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at a final spacing class of 1 slot size classes

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Daniel Micay 2019-04-10 16:32:24 -04:00
parent 7a89a7b8c5
commit 086eb1fee4
3 changed files with 27 additions and 17 deletions
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35
README.md
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@ -407,17 +407,18 @@ The slab allocation size classes end at 16384 since that's the final size for
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.
The `CONFIG_EXTENDED_SIZE_CLASSES` option extends the size classes up to
131072, 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 |
| - | - | - | - | - |
@ -425,10 +426,14 @@ most of the performance advantage, but while retaining the isolation.
| 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% |
| 40960 | 19.99755859375% | 1 | 40960 | 0.0% |
| 49152 | 16.664632161458343% | 1 | 49152 | 0.0% |
| 57344 | 14.283970424107139% | 1 | 57344 | 0.0% |
| 65536 | 12.49847412109375% | 1 | 65536 | 0.0% |
| 81920 | 19.998779296875% | 1 | 81920 | 0.0% |
| 98304 | 16.6656494140625% | 1 | 98304 | 0.0% |
| 114688 | 14.284842354910708% | 1 | 114688 | 0.0% |
| 131072 | 12.499237060546875% | 1 | 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.

6
calculate_waste.py
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@ -12,7 +12,8 @@ size_classes = [
5120, 6144, 7168, 8192,
10240, 12288, 14336, 16384,
20480, 24576, 28672, 32768,
40960, 49152, 57344, 65536
40960, 49152, 57344, 65536,
81920, 98304, 114688, 131072,
]
size_class_slots = [
@ -25,7 +26,8 @@ size_class_slots = [
8, 8, 8, 8,
6, 5, 4, 4,
2, 2, 2, 2,
2, 2, 2, 2
1, 1, 1, 1,
1, 1, 1, 1,
]
fragmentation = [100 - 1 / 16 * 100]

3
h_malloc.c
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@ -119,6 +119,7 @@ static const u32 size_classes[] = {
#if CONFIG_EXTENDED_SIZE_CLASSES
/* 4096 */ 20480, 24576, 28672, 32768,
/* 8192 */ 40960, 49152, 57344, 65536,
/* 16384 */ 81920, 98304, 114688, 131072,
#endif
};
@ -135,6 +136,7 @@ static const u16 size_class_slots[] = {
#if CONFIG_EXTENDED_SIZE_CLASSES
/* 4096 */ 2, 2, 2, 2,
/* 8192 */ 1, 1, 1, 1,
/* 16384 */ 1, 1, 1, 1,
#endif
};
@ -152,6 +154,7 @@ static const char *const size_class_labels[] = {
#if CONFIG_EXTENDED_SIZE_CLASSES
/* 4096 */ "malloc 20480", "malloc 24576", "malloc 28672", "malloc 32768",
/* 8192 */ "malloc 40960", "malloc 49152", "malloc 57344", "malloc 65536",
/* 16384 */ "malloc 81920", "malloc 98304", "malloc 114688", "malloc 131072",
#endif
};