Garbage collection doesn't trigger on MtlArrays

[Jromano1997]

Hello,

I'm fairly new to Metal and GPU coding in general, so I hope this issue isn't only due to my lack of knowledge.

On my MacBook Air M1:
Metal.versioninfo()

macOS 15.2.0, Darwin 24.2.0

Toolchain:

• Julia: 1.11.2
• LLVM: 16.0.6

Julia packages:

• Metal.jl: 1.5.1
• GPUArrays: 11.2.1
• GPUCompiler: 1.1.0
• KernelAbstractions: 0.9.31
• ObjectiveC: 3.2.0
• LLVM: 9.1.3
• LLVMDowngrader_jll: 0.6.0+0

1 device:

• Apple M1 (1.501 GiB allocated)

If I run:

using Metal
N=2^13
X=rand(Float32,N,N)
mtl_X=MtlArray(X);

u=sum(exp.(mtl_X));

I see an increase in the used memory from the activity monitor,
memory which is not freed until I close Julia.

(Image: Activity monitor resulting from running the line `u=sum(exp.(mtl_X));` multiple times)

If then I run:

n=30
for _ in 1:n Metal.@sync u=sum(exp.(mtl_X)) end

the program fills the RAM and my laptop freezes.

Note that if I run instead:

mtl_A=similar(mtl_X)

n=300
for _ in 1:n Metal.@sync begin
      mtl_A.=exp.(mtl_X)
      u=sum(mtl_A) 
      end
end

the code runs just fine without any freezing. I thus suspect that u=sum(exp.(mtl_X)) is allocating an MtlMatrix for exp.(mtl_X), which the garbage collection is unable to free. Is this standard behaviour?
Shouldn't the garbage collector be able to free the memory that he allocates?