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Ruby Programming/Reference/Objects/IO/Fiber
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A Fiber is a unit of concurrency (basically a manually controlled thread). It is a new construct in 1.9
1.8 basically used green threads similar, to fibers, but would pre-empt them, which 1.9 does not do.
See its description.
Several useful things have been built using fibers, like neverblock, the revactor gem, et al.