# Why there is a mutex in sim2\_actor
**URL:** https://forums.foundationdb.org/t/why-there-is-a-mutex-in-sim2-actor/2893
**Category:** FoundationDB Core
**Created:** [September 18, 2021, 6:07am UTC](https://forums.foundationdb.org/t/why-there-is-a-mutex-in-sim2-actor/2893 "2021-09-18T06:07:53Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@ming535](https://forums.foundationdb.org/u/ming535)
#### Post date: [September 18, 2021, 6:07am UTC](https://forums.foundationdb.org/t/why-there-is-a-mutex-in-sim2-actor/2893/1 "2021-09-18T06:07:53Z")
Hi, I was looking into code of fdb’s simulation related stuff, and I saw that Sim2 actually uses mutex. My understanding is that the whole system is running inside a thread when in simulation model; so why there is a need to use mutex? For example, in sim2\_actor.cpp
```auto
mutex.enter();
tasks.push(Task(time + seconds, taskID, taskCount++, machine, f));
mutex.leave();
```
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### Author:  [@jzhou](https://forums.foundationdb.org/u/jzhou)
#### Post date: [September 25, 2021, 4:41pm UTC](https://forums.foundationdb.org/t/why-there-is-a-mutex-in-sim2-actor/2893/2 "2021-09-25T16:41:20Z")
That’s a good question. I tried to run a simulation using `lldb` and added breakpoints in the `runLoop()`. From what I can tell, there is indeed only one thread running.
@Evan, do you know the reason of using mutex here?
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### Author:  [@alexmiller](https://forums.foundationdb.org/u/alexmiller)
#### Post date: [September 27, 2021, 9:00pm UTC](https://forums.foundationdb.org/t/why-there-is-a-mutex-in-sim2-actor/2893/3 "2021-09-27T21:00:51Z")
There’s things that need to be able to queue up work into flow. When sim2 is used from an fdb client, then I’d think most operations would work this way? However, I still think fdbserver will occasionally activate work from other threads (e.g. when libeio completes a future?)
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### Author:  [@ajbeamon](https://forums.foundationdb.org/u/ajbeamon)
#### Post date: [September 30, 2021, 7:22pm UTC](https://forums.foundationdb.org/t/why-there-is-a-mutex-in-sim2-actor/2893/4 "2021-09-30T19:22:35Z")
Apparently I wrote this a long time ago while adding some sort of support for running threads in simulation. It looks like there are a couple unit tests in the multi-version client that might make use of this, and there’s a workload that uses it as well that I don’t think we actually run in simulation.