The STM system we created in the first recipe of this chapter, Managing program complexity with STM, has one subtle problem: threads attempting to reference and update total-hu and total-fams are contending for these two values unnecessarily. Since everything comes down to accessing these two resources, a lot of tasks are probably being retried.

But they don't need to be. Both are simply updating those values with commutative functions (#(+ sum-? %)). The order that these updates are applied in doesn't matter. And since we block until all processing is done, we don't have to worry about the two references getting out of sync. They'll get back together eventually, before we access their values, and that's good enough for this situation.

To handle this use case (updating references with a commutative function), instead of alter, we should use commute. The alter function updates the references on the spot; commute queues the update to happen later, when...