[manwe150]

True, I don’t remember the details for writes, and the complexity of managing high/low water marks makes it even trickier for optimal code. And large kernel send buffers here mostly avoid the performance problem here anyways. But on a short write, I am not sure I see the value in testing for EAGAIN over looping through epoll and getting a fresh set of events for everything instead of just this one fd

Right, for reads, epoll will happily tell you if there is more data still there. If the read buffer size is reasonable, short reads should not be common. And if the buffer is huge, a trip around the event loop is probably better at that point to avoid starvation of the other events