# Do views interfere with thread synchronization?
**URL:** https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379
**Category:** General Usage
**Tags:** multithreading, views
**Created:** [January 28, 2022, 4:15pm UTC](https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379 "2022-01-28T16:15:52Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [January 28, 2022, 4:15pm UTC](https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379/1 "2022-01-28T16:15:52Z")
I have this code which attempts compute a matrix-vector product using submatrices.
```julia
tasks = [];
for th in 1:length(tbs)
tb = tbs[th]
push!(tasks, Threads.@spawn begin
mul!(tb.result, tb.kcolumns, tb.uv)
end);
end
```
This works fine when materialize the submatrices, i.e. when `tb.kcolumns` is `K[:, colrange]`, but it does not work when I use views, i.e. when `tb.kcolumns` is `view(K, :, colrange)`.
Do views change the thread behavior?
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [January 28, 2022, 6:34pm UTC](https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379/2 "2022-01-28T18:34:26Z")
> [@PetrKryslUCSD](#):
>
> Do views change the thread behavior?
If they change threading behavior I’d suspect a data race. More information would help to understand that better, of course.
---
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [January 28, 2022, 9:29pm UTC](https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379/3 "2022-01-28T21:29:50Z")
Alas, my attempts to construct a MWE have failed to reproduce the behavior (the threads get stuck forever).
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [January 28, 2022, 9:35pm UTC](https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379/4 "2022-01-28T21:35:39Z")
Which sounds like a problem in itself. Should we have a look?
---
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [January 28, 2022, 9:36pm UTC](https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379/5 "2022-01-28T21:36:53Z")
OK. Here is the code of the M(non)WE:
```julia
module example
using LinearAlgebra
using SparseArrays
struct ThreadBuffer{KVT}
kcolumns::KVT
uv::SubArray{Float64, 1, Vector{Float64}, Tuple{UnitRange{Int64}}, true}
result::Vector{Float64}
end
function parmul!(R, tbs)
tasks = [];
for th in 1:length(tbs)
tb = tbs[th]
push!(tasks, Threads.@spawn begin
mul!(tb.result, tb.kcolumns, tb.uv)
end);
end
Threads.wait(tasks[1]);
R .= tbs[1].result
for th in 2:length(tbs)
Threads.wait(tasks[th]);
R .+= tbs[th].result
end
R
end
function parloop!(N)
K = sprand(N, N, 0.1)
U1 = rand(N)
R = fill(0.0, N)
nth = Base.Threads.nthreads()
@info "$nth threads used"
chunk = Int(floor(N / nth))
threadbuffs = ThreadBuffer[];
for th in 1:nth
colrange = th < nth ? (chunk*(th-1)+1:chunk*(th)+1-1) : (chunk*(th-1)+1:length(U1))
push!(threadbuffs, ThreadBuffer(view(K, :, colrange), view(U1, colrange), deepcopy(U1)));
# push!(threadbuffs, ThreadBuffer(K[:, colrange], view(U1, colrange), deepcopy(U1)));
end
R .= parmul!(R, threadbuffs)
@show norm(R - K*U1)
true
end
end # module
nothing
using .example; example.parloop!(10000)
```
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [January 28, 2022, 9:40pm UTC](https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379/6 "2022-01-28T21:40:44Z")
Interesting. Returns immediately for me with
```julia
[ Info: 5 threads used
norm(R - K * U1) = 2.0796050396124542e-11
true
```
on
```julia
julia> versioninfo()
Julia Version 1.8.0-DEV.1309
Commit 89f23325aa (2022-01-13 19:48 UTC)
Platform Info:
OS: Windows (x86_64-w64-mingw32)
CPU: Intel(R) Core(TM) i7-10710U CPU @ 1.10GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-13.0.0 (ORCJIT, skylake)
Environment:
JULIA_EDITOR = code
JULIA_NUM_THREADS = 5
```
---
### Author:  [@PetrKryslUCSD](https://discourse.julialang.org/u/PetrKryslUCSD)
#### Post date: [January 28, 2022, 9:42pm UTC](https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379/7 "2022-01-28T21:42:09Z")
Correct: That is why it is a non-working example.
---
### Author:  [@goerch](https://discourse.julialang.org/u/goerch)
#### Post date: [January 28, 2022, 10:17pm UTC](https://discourse.julialang.org/t/do-views-interfere-with-thread-synchronization/75379/8 "2022-01-28T22:17:59Z")
Code looks fine too me. Only thing I’m wondering: why `deepcopy(U1)` instead of `similar(U1)`?