A filled torus (a doughnut) is a 3-manifold homeomorphic to \(S^1 \times D^2\), where \(D^2\) is the 2-dimensional disk. There exists a deformation retract from the doughnut to a circle, so the fundamental group of the doughnut is \(\pi_1(S^1 \times D^2) \cong \mathbb{Z}\).
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I figured the Secure Snake Home community would be excited to have a new server to play on, so I wanted to support at least a thousand concurrent players. But early performance profiling was bad. I was using something like a full core for every 40 users.