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improve explanation of jemalloc size classes

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Daniel Micay 2018-11-19 00:11:15 -05:00
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README.md
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@ -309,12 +309,17 @@ 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).
The choice of size classes is the same as jemalloc, but with a much different approach to the
slabs containing them:
> size classes are multiples of the quantum [16], spaced such that there are four size classes for
> each doubling in size, which limits internal fragmentation to approximately 20% for all but the
> smallest size classes
The choice of size classes for slab allocation is the same as jemalloc, which
is a careful balance between minimizing internal and external fragmentation. If
there are more size classes, more memory is wasted on free slots available only
to allocation requests of those sizes (external fragmentation). If there are
fewer size classes, the spacing between them is larger and more memory is
wasted due to rounding up to the size classes (internal fragmentation). There
are 4 special size classes for the smallest sizes (16, 32, 48, 64) that are
simply spaced out by the minimum spacing (16). Afterwards, there are four size
classes for every power of two spacing which results in bounding the internal
fragmentation below 20% for each size class. This also means there are 4 size
classes for each doubling in size.
| size class | worst case internal fragmentation | slab slots | slab size | internal fragmentation for slabs |
| - | - | - | - | - |