A runtime system for distributing work between multiple threads in multi-socket shared memory machines that may support fine-grained scheduling of parallel loops. The runtime system may implement a request combining technique in which a representative thread requests work on behalf of other threads. The request combining technique may be asynchronous; a thread may execute work while waiting to obtain additional work via the request combining technique. Loops can be nested within one another, and the runtime system may provide control over the way in which hardware contexts are allocated to the loops at the different levels. An “inside out” approach may be used for nested loops in which a loop indicates how many levels are nested inside it, rather than a conventional “outside in” approach to nesting.