From b31e8dacb1e5ef28425f2bfa87c35eb82db79719 Mon Sep 17 00:00:00 2001 From: Daniel Micay Date: Wed, 10 Apr 2019 08:42:32 -0400 Subject: [PATCH] document extended size classes --- README.md | 36 +++++++++++++++++++++++++++++++----- calculate_waste.py | 8 ++++++-- 2 files changed, 37 insertions(+), 7 deletions(-) diff --git a/README.md b/README.md index 6291d22..c2d9aad 100644 --- a/README.md +++ b/README.md @@ -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. diff --git a/calculate_waste.py b/calculate_waste.py index 07dda15..b2f5ec6 100755 --- a/calculate_waste.py +++ b/calculate_waste.py @@ -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]