# Segfault while loading images in multiple threads

**URL:** <https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878>\
**Category:** New to Julia\
**Tags:** images, multithreading\
**Created:** [October 23, 2020, 11:58am UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878 "2020-10-23T11:58:04Z")\
**Posts on this page:** 15\
**Page:** 1

<div class="post-metadata">

**Author:** ![sigmike](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sigmike/32/18893_2.png) [@sigmike](https://discourse.julialang.org/u/sigmike)\
**Post date:** [October 23, 2020, 11:58am UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/1 "2020-10-23T11:58:04Z")

</div>

Hello,

I tried to process images in parallel but the `load` operation results in a Segmentation Fault. Is loading images not thread safe?

```julia
using Images, FileIO
Threads.@threads for path in readdir("img"; join=true)
  load(path)
end

```

The `img` directory contains only PNG images. Running the above script with a single thread works, but with multiple threads (`julia --threads 4`) it gives me a segmentation fault:

```julia
signal (11): Segmentation fault
in expression starting at none:1
in expression starting at none:1
unknown function (ip: 0x7fbd79b0d298)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0e80a)
unknown function (ip: 0x7fbd79b0d294)
unknown function (ip: 0x7fbd79b0d725)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0c8b7)
unknown function (ip: 0x7fbd79b0e80a)
unknown function (ip: 0x7fbd79b0d766)
unknown function (ip: 0x7fbd79b0d725)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0c8b7)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0d766)
unknown function (ip: 0x7fbd79b0e80a)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0d725)
unknown function (ip: 0x7fbd79b0d32d)
unknown function (ip: 0x7fbd79b0d9ec)
unknown function (ip: 0x7fbd79b0e80a)
unknown function (ip: 0x7fbd79b0dafc)
unknown function (ip: 0x7fbd79b0d725)
unknown function (ip: 0x7fbd79b0ea0a)
unknown function (ip: 0x7fbd79b0d9ec)
unknown function (ip: 0x7fbd79b12347)
unknown function (ip: 0x7fbd79b0dafc)
jl_restore_incremental at /usr/bin/../lib/libjulia.so.1 (unknown line)
unknown function (ip: 0x7fbd79b0ea0a)
unknown function (ip: 0x7fbd79b12347)
jl_restore_incremental at /usr/bin/../lib/libjulia.so.1 (unknown line)
_include_from_serialized at ./loading.jl:681
_require_search_from_serialized at ./loading.jl:782
_require_search_from_serialized at ./loading.jl:782
Errors encountered while loading "/path/img/000811.png".
All errors:Errors encountered while loading "/path/img/000001.png".

===========================================Errors encountered while loading "/path/img/001081.png".

All errors:
===========================================
All errors:
===========================================
_require at ./loading.jl:1007
_require at ./loading.jl:1007
require at ./loading.jl:928
require at ./loading.jl:928
require at ./loading.jl:923
require at ./loading.jl:923
unknown function (ip: 0x7fbd79b1cac0)
unknown function (ip: 0x7fbd79b1cac0)
unknown function (ip: 0x7fbd79b1e16e)
unknown function (ip: 0x7fbd79b1e16e)
jl_toplevel_eval_in at /usr/bin/../lib/libjulia.so.1 (unknown line)
jl_toplevel_eval_in at /usr/bin/../lib/libjulia.so.1 (unknown line)
eval at ./boot.jl:331 [inlined]
topimport at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:13
eval at ./boot.jl:331 [inlined]
topimport at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:13
checked_import at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:30
checked_import at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:30
#load#28 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:195
#load#28 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:195
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:184 [inlined]
#load#14 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
macro expansion at /path/open_images.jl:4 [inlined]
#3#threadsfor_fun at ./threadingconstructs.jl:81
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:184 [inlined]
#load#14 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
macro expansion at /path/open_images.jl:4 [inlined]
#3#threadsfor_fun at ./threadingconstructs.jl:81
#3#threadsfor_fun at ./threadingconstructs.jl:48
#3#threadsfor_fun at ./threadingconstructs.jl:48
unknown function (ip: 0x7fbd49e314ac)
unknown function (ip: 0x7fbd49e314ac)
unknown function (ip: 0x7fbd79b053b9)
unknown function (ip: 0x7fbd79b053b9)
unknown function (ip: (nil))
nknown function (ip: (nil))
Allocations: 11229080 (Pool: 11225888; Big: 3192); GC: 8
Allocations: 11229080 (Pool: 11225888; Big: 3192); GC: 8
Segmentation fault (core dumped)

```

I’m using julia version 1.5.2.

---

<div class="post-metadata">

**Author:** ![pbayer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pbayer/32/11675_2.png) [@pbayer](https://discourse.julialang.org/u/pbayer)\
**Post date:** [October 23, 2020, 12:58pm UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/2 "2020-10-23T12:58:46Z")

</div>

do you use the `lock=true` argument in [`Base.open`](https://docs.julialang.org/en/v1/base/io-network/#Base.open)?

---

<div class="post-metadata">

**Author:** ![sigmike](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sigmike/32/18893_2.png) [@sigmike](https://discourse.julialang.org/u/sigmike)\
**Post date:** [October 23, 2020, 1:08pm UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/3 "2020-10-23T13:08:56Z")

</div>

I just tried this and got a similar error:

```julia
Threads.@threads for path in readdir("img"; join=true)
    open(path, "r"; lock = true) do io
        load(io)
    end
end

```

```julia
Errors encountered while loading nothing.
All errors:
===========================================
Errors encountered while loading nothing.
Errors encountered while loading nothing.All errors:

===========================================Errors encountered while loading nothing.

Errors encountered while loading nothing.All errors:

All errors:
===========================================
All errors:
===========================================
===========================================
Errors encountered while loading nothing.
All errors:
===========================================
concurrency violation detectedconcurrency violation detected
===========================================

===========================================concurrency violation detected

===========================================concurrency violation detected

===========================================
Fatal error:

Fatal error:
concurrency violation detected
===========================================
concurrency violation detected
===========================================

Fatal error:
concurrency violation detectedconcurrency violation detected
===========================================

===========================================concurrency violation detected

===========================================concurrency violation detected

Fatal error:
===========================================

Fatal error:
concurrency violation detected
===========================================
concurrency violation detected
===========================================

Fatal error:

signal (11): Segmentation fault
in expression starting at none:1
unknown function (ip: 0x7f01122bfe6c)
unknown function (ip: 0x7f01122c6817)
jl_restore_incremental at /usr/bin/../lib/libjulia.so.1 (unknown line)
_include_from_serialized at ./loading.jl:681
_require_search_from_serialized at ./loading.jl:782
_tryrequire_from_serialized at ./loading.jl:712
_require_search_from_serialized at ./loading.jl:771
_tryrequire_from_serialized at ./loading.jl:712
_require_search_from_serialized at ./loading.jl:771
_require at ./loading.jl:1007
require at ./loading.jl:928
require at ./loading.jl:923
unknown function (ip: 0x7f01122d0ac0)
unknown function (ip: 0x7f01122d216e)
jl_toplevel_eval_in at /usr/bin/../lib/libjulia.so.1 (unknown line)
eval at ./boot.jl:331 [inlined]
topimport at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:13
checked_import at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:30
#load#28 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:195
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:184 [inlined]
#load#14 at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
load at /home/someuser/.julia/packages/FileIO/wN5rD/src/loadsave.jl:133 [inlined]
#1 at /path/open_images.jl:5 [inlined]
#open#287 at ./io.jl:325
open##kw at ./io.jl:323 [inlined]
macro expansion at /path/open_images.jl:4 [inlined]
#3#threadsfor_fun at ./threadingconstructs.jl:81
#3#threadsfor_fun at ./threadingconstructs.jl:48
unknown function (ip: 0x7f00da62ef3c)
unknown function (ip: 0x7f01122b93b9)
unknown function (ip: (nil))
Allocations: 13377958 (Pool: 13373909; Big: 4049); GC: 10
Segmentation fault (core dumped)

```

---

<div class="post-metadata">

**Author:** ![pbayer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pbayer/32/11675_2.png) [@pbayer](https://discourse.julialang.org/u/pbayer)\
**Post date:** [October 23, 2020, 2:27pm UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/4 "2020-10-23T14:27:58Z")

</div>

Perhaps you could give my new actor library [`YAActL`](https://github.com/pbayer/YAActL.jl) a try (see the announcement):

The following installs a very simple file server:

```julia
julia> using YAActL, FileIO

julia> fs = Actor(load)
Channel{YAActL.Message}(sz_max:32,sz_curr:0)

```

then you can do:

```julia
julia> @threads for file in readdir("img"; join=true)
           img = call!(fs, file)
           # then do the processing in parallel
       end

```

which is the same as your code above, but works (without lock). It opens the files sequentially and serves the content over the `fs` channel. Then the threads can compute in parallel. This is threadsafe by design.

---

<div class="post-metadata">

**Author:** ![sigmike](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sigmike/32/18893_2.png) [@sigmike](https://discourse.julialang.org/u/sigmike)\
**Post date:** [October 23, 2020, 2:51pm UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/5 "2020-10-23T14:51:39Z")

</div>

It works, but if I understand correctly the loading and decoding is now done in a single thread. Can’t Julia load images in parallel?

---

<div class="post-metadata">

**Author:** ![pbayer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pbayer/32/11675_2.png) [@pbayer](https://discourse.julialang.org/u/pbayer)\
**Post date:** [October 23, 2020, 3:00pm UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/6 "2020-10-23T15:00:10Z")

</div>

Thank you for trying!

1. The loading is done in one thread (the one the actor resides on). It serves the images over the fs channel to the threads that called it.
2. The threads then do the processing of the images (in your case 4 at a time) in parallel.

This makes only sense if the processing takes significantly longer than the loading. Otherwise you won’t gain much by multithreading. See [Amdahl’s law](https://en.wikipedia.org/wiki/Amdahl%27s_law). The reading of the files can’t be parallized. It’s the same with a lock. It creates a queue which makes tasks wait until they can read the file.

You can parallelize all operations after the file access since only the reading needs to happen in a single thread. But this must be implemented in a library or by the user. Amdahl’s law suggests to keep the part that cannot parallelized as small as possible.

---

<div class="post-metadata">

**Author:** ![sigmike](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sigmike/32/18893_2.png) [@sigmike](https://discourse.julialang.org/u/sigmike)\
**Post date:** [October 23, 2020, 3:51pm UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/7 "2020-10-23T15:51:00Z")

</div>

> [@pbayer](#):
>
> The reading of the files can’t be parallized.

That’s probably true in most situations, including mine, but there’s still the decoding step that could be parallelized. I guess I could separate the loading and decoding, but that doesn’t seem to be very easy since `load` only accepts a filename or a stream.

Anyway I was able to run my process. And surprisingly saving the resulting images in multiple threads does work.

Thank you for your help.

---

<div class="post-metadata">

**Author:** ![GunnarFarneback](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gunnarfarneback/32/1827_2.png) [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)\
**Post date:** [October 23, 2020, 5:52pm UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/8 "2020-10-23T17:52:06Z")

</div>

> [@pbayer](#):
>
> The reading of the files can’t be parallized.

Please remind me what makes that impossible.

---

<div class="post-metadata">

**Author:** ![pbayer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pbayer/32/11675_2.png) [@pbayer](https://discourse.julialang.org/u/pbayer)\
**Post date:** [October 23, 2020, 8:26pm UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/9 "2020-10-23T20:26:37Z")

</div>

I perhaps should have written that it is not practical:

Hardware file access works sequentially. If you allow parallel cores concurrently access the file system, then somewhere in between you have to manage the switching, to save and restore permanently status for each concurrent file access. This is possible but involves considerable complexity and may be error prone.

See also: [A related stack overflow question](https://stackoverflow.com/questions/10676299/fastest-file-reading-in-a-multi-threaded-application)  
Interesting too: [Multi-threaded File IO](https://jrtechs.net/programming/multi-threaded-file-io)

---

<div class="post-metadata">

**Author:** ![GunnarFarneback](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gunnarfarneback/32/1827_2.png) [@GunnarFarneback](https://discourse.julialang.org/u/GunnarFarneback)\
**Post date:** [October 24, 2020, 10:15am UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/10 "2020-10-24T10:15:22Z")

</div>

So no reason not to try to split the load over a couple of threads then, in particular as there’s decoding to do as well. It seems to me like there’s some thread unsafety in FileIO because reading directly with PNGFiles works fine for me.

```julia
using PNGFiles
filenames = filter(x->endswith(x, ".png"), readdir("."; join=true));
x = Vector{Any}(undef, length(filenames));
@sync for (i, path) in enumerate(filenames)
    Threads.@spawn begin
        x[i] = PNGFiles.load(path)
    end
end

```

This is probably not a good way to get performance but at least there should be no fundamental problem with multithreading it.

Update: When I try this with 100 png files of size 4096x4096 and 8 threads I get a factor 4 speedup over sequential read. Seems fairly decent.

---

<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 24, 2020, 11:56am UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/11 "2020-10-24T11:56:58Z")

</div>

FileIO and ImageIO do lazy loading of the underlying file IO packages, so what may be happening is multiple threads trying to load the package at the same time, if it hasn’t happened already in the session.

The first example may work if the underlying IO package is first loaded on the main thread, either by invoking a single image load first, or by explicitly `using ImageIO, PNGFiles` or `ImageMagick` etc

```julia
using Images, FileIO, ImageIO, PNGFiles
Threads.@threads for path in readdir("img"; join=true)
  load(path)
end

```

Note that if the file format changes, it may try to load different packages and hit the same problem.

---

<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 24, 2020, 1:01pm UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/12 "2020-10-24T13:01:34Z")

</div>

These two should be a fix to make the original example work with the `ImageIO` backend. Only the first is needed if you’re using either the `ImageMagick` or `QuartzImageIO` backend.  
[https://github.com/JuliaIO/FileIO.jl/pull/276](https://github.com/JuliaIO/FileIO.jl/pull/276)  
[https://github.com/JuliaIO/ImageIO.jl/pull/11](https://github.com/JuliaIO/ImageIO.jl/pull/11)

---

<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 25, 2020, 5:37am UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/13 "2020-10-25T05:37:55Z")

</div>

Master on both packages should now be fixed.

@sigmike would you mind checking with your setup, and we can release if fixed.

In a fresh session:

```julia
pkg> add FileIO#master ImageIO#master
julia> using Images, FileIO
julia> Threads.@threads for path in readdir("img"; join=true)
    load(path)
end

```

Thanks!

---

<div class="post-metadata">

**Author:** ![sigmike](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/sigmike/32/18893_2.png) [@sigmike](https://discourse.julialang.org/u/sigmike)\
**Post date:** [October 25, 2020, 8:51am UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/14 "2020-10-25T08:51:01Z")

</div>

Yes it works. And it was about 3 times faster to `load` all the images with 4 threads than with 1.

Thank you.

---

<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 25, 2020, 11:57am UTC](https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/15 "2020-10-25T11:57:12Z")

</div>

Great thanks