How to do custom allocators

I’m not sure. There is some intrusive linked list even in Julia core. It seems to be useful to implement a queue.

I’m just wondering what are the modification patterns where a custom allocator in addition to the built-in one offers an advantage. Of course, using lists as stacks or queues is one such case but the solution for that case is basically known.

Maybe one case is when it’s known that the structure isn’t going to contain more than N items, and a pool of O(N) pre-allocated objects offers some advantages over the Julia allocator.

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Hi Vasily,

let me try to motivate this with your simple List definition

mutable struct Cons{T}
    car::T
    cdr::Union{Nothing, Cons{T}}
end
const List{T} = Union{Nothing, Cons{T}}

T = Tuple{Int, StaticArrays.SVector{100, Float64}} and rather artificial tests similar to

l = nil()
for j in 1:n
    l = cons((j, p) , l)
end
s = 0
while l != nothing
    s += l.car[1]
    l = l.cdr
end

you can either use the Julia allocator resulting in

 List without allocator                   442.167 ms (1000000 allocations: 854.49 MiB)

or a fixed size allocator (if you know the maximal number of elements)

  with fixed allocator                     83.345 ms (0 allocations: 0 bytes)

or a resizable allocator

  with resizable allocator                 75.376 ms (0 allocations: 0 bytes)

or an allocator keeping an internal free list (if you want to remove and readd elements to the list)

  with fixed free list allocator           98.112 ms (0 allocations: 0 bytes)
  with resizable free list allocator       88.990 ms (0 allocations: 0 bytes)

or an corresponding SOA allocator

  with fixed SOA allocator                 56.665 ms (0 allocations: 0 bytes)
  with resizable SOA allocator             56.830 ms (0 allocations: 0 bytes)
  with fixed free list SOA allocator       68.195 ms (0 allocations: 0 bytes)
  with resizable free list SOA allocator   73.258 ms (0 allocations: 0 bytes)

For me these are only abstractions of common preallocation patterns. I see a tradeoff of programming convenience vs. a bit of performance gain, because you have to rework your data structures.