# How to GC a SharedArray

**URL:** <https://discourse.julialang.org/t/how-to-gc-a-sharedarray/48699>\
**Category:** General Usage\
**Tags:** distributed, sharedarrays\
**Created:** [October 20, 2020, 7:24pm UTC](https://discourse.julialang.org/t/how-to-gc-a-sharedarray/48699 "2020-10-20T19:24:03Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![ianshmean](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ianshmean/32/216042_2.png) [@ianshmean](https://discourse.julialang.org/u/ianshmean)\
**Post date:** [October 20, 2020, 7:24pm UTC](https://discourse.julialang.org/t/how-to-gc-a-sharedarray/48699/1 "2020-10-20T19:24:03Z")

</div>

What’s the right way to garbage collect a `SharedArray`? This doesn’t work. Julia consumes 8 GB memory indefinitely (Version 1.6.0-DEV.1237)

```julia
using SharedArrays
begin
    a = SharedArray{Float64}(10000000)
    @distributed for i = 1:10
        a[i] = i
    end
end
GC.gc()

```

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<div class="post-metadata">

**Author:** ![ianshmean](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ianshmean/32/216042_2.png) [@ianshmean](https://discourse.julialang.org/u/ianshmean)\
**Post date:** [October 20, 2020, 7:31pm UTC](https://discourse.julialang.org/t/how-to-gc-a-sharedarray/48699/2 "2020-10-20T19:31:05Z")

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Moving to a `let` made this work. This MWE doesn’t seem to be representative of the issue I’m seeing

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<div class="post-metadata">

**Author:** ![tim.holy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tim.holy/32/52_2.png) [@tim.holy](https://discourse.julialang.org/u/tim.holy)\
**Post date:** [October 20, 2020, 8:57pm UTC](https://discourse.julialang.org/t/how-to-gc-a-sharedarray/48699/3 "2020-10-20T20:57:45Z")

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Maybe try `a = nothing; GC.gc()`. If you’ve bound the array to a named variable on the workers, I suspect you’ll have to assign that name something else there too.

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<div class="post-metadata">

**Author:** ![ianshmean](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ianshmean/32/216042_2.png) [@ianshmean](https://discourse.julialang.org/u/ianshmean)\
**Post date:** [October 21, 2020, 4:14pm UTC](https://discourse.julialang.org/t/how-to-gc-a-sharedarray/48699/4 "2020-10-21T16:14:12Z")

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Thanks for the tip. We’d `nothing`-ed it locally but not on the workers.

However, the more we look into this, the more it looks like a memory leak that can be replicated without any `Distributed` involvement, and we’re yet to find a good MWE (emphasis on the M)

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<div class="post-metadata">

**Author:** ![Octogonapus](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/octogonapus/32/9489_2.png) [@Octogonapus](https://discourse.julialang.org/u/Octogonapus)\
**Post date:** [October 21, 2020, 5:26pm UTC](https://discourse.julialang.org/t/how-to-gc-a-sharedarray/48699/5 "2020-10-21T17:26:18Z")

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I am debugging this problem with Ian so I am going to jump in here. I ran the program under `strace` to see what is happening (kernel `5.8.15-201.fc32.x86_64`). Here are my findings.

When the SharedArray is created, it gets a shared memory object, mmaps it, and unlinks it.

```julia
openat(AT_FDCWD, "/dev/shm/jl326782ohMyKWxhO9Rm4KiyE8Qr", O_RDWR|O_CREAT|O_NOFOLLOW|O_CLOEXEC, 0600) = 29
mmap(NULL, 1365245952, PROT_READ|PROT_WRITE, MAP_SHARED, 29, 0) = 0x7f0c6ac6f000
munmap(0x7f0c6ac6f000, 1365245952) = 0
close(29) = 0

openat(AT_FDCWD, "/dev/shm/jl326782ohMyKWxhO9Rm4KiyE8Qr", O_RDWR|O_NOFOLLOW|O_CLOEXEC) = 32
fcntl(32, F_GETFL) = 0x28002 (flags O_RDWR|O_LARGEFILE|O_NOFOLLOW)
mmap(NULL, 1365245952, PROT_READ|PROT_WRITE, MAP_SHARED, 32, 0) = 0x7f0c1926f000
unlink("/dev/shm/jl326782ohMyKWxhO9Rm4KiyE8Qr") = 0

```

The region starting at `0x7f0c1926f000` is not munmaped until the program exits. It is not munmaped when the GC runs. The last line in the program (`write(22, "DONEDONEDONEDONEDONEDONEDONEDONE", 32DONEDONEDONEDONEDONEDONEDONEDONE) = 32`) runs before this munmap call

```julia
munmap(0x7f0c1926f000, 1365245952) = 0

```

I am confident that when the GC runs the SharedArray is finalized because the ref put in `SharedArrays.sa_refs` is deleted, which only happens in the finalizer `finalize_refs`. Finalizing the SharedArray should also finalize its member `s`, though. `Mmap.mmap` registers a finalizer for `s` that calls `munmap`, so I don’t know why this is not called.

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<div class="post-metadata">

**Author:** ![dridz](https://avatars.discourse-cdn.com/v4/letter/d/e47774/32.png) [@dridz](https://discourse.julialang.org/u/dridz)\
**Post date:** [October 27, 2020, 3:56pm UTC](https://discourse.julialang.org/t/how-to-gc-a-sharedarray/48699/6 "2020-10-27T15:56:56Z")

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Hi,  
I think I am facing a similar issue. Here is another MWE:

```julia
using Distributed, SharedArrays

function parallel_compute()
    shared_array = SharedArray{Float64,1}(100_000)
    @sync @distributed for i in 1:100_000
        shared_array[i] = randn()
    end
    return sum(shared_array)
end

addprocs(4)

for t = 1:10
    x = parallel_compute()
    println(Sys.free_memory()/2^20)
end

rmprocs(workers())

```

when I run this code, the amount of free RAM available (printed in MiB in the main loop) keeps decreasing:

```julia
493.28515625
492.31640625
491.46875
478.390625
477.6640625
476.81640625
463.73828125
463.01171875
462.52734375
461.55859375

```

If I push it further (with more iterations and a more complex `parallel_compute` function), my code breaks because of memory shortage. I would also expect that the GC would help getting some free space after `parallel_compute` is called but it seems that it doesn’t. I tried `sharred_array = nothing; GC.gc()` at the end of `parallel_compute` but it didn’t help. Is this the expected behavior ? Am I missing something ?

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<div class="post-metadata">

**Author:** ![dridz](https://avatars.discourse-cdn.com/v4/letter/d/e47774/32.png) [@dridz](https://discourse.julialang.org/u/dridz)\
**Post date:** [October 28, 2020, 9:23am UTC](https://discourse.julialang.org/t/how-to-gc-a-sharedarray/48699/7 "2020-10-28T09:23:08Z")

</div>

I solved my problem with

```julia
@everywhere shared_array = nothing
@everywhere GC.gc()

```

I was missing that it should be called on all workers, sorry for the duplicate question.