[3pt] Add 'slab_alloc_granularity' option to box.cfg{}

[TarantoolBot]

Related dev. issue(s): tarantool/tarantool#5518

Product: Tarantool
Since: 2.8.1
Root document: Configuring the storage
SME: ?

Details

Add slab_alloc_granularity option to box.cfg{} that allows user to set
multiplicity of memory allocation in small allocator. slab_alloc_granularity
determines not only alignment of objects, but also size of the objects in
the pool. Thus, the greater the slab_alloc_granularity, the greater the
memory loss per one memory allocation, but tuples with different sizes
are allocated from the same mempool, and we do not lose memory on
the slabs, when we have highly distributed tuple sizes. This is somewhat
similar to a large alloc factor. The smaller the slab_alloc_granularity, the less
memory loss per allocation, if the user has many small tuples of approximately
the same size, it will be nice to set slab_alloc_granularity == 4 to save memory.

This option must be set once during start, default value == sizeof(intptr_t)
(8 on 64 bit platforms), as it was before, when this option was not provided.
slab_alloc_granularity must be exponent of two and >= 4. Together with the
slab_alloc_factor, this option gives you full control over the behavior of small allocator.

Add the reference to slab_alloc_granularity in the slab_alloc_factor description:
"See also slab_alloc_granularity".

Closes #5518
Requested by @mechanik20051988 in tarantool/tarantool@53c0e91.