Scheduling Algorithms

Scheduling Algorithms

Types of Scheduling Algorithms

The following are the three types of scheduling algorithms.

1- Round-Robin Scheduling

Distributes each request sequentially around the pool of real servers. Using this algorithm, all the real servers are treated as equals without regard to capacity or load. This scheduling model resembles round-robin DNS but is more granular due to the fact that it is network-connection based and not host-based. LVS round-robin scheduling also does not suffer the imbalances caused by cached DNS queries. The framework includes multiple mechanisms: fast relay, backward-pressure congestion control, receiver-initiated transmission scheduling, queue space limitation, and Round Robin scheduling. The fast relay assigns high priority of channel access to the downstream nodes when they receive packets, which can reduce a lot of intra-flow contentions. The backward-pressure congestion control gives transmission opportunity to the congested node while keeping its upstream nodes from transmissions. This could not only reduce a lot of contentions in the congested area, but also quickly eliminate the congestion. It is also a quick method to notify the source to slow the sending rate down by exploiting the RTS/CTS of the IEEE 802.11 MAC protocol. The receiver-initiated transmission scheduling scheme uses a three-way handshake to resume the blocked flow at the upstream nodes when the congestion is cleared. It is a timely and economical approach with even less control overhead than the normal four-way handshake transmission in the IEEE 802.11 protocol. The queue space limitation for each flow prevents the irresponsible application as well as the congested flows from occupying the whole queue space and leaves the resource for other responsible applications instead of the congested flows. The Round Robin scheduling is adopted in the queue management to further address the unfairness problem due to greedy sources. Thus, altogether all above mechanisms provide a framework of distributed flow control and medium access control designed to reduce the MAC layer contentions and eliminate the congestion.

Our contribution is to devise these mechanisms for the shared channel environment in the multihop ad hoc networks, and incorporate them into the IEEE 802.11 DCF protocol. Extensive simulation studies are carried out to validate their performance. It turns out that our scheme could maintain stable performance with high throughput independent of traffic status, and improve the aggregated throughput by up to more than 12 times especially for the multihop flows under heavy traffic load. At the same time, it also improves the fairness among flows in terms of end-to-end throughput, and has much shorter delay and much lower control overhead compared to IEEE 802.11 DCF protocol. Moreover, it is scalable for large networks where there are more multihop flows with longer paths.

Weighted Round-Robin Scheduling

Distributes each request sequentially around the pool of real servers but gives more jobs to servers with greater capacity. Capacity is indicated by a user-assigned weight factor, which is then adjusted upward or downward by dynamic load information. Weighted round-robin scheduling is a preferred choice if there are significant differences in the capacity of real servers ...