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document extended size classes

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Daniel Micay 2019-04-10 08:42:32 -04:00
parent 6c31f6710a
commit b31e8dacb1
2 changed files with 37 additions and 7 deletions
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36
README.md
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@ -168,6 +168,10 @@ for the chosen values are not written yet, so use them at your own peril:
* `CONFIG_STATS`: `false` (default) to control whether stats on allocation /
deallocation count and active allocations are tracked. This is currently only
exposed via the mallinfo APIs on Android.
* `CONFIG_EXTENDED_SIZE_CLASSES`: `true` (default) to control whether small
size class go up to 64k instead of the minimum requirement for avoiding
memory waste of 16k. The option to extend it even further will be offered in
the future when better support for larger slab allocations is added.
* `CONFIG_LARGE_SIZE_CLASSES`: `true` (default) to control whether large
allocations use the slab allocation size class scheme instead of page size
granularity (see the section on size classes below)
@ -398,11 +402,33 @@ preliminary set of values.
| 14336 | 14.278738839285708% | 4 | 57344 | 0.0% |
| 16384 | 12.493896484375% | 4 | 65536 | 0.0% |
The slab allocation size classes currently end at 16384 since that's the final
size for 2048 byte spacing and the next spacing class matches the page size of
4096 bytes on the target platforms. This is the minimum set of small size
classes required to avoid substantial waste from rounding. Further slab
allocation size classes may be offered as an option in the future.
The slab allocation size classes end at 16384 since that's the final size for
2048 byte spacing and the next spacing class matches the page size of 4096
bytes on the target platforms. This is the minimum set of small size classes
required to avoid substantial waste from rounding.
The `CONFIG_EXTENDED_SIZE_CLASSES` option extends the size classes up to 65536,
with a final spacing class of 16384. This offers improved performance compared
to the minimum set of size classes. The security story is complicated, since
the slab allocation has both advantages like size class isolation completely
avoiding reuse of any of the address space for any other size classes or other
data. It also has disadvantages like caching a small number of empty slabs and
deterministic guard sizes. The cache will be configurable in the future, making
it possible to disable slab caching for the largest slab allocation sizes, to
force unmapping them immediately and putting them in the slab quarantine, which
eliminates most of the security disadvantage at the expense of also giving up
most of the performance advantage, but while retaining the isolation.
| size class | worst case internal fragmentation | slab slots | slab size | internal fragmentation for slabs |
| - | - | - | - | - |
| 20480 | 19.9951171875% | 2 | 40960 | 0.0% |
| 24576 | 16.66259765625% | 2 | 49152 | 0.0% |
| 28672 | 14.2822265625% | 2 | 57344 | 0.0% |
| 32768 | 12.4969482421875% | 2 | 65536 | 0.0% |
| 40960 | 19.99755859375% | 2 | 81920 | 0.0% |
| 49152 | 16.664632161458343% | 2 | 98304 | 0.0% |
| 57344 | 14.283970424107139% | 2 | 114688 | 0.0% |
| 65536 | 12.49847412109375% | 2 | 131072 | 0.0% |
The `CONFIG_LARGE_SIZE_CLASSES` option controls whether large allocations use
the same size class scheme providing 4 size classes for every doubling of size.

8
calculate_waste.py
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@ -10,7 +10,9 @@ size_classes = [
1280, 1536, 1792, 2048,
2560, 3072, 3584, 4096,
5120, 6144, 7168, 8192,
10240, 12288, 14336, 16384
10240, 12288, 14336, 16384,
20480, 24576, 28672, 32768,
40960, 49152, 57344, 65536
]
size_class_slots = [
@ -21,7 +23,9 @@ size_class_slots = [
16, 16, 16, 16,
8, 8, 8, 8,
8, 8, 8, 8,
6, 5, 4, 4
6, 5, 4, 4,
2, 2, 2, 2,
2, 2, 2, 2
]
fragmentation = [100 - 1 / 16 * 100]