage of large objects.
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For zspage chain size of 8, huge class watermark becomes 3632 bytes:::
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class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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202 3264 0 .. 0 0 0 0 4 0
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211 3408 0 .. 0 0 0 0 5 0
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217 3504 0 .. 0 0 0 0 6 0
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222 3584 0 .. 0 0 0 0 7 0
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225 3632 0 .. 0 0 0 0 8 0
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254 4096 0 .. 0 0 0 0 1 0
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...
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For zspage chain size of 16, huge class watermark becomes 3840 bytes:::
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class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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202 3264 0 .. 0 0 0 0 4 0
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206 3328 0 .. 0 0 0 0 13 0
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207 3344 0 .. 0 0 0 0 9 0
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208 3360 0 .. 0 0 0 0 14 0
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211 3408 0 .. 0 0 0 0 5 0
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212 3424 0 .. 0 0 0 0 16 0
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214 3456 0 .. 0 0 0 0 11 0
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217 3504 0 .. 0 0 0 0 6 0
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219 3536 0 .. 0 0 0 0 13 0
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222 3584 0 .. 0 0 0 0 7 0
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223 3600 0 .. 0 0 0 0 15 0
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225 3632 0 .. 0 0 0 0 8 0
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228 3680 0 .. 0 0 0 0 9 0
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230 3712 0 .. 0 0 0 0 10 0
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232 3744 0 .. 0 0 0 0 11 0
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234 3776 0 .. 0 0 0 0 12 0
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235 3792 0 .. 0 0 0 0 13 0
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236 3808 0 .. 0 0 0 0 14 0
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238 3840 0 .. 0 0 0 0 15 0
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254 4096 0 .. 0 0 0 0 1 0
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...
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Overall the combined zspage chain size effect on zsmalloc pool configuration:::
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pages per zspage number of size classes (clusters) huge size class watermark
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4 69 3264
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5 86 3408
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6 93 3504
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7 112 3584
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8 123 3632
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9 140 3680
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10 143 3712
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11 159 3744
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12 164 3776
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13 180 3792
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14 183 3808
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15 188 3840
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16 191 3840
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A synthetic test
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----------------
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zram as a build artifacts storage (Linux kernel compilation).
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* `CONFIG_ZSMALLOC_CHAIN_SIZE=4`
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zsmalloc classes stats:::
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class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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Total 13 .. 51 413836 412973 159955 3
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zram mm_stat:::
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1691783168 628083717 655175680 0 655175680 60 0 34048 34049
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* `CONFIG_ZSMALLOC_CHAIN_SIZE=8`
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zsmalloc classes stats:::
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class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable
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...
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Total 18 .. 87 414852 412978 156666 0
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zram mm_stat:::
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1691803648 627793930 641703936 0 641703936 60 0 33591 33591
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Using larger zspage chains may result in using fewer physical pages, as seen
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in the example where the number of physical pages used decreased from 159955
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to 156666, at the same time maximum zsmalloc pool memory usage went down from
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655175680 to 641703936 bytes.
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However, this advantage may be offset by the potential for increased system
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memory pressure (as some zspages have larger chain sizes) in cases where there
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is heavy internal fragmentation and zspool compaction is unable to relocate
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objects and release zspages. In these cases, it is recommended to decrease
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the limit on the size of the zspage chains (as specified by the
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CONFIG_ZSMALLOC_CHAIN_SIZE option).
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