Garbage collection of SharedArrays

[usefulhyun]

Hi, there

I think that automatic garbage collection does not work for SharedArray Datatype.
Only explicit calling gc function deallocate SharedArrays that are useless.

What I am using version 0.6.2
The below code works well.

function main()
sz = 10000

for i=1:100
    sm = SharedMatrix{Float64}(sz, sz)
    # SharedMatrix is not allocated before they use.
    # So explicit access is required to make them being allocated.
    for x=1:sz
        for y=1:sz
            sm[x,y] = i
        end
    end
    println(i, " ", sm[1,1])
    gc()
end
end # main
main()

However, the below code makes a crash.

function main()
sz = 10000

for i=1:100
    sm = SharedMatrix{Float64}(sz, sz)
    for x=1:sz 
        for y=1:sz
            sm[x,y] = i
        end
    end
    println(i, " ", sm[1,1])
    #gc() without explicit gc() call.
end
end # main
main()

The reason of this crash is the limitation of allocating shared memory.

[yeonsookimdev]

Same error with #26222
The error occurs when the total usage of shared memory exceeds the system limit.
(e.g /dev/shm in Linux)

I think Julia GC needs to consider this limitation of shared memory also.

[ViralBShah]

Dup of JuliaLang/Distributed.jl#35