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I would guess contention on sync primitives in the dispatcher. The queues look like they need to be locked before they can be modified:

    // Scheduling helpers.  Sched must be locked.
    static void gput(G*);   // put/get on ghead/gtail
This is the comment on the lock:

    in the uncontended case,
     * as fast as spin locks (just a few user-level instructions),
     * but on the contention path they sleep in the kernel.
     * a zeroed Lock is unlocked (no need to initialize each lock).
A better approach for multicore would probably be work-stealing queues.

From what I know of the authors, they would have picked the simpler solution until it actually turns out to be a problem in the real world.

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