Shrapnel maintains a concept called “selfishness”. This mechanism is used to prevent a coroutine from yielding too often (or from running for too long). This is currently only relevant to socket objects and socket I/O.
Each coroutine is given a set number of “free passes” each time it tries to do I/O on a socket. If there is data immediately available on the socket, then the coroutine may immediately receive that data. If Shrapnel did not implement any “selfishness” limits, and that coroutine is in a loop repeatedly calling read and there is always data available to the socket, then that coroutine would run continuously without letting its fellow coroutines a chance to run.
By default, every coroutine has a selfishness limit of 4. That means it is allowed to do 4 I/O operations before it is forced to yield. Of course, if it attempts to do an I/O operation that would block (such as if there is no data available on a socket), then it will yield immediately.
You can set the default selfishness limit for all new coroutines with the coro.set_selfishness() function. You can also change a coroutine’s limit with the coro.coro.set_max_selfish_acts() method.