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clarify documentation about the 0 byte size class

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Daniel Micay 2018-11-03 04:40:13 -04:00
parent f949698c7a
commit d5b773b2bc
2 changed files with 8 additions and 5 deletions
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11
README.md
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@ -213,7 +213,8 @@ was a bit less important and if a core goal was finding latent bugs.
with a strong keyed hash due to performance limitations)
* Possible slab locations are skipped and remain memory protected, leaving slab
size class regions interspersed with guard pages
* Zero size allocations are memory protected
* Zero size allocations are a dedicated size class with the entire region
remaining non-readable and non-writable
* Protected allocator state (including all metadata)
* Address space for state is entirely reserved during initialization and
never reused for allocations or anything else
@ -277,8 +278,10 @@ overhead and can easily dwarf the cost of an efficient CSPRNG.
# Size classes
The zero byte size class is a special case of the smallest regular size class. It's allocated in a
separate region with the memory left non-readable and non-writable.
The zero byte size class is a special case of the smallest regular size class.
It's allocated in a dedicated region like other size classes but with the slabs
never being made readable and writable so the only memory usage is for the slab
metadata.
The slab slot count for each size class is not yet finely tuned beyond choosing values avoiding
internal fragmentation for slabs (i.e. avoiding wasted space due to page size rounding).
@ -292,7 +295,7 @@ slabs containing them:
| size class | worst case internal fragmentation | slab slots | slab size | worst case internal fragmentation for slabs |
| - | - | - | - | - |
| 16 | 100% | 256 | 4096 | 0.0% |
| 16 | 93.75% | 256 | 4096 | 0.0% |
| 32 | 46.875% | 128 | 4096 | 0.0% |
| 48 | 31.25% | 85 | 4096 | 0.390625% |
| 64 | 23.4375% | 64 | 4096 | 0.0% |

2
calculate_waste.py
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@ -24,7 +24,7 @@ size_class_slots = [
6, 5, 4, 4
]
fragmentation = [100]
fragmentation = [100 - 1 / 16 * 100]
for i in range(len(size_classes) - 1):
size_class = size_classes[i + 1]