# JuMP and CPLEX - Setting number of threads
**URL:** https://discourse.julialang.org/t/jump-and-cplex-setting-number-of-threads/83747
**Category:** Optimization (Mathematical)
**Tags:** jump, cplex
**Created:** [July 4, 2022, 3:58pm UTC](https://discourse.julialang.org/t/jump-and-cplex-setting-number-of-threads/83747 "2022-07-04T15:58:27Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@vmk97](https://discourse.julialang.org/u/vmk97)
#### Post date: [July 4, 2022, 3:58pm UTC](https://discourse.julialang.org/t/jump-and-cplex-setting-number-of-threads/83747/1 "2022-07-04T15:58:27Z")
I am trying to solve a mixed integer linear program by branch-and-cut using JuMP and CPLEX.
I am having some trouble setting the number of threads for CPLEX.
I am trying to set the parameter CPXPARAM\_Threads using set\_optimizer\_attribute in the following way:
```julia
model = Model(CPLEX.Optimizer)
set_optimizer_attribute(model, "CPXPARAM_Threads", 4)
```
However, in the log I get the following:
```julia
Version identifier: 20.1.0.0 | 2020-11-10 | 9bedb6d68
CPXPARAM_Threads 1
...
Parallel mode: none, using 1 thread.
...
```
I don’t understand why the value of 4 that I provided turns into 1.
I would like to run CPLEX in parallel mode with more than one thread.
Thanks in advance.
**Obs** : I have CPLEX Optimization Studio v20.1 installed which I got through an academic license.
I am running Julia on version 1.7.3, JuMP on version 0.21.10 and CPLEX on version 0.7.8.
---
### Author:  [@gdalle](https://discourse.julialang.org/u/gdalle)
#### Post date: [July 4, 2022, 5:58pm UTC](https://discourse.julialang.org/t/jump-and-cplex-setting-number-of-threads/83747/2 "2022-07-04T17:58:59Z")
Not sure what is going on here, but maybe you could try setting [this CPLEX parameter](https://www.ibm.com/docs/en/icos/12.8.0.0?topic=parameters-parallel-mode-switch) too?
Did you launch Julia with a single thread or several?
---
### Author:  [@vmk97](https://discourse.julialang.org/u/vmk97)
#### Post date: [July 4, 2022, 9:34pm UTC](https://discourse.julialang.org/t/jump-and-cplex-setting-number-of-threads/83747/3 "2022-07-04T21:34:05Z")
Thanks, Guillaume.
Yes, I did try to set the CPXPARAM\_Parallel parameters as well, but it didn’t work either.
I think the answer is [here](https://github.com/jump-dev/JuMP.jl/issues/904).
I am passing a custom callback function to CPLEX for handling both lazy constraints and cuts.
According to the link, apparently CPLEX automatically turns off multi-threading when given callbacks.
---
### Author:  [@odow](https://discourse.julialang.org/u/odow)
#### Post date: [July 4, 2022, 11:34pm UTC](https://discourse.julialang.org/t/jump-and-cplex-setting-number-of-threads/83747/4 "2022-07-04T23:34:01Z")
Did this warning trigger?
>
Perhaps CPLEX started automatically turning off multithreading with callbacks.
But yes, you cannot use threading if you have a callback. The underlying problem is that callbacks from Julia into C are not thread safe, so there is no way to implement this.
---
### Author:  [@vmk97](https://discourse.julialang.org/u/vmk97)
#### Post date: [July 5, 2022, 12:56pm UTC](https://discourse.julialang.org/t/jump-and-cplex-setting-number-of-threads/83747/5 "2022-07-05T12:56:37Z")
First of all, thank you, Guillaume and Oscar.
No, the warning is not triggered. I guess MOI gets the number of threads already forced to one and hence no warning.
Indeed when I run the model with no callback I am able to set the number of threads as I please.
> callbacks from Julia into C are not thread safe
Is this specific to Julia? Is it the same for other programming languages?