Fair Arbitration Scheme for Arbitrating Between Multiple Nodes in a Computer System Seeking Control of a Common Bus

The present invention relates to the field of interconnect buses for computers and more particularly, to a fair arbitration scheme for determining which of a plurality of nodes attached to the bus may take control of the bus.

In an arrangement in which a plurality of nodes are connected to a common bus, some kind of scheme is needed to select which node will be able to use the bus at any given time. One scheme which has been used in prior systems is to assign a fixed priority to each of the nodes connected to the bus so that if two of the nodes desire to use the bus at the same time, the node having the higher priority will win every time. The advantage of using a fixed priority is that the nodes which need to use the bus the most have the most opportunity to do so. The scheme has an inherent disadvantage in that the higher priority nodes can effectively monopolize the use of the bus so that lower priority nodes may never have the opportunity to use the bus.

Another scheme which has been used in the past is a round robin arbitration. Each node is sequentially enabled in round robin fashion so that there is an equal sharing of the bus between the nodes. Although this provides equal access to all of the nodes, the scheme can be inefficient since nodes which need to use the bus more often than other nodes have to wait for the round robin to complete before being able to use the bus again. However, the nodes which use the bus less frequently are less likely to actually use the bus during their turn, so there is often a waste of the time reserved for nodes which use the bus less frequently.

It is therefore an objective of the present invention to provide a fair arbitration scheme in which all of the nodes connected to a bus have an equal opportunity to use the bus, but without the inefficiencies created by reserving time for each node to use the bus.

This and other objectives are achieved by the present invention by the provision of a method for selecting a particular node from a plurality of nodes connected to a common bus to allow the node to use the bus. The method comprises the step of enabling the nodes, each node having a pre-determined priority. The bus is monitored for a bus idle condition, and it is then determined which of the nodes are enabled message nodes, which is defined as an enabled node having a message to send on the bus. There is then arbitration between the enabled message nodes after the bus is in the bus idle condition for a first period of time, such that the enabled message node having the highest pre-determined priority among the enabled message nodes is selected to use the bus. The selected enabled message node is disabled for purpose of arbitration while it proceeds with the selection of a target and transfer of information with that target. The above procedure (except for the enabling of all the nodes) is repeated until all the enabled message nodes have been disabled. All of the nodes are again enabled when the bus is in the bus idle condition for a second period of time, which is longer than the first period of time.

FIG. 1 shows a block diagram of the present invention.

FIG. 2 is a phase diagram showing the various phases of the bus of FIG. 1 according to the present invention.

An embodiment of the present invention is illustrated in FIG. 1, in which a storage interconnect bus 10 is connected to four nodes 12, 14, 16 and 18. These nodes 12-18 can be different types of devices, such as central processing units, peripheral memory storage units, etc. Although four nodes are shown in this exemplary embodiment, different numbers of nodes can be used without departing from the scope of the invention.

Each of the nodes 12, 14, 16 and 18 includes an arbitrator 20, 22, 24 and 26 and a monitor and enabler unit 28, 30, 32 and 34. Each of the nodes 12, 14, 16 and 18 is assigned a priority, which fixes the priority of the nodes for arbitration purposes. This can be accomplished, for example, by providing a priority identification bit in each node and comparing the priority identification bits of each enabled node (in this case nodes 12 and 16) to determine which enabled node has the highest priority. In the illustrated example, node 12 has the highest priority, node 14 the next lower, node 16 the next lower and node 18 the lowest priority. Thus, during an arbitration in which node 12 and node 16 are both attempting to use the bus 10, node 12 will win because it is the higher priority device. Devices 14 and 18 which have not asserted their busy signal and priority will continue monitoring the bus which is no longer idle due to the busy signals asserted by nodes 12 and 16.

In the arbitration block 46, the nodes 12-18 determine which of the nodes 12 and 16 wins the control of the bus 10. This is done by comparing in the arbitrators 20-26 the priority asserted by the nodes 12 and 16. Since node 12 has a higher priority, it is the winner of the arbitration and takes control of the bus 10 which now enters the selection phase block 50. All the losers of the arbitration (in this case node 16) enter a wait phase 48 and monitor the bus 10 for the bus idle condition to re-appear.

The winner of the arbitration, node 12, clears its own enable flag upon winning the arbitration so that the node 12 will be disabled from arbitrating until it becomes enabled. Although it is now disabled for purposes of arbitration, node 12 continues to proceed with a selection and transfer process over the bus 10. When the bus enters the selection phase 50, node 12 selects another node, i.e. a target node, for a transfer of data. The bus 10 then enters the transfer phase 52 and node 12 receives a transfer from the target node. Regardless of the success of the transfer, the now disabled node 12 will enter the wait phase 48 if it has a message to send, i.e. is a disabled message node.

Now assume that the now disabled message node 12 and enabled nodes 14 and 16 all desire to take control of the bus 10. Upon a bus idle condition being detected for 800 nsec (t.sub.1), the arbitration phase 46 is entered by nodes 14 and 16. Node 12, which is disabled, must wait until the bus is idle for a longer time span t.sub.2 (shown by path 44, in the example 1600 nsec), before the node is reselected as a priority node.

The present invention overcomes the disadvantages of the prior art fixed priority and round robin arbitration schemes discussed above by the use of a fair arbitration scheme. The invention implements the fair arbitration scheme by enabling and disabling the nodes 12, 14, 16 and 18 for purposes of arbitration. Basically, only enabled nodes can participate in arbitration for the bus, and once a node wins an arbitration and takes control of the bus 10, the node becomes disabled and cannot participate in an arbitration until all of the other enabled nodes connected to the bus have had an opportunity to use the bus.

The operation of the fair arbitration scheme of the present invention will be described using the state diagram of FIG. 2. Initially, it is assumed that all of the enable flags of the nodes 12-18 are set, so that all of the nodes 12-18 are enabled and able to participate in arbitration. Nodes 12-18 continuously monitor the state of the bus 10 with the monitor and enabler units 28, 30, 32 and 34. Since we have started our example from the beginning of operation, the bus will be in a bus idle state as shown by box 40. This state is detected by each of the monitor and enabler units 28-34. The enabled nodes 12-18 assert a bus busy signal and their priority when they have a need to use the bus 10, i.e. when an enabled node wants to have a transfer of information with another node, so that these are now enabled message nodes. For now, it is assumed that only enabled nodes 12 and 16 have a need to use the bus. Since both of the nodes 12 and 16 are enabled message nodes, the phase of the bus 10 follows along an enabled path 42, which in the described embodiment takes 800 nsec (t.sub.1), to the arbitration block 46. The other again becoming enabled to enter arbitration. The two nodes 12 and 16 (the enabled message nodes) arbitrate as described before after the first time period t.sub.1, with node 12 winning the arbitration. After the transfer involving node 12 has been completed, it too is disabled and must wait until the bus is idle for 1600 nsec (t.sub.2) before it is enabled again. However, the bus will not be idle for this longer time span since the enabled message node 16 will enter into arbitration after the bus is idle for 800 nsec (t.sub.1). Since there are no other higher priority enabled nodes which desire to control the bus, node 16 wins with the transfer operating as described above.

At this point in the example, assume that, the three nodes 12-16 are all disabled, while node 18 is enabled. If node 18 needed to take control of the bus, it could do so after node 16 completes its transfer and the bus 10 is idle again for the first time period t.sub.1. However, assume that the node 18 does not need to take control of the bus at this time. All of the devices 12-18 continue to monitor the bus and since no enabled node has taken control of the bus after the bus idle condition has been detected for a period of 1600 nsec (t.sub.2), all the disabled nodes 12-16 once again become enabled and therefore are able to participate in an arbitration. The operation of the bus then proceeds again as described above.

As one can see, if all of the devices are enabled and want to take control of the bus, the fair arbitration scheme acts as a pure round robin scheme since the highest priority nodes will be disabled in turn after they control the bus, until each of the nodes has had an opportunity to take control of the bus. The fair arbitration scheme is more efficient, however, since nodes which use the bus more frequently are not forced to wait through time periods reserved for nodes which do not wish to take control of the bus.