# How to create a parallel task?
**URL:** https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194
**Category:** General Usage
**Tags:** multithreading
**Created:** [November 28, 2021, 10:39am UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194 "2021-11-28T10:39:28Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@tomhaber](https://discourse.julialang.org/u/tomhaber)
#### Post date: [November 28, 2021, 10:39am UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194/1 "2021-11-28T10:39:28Z")
I wanted to create a task that runs in parallel with the rest of the code. Seems pretty straightforward, but I’m running into issues I don’t understand. For some reason the `sleep` in the following code gives an exception, which I then cannot print. Additionally, the code behaves quite differently inside the REPL and non-interactively. Seems like a scheduling issue…
```julia
function memusage()
l = ReentrantLock()
peak_usage = 0
done = Threads.Atomic{Bool}(false)
t = Threads.@spawn begin
try
while !done[]
try
lock(l)
peak_usage = max(peak_usage, get_memusage())
# ccall(:printf, Int, (Ptr{UInt8},Int64), "%lu\n", peak_usage)
finally
unlock(l)
end
# sleep(0.01) # does not work, scheduling issue?
ccall(:usleep, Int, (Int32,), 1000) # works but ugly and stalls thread
end
catch e # exception from the sleep
ccall(:printf, Int, (Ptr{UInt8},), "error\n")
# println(e) # does not work
end
end
function query_memusage(stop::Bool=false)
rv = try
lock(l)
rv = peak_usage
peak_usage = get_memusage()
if stop
done[] = true
end
rv
finally
unlock(l)
end
rv
end
end
```
Any help would be useful. Thanks
---
### Author:  [@jpsamaroo](https://discourse.julialang.org/u/jpsamaroo)
#### Post date: [November 28, 2021, 2:22pm UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194/2 "2021-11-28T14:22:15Z")
Can you also provide the definition of `get_memusage()`? Otherwise it’s not clear where the problem would be.
---
### Author:  [@tomhaber](https://discourse.julialang.org/u/tomhaber)
#### Post date: [November 28, 2021, 3:32pm UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194/3 "2021-11-28T15:32:41Z")
Of course, this one doesn’t seem to contribute to the problem. It’s mostly `sleep` throwing some unknown exception. BTW: `printf` also fails to print the exception type. I guess the interpolation fails.
```julia
function get_memusage(pid=getpid())
open("/proc/$pid/statm") do io
line = readline(io)
parts = split(line)
vmrss = parse(Int64, parts[2])
4096 * vmrss
end
end
```
---
### Author:  [@jpsamaroo](https://discourse.julialang.org/u/jpsamaroo)
#### Post date: [November 28, 2021, 8:46pm UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194/4 "2021-11-28T20:46:50Z")
If I define `get_memusage` at the top, this works perfectly fine for me with `sleep` and `println` with any number of threads. How are you running this, and on what Julia version? I’m on the 1.6 release branch.
---
### Author:  [@tomhaber](https://discourse.julialang.org/u/tomhaber)
#### Post date: [November 28, 2021, 9:42pm UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194/5 "2021-11-28T21:42:20Z")
hmm, can you uncomment the ccall to printf to see if the thread didn’t quietly die as it does for me?
I’m running 1.6.3 and 1.6.1
---
### Author:  [@jpsamaroo](https://discourse.julialang.org/u/jpsamaroo)
#### Post date: [November 28, 2021, 9:45pm UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194/6 "2021-11-28T21:45:54Z")
Yeah, they work fine for me, including the call to `usleep`. What do you see that makes you think `sleep` is throwing an error?
---
### Author:  [@tkf](https://discourse.julialang.org/u/tkf)
#### Post date: [November 28, 2021, 10:41pm UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194/7 "2021-11-28T22:41:33Z")
If you keep the reference to the task `t` somewhere (e.g., save it to a global variable `global TASK = t`), you can later display it on the REPL (or call `display(TASK)` when not using REPL). If it had an error, it should be possible to see the stacktrace. Sharing it can help people here debug it. Demo:
```julia
julia> TASK = Threads.@spawn error("hello");
julia> # Then later see the stacktrace:
julia> TASK
Task (failed) @0x00007f66678e9e40
hello
Stacktrace:
[1] error(s::String)
@ Base ./error.jl:33
[2] (::var"#53#54")()
@ Main ./threadingconstructs.jl:169
```
Also, let me note that calling `usleep` brings down an entire worker thread in the Julia runtime. It can’t execute other Julia `Task`s anymore.
> [@tomhaber](#):
>
> Additionally, the code behaves quite differently inside the REPL and non-interactively.
REPL creates an additional task so something like this can happen sometimes.
---
### Author:  [@tomhaber](https://discourse.julialang.org/u/tomhaber)
#### Post date: [November 29, 2021, 8:40am UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194/8 "2021-11-29T08:40:37Z")
I should clarify: with `usleep` it “works” (but rather badly) and with `sleep` I sometimes get errors.
Looking further into this, it looks like the main task doing actual work (reading/computing) is blocking the task (in jl\_task\_get\_next) from progressing. It gets stuck in sleep for hundreds of seconds, as long as it takes for the main task to do some IO.
gdb backtrace of the worker thread:
```julia
futex_wait_cancelable (private=, expected=0, futex_word=0x7efe534970c0) at ../sysdeps/nptl/futex-internal.h:183
__pthread_cond_wait_common (abstime=0x0, clockid=0, mutex=0x7efe53497070, cond=0x7efe53497098) at pthread_cond_wait.c:508
__pthread_cond_wait (cond=0x7efe53497098, mutex=0x7efe53497070) at pthread_cond_wait.c:638
0x00007efe674330d8 in uv_cond_wait (cond=0x7efe53497098, mutex=0x7efe53497070) at src/unix/thread.c:847
0x00007efe67348fac in jl_task_get_next (trypoptask=0x7efe5a985d90 , q=0x7efe549a2970) at /buildworker/worker/package_linux64/build/src/partr.c:508
0x00007efe590333d2 in julia_poptask_29151 () at task.jl:760
0x00007efe594b19c2 in wait () at task.jl:768
julia_wait_33659 () at condition.jl:106
0x00007efe58eeb14c in japi1__trywait_30682 () at asyncevent.jl:111
0x00007efe58eeb45a in wait () at asyncevent.jl:129
julia_sleep_29475 () at asyncevent.jl:214
```
---
### Author:  [@lawless-m](https://discourse.julialang.org/u/lawless-m)
#### Post date: [November 29, 2021, 1:44pm UTC](https://discourse.julialang.org/t/how-to-create-a-parallel-task/72194/9 "2021-11-29T13:44:08Z")
No idea if it will solve the sleep problem but you have the lock inside the try
[https://docs.julialang.org/en/v1/base/parallel/#Base.ReentrantLock](https://docs.julialang.org/en/v1/base/parallel/#Base.ReentrantLock)
_but beware of inverting the try/lock order or missing the try block entirely (e.g. attempting to return with the lock still held):_
```julia
lock(l)
try
finally
unlock(l)
end
```