Why there is a mutex in sim2_actor

Development FoundationDB Core
1

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

mutex.enter();
tasks.push(Task(time + seconds, taskID, taskCount++, machine, f));
mutex.leave();
2

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?

3

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?)

4

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.

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