It sounds backwards, but it's the situation I'm currently trying to solve on my dedicated server.
Everything I've read about fixed timestep assumed it was on a client, where you're already running in a loop as fast as you can to render the game, and you are just inserting the fixed timestep updates within that.
Problem is, on the server, that's the only thing being done in the loop, aside from checking for network activity. So my server ends up using 100% CPU, looping needlessly while waiting for the next fixed timestep.
The only way I know to fix this is by adding a sleep to the loop, but I know how inaccurate that delay can be, which would throw off the accuracy of the timesteps.
Is there a better way to keep it from burning CPU? Or is sleep what most dedicated servers are doing anyway?