Performance impact of coarse timer granularities on QoS guarantees in Unix-Based systems

Owing to the Internet´s rapid expansion and fast advancing PC technology, there are many PC-based network systems now. For increasingly many applications running over the Internet, guaranteeing QoS on these PC-based systems has become an issue of some concern. In this paper, we investigate QoS failures that occur on PC-based systems and focus on one aspect of the problem that arises from coarse timer granularities. While it is usually assumed that packet schedulers in routers have sufficiently fine-grain timers, network systems frequently have timers of coarse granularity. Therefore, users cannot obtain the desired QoS even if they reserve the required bandwidth for transmission. Based on the investigation of QoS failures due to coarse timer granularities, we experiment with two methods to cure the problems. We implement them into real PC Unix-based systems and show that they can satisfy QoS requirements of TCP connections by helping them transmit the traffic at the reserved bandwidth.