I am trying to find in the literature how to solve the following problem:

Suppose we have a list of $n$ jobs. Some of the jobs should be scheduled after other jobs (i.e., job a should be done after job b), other jobs need to be mutually exclusive (i.e., job c cannot be done concurrently with job e). Each job $i$ has a constant execution time, $T_i$. Our goal to find a scheduling for the jobs, that minimazes the time to execute all jobs.

I know that when all jobs scheduled after other jobs, this problem can be using topology ordering and dynamic programming. Also, I found this problem is highly related to makespane problem with infinite identical stations, with a contrain on the order of jobs. I did not found yet a problem given a ording/mutual exelcusive relationship between jobs.

How can we solve this problem?

  • $\begingroup$ Do you have multiple identical machines or different speeds? $\endgroup$
    – ttnick
    Commented Jun 5, 2020 at 13:52
  • $\begingroup$ Infinite identical machines (I have edited the question) $\endgroup$ Commented Jun 5, 2020 at 13:54

1 Answer 1


OK, I found that the problem is NP-hard. Given that all jobs have only mutually exclusive relationship, and $T_i=1$ for every job $i$, the problem is eqivalent to the finding the orientation of a graph $G$, that has the minimal longest path. According to Gallai–Hasse–Roy–Vitaver theorem, this is equivalent to finding a chromatic number of $G$, which is NP-hard.


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