Abstract:
A reliable and scalable system and method of broadcasting information to other computer nodes in a communication network requires only O(2) time steps. According to one aspect, after broadcasting data in O(1) steps to all nodes in the network, the system and method provides a distributed reliability protocol to ensure data delivery which only requires an additional O(1) steps. Therefore, unlike in prior art approaches where the root or co-root is responsible for the reliable data delivery, each node in the network takes on responsibility to deliver the message to a partner/neighborhood node. The broadcasting method and system of the can be used as building block for most collective/distributive operations, and provides a significant performance advantage in parallel computer systems that have multicast/broadcast capabilities.

Description:
TECHNICAL FIELD 
     The present invention relates generally to data broadcasting, and in particular to reliable and fast broadcasting of information in a system comprising a plurality of nodes. 
     BACKGROUND 
     Parallel computing systems such as computer clusters, Symmetric Multiprocessor (SMP) computers and other architectures are becoming increasingly affordable and cost-effective for a wide range of applications. Many of these systems are built from commodity and relatively inexpensive computer parts connected with high speed networks (such as LANs, proprietary interconnects, etc.) or System Area Networks (SANs). These types of computing systems compete with the historically expensive custom-built supercomputers. 
     Most programming models provide for writing parallel applications, and many parallel applications take advantage of collective/distributive communication operations. Moreover, many scientific applications rely exclusively on collective/distributive operations for their communication. Thus, providing a high performance and scalable collective/distributive communication method for broadcasting information is critical for the operation and success of commodity based parallel computing systems. 
     A key issue in providing a high performance and scalable collective/distributive communication method is to reliably and quickly broadcast a message from a root process to multiple other processes in a group. Current methods need at least O(log n) steps to deliver a message reliably to n nodes. This means the effort and time to reliably broadcast data increases logarithmically with the number of nodes. 
       FIGS. 1A to 1C  are block diagrams for illustrating in more detail the conventional approaches to broadcasting data in a parallel computing scheme. 
     As shown in  FIG. 1A , the simplest method to broadcast data is to send it one by one to each recipient in the group. With n total number of nodes, this method will take n sends and n receives. The number of steps is thus O(2n). 
     A straightforward improvement to the method in  FIG. 1A  is to use the broadcast/multicast capability of certain interconnects (such as Ethernet, Infiniband, etc.). However, such broadcast and multicast protocols support only unreliable datagram services which do not guarantee reliable data delivery. To take advantage of hardware supported broadcast/multicast capability therefore, an additional reliability protocol needs to be in place. All known approaches and prior art implement this reliability protocol as either (1) a simple method where every recipient sends an acknowledgement directly back to the root or (2) a reverse tree-based confirmation method where the leaves start sending the confirmation to co-roots and finally informing the main root that the message was delivered. 
     For example, in connection with the first improvement as shown in  FIG. 1B , the root node sends data using multicast, while the other nodes are waiting for it. If the message is received, an acknowledgement (ACK) is sent back to the root node. The root blocks and waits for all ACKs to be received. If not all ACKs arrive within a certain period of time, it times out and re-transmits the message. This method is O(n+1), where sending is O(1) and returning the confirmations is O(n). 
     The second improvement takes advantage of tree-based algorithms, which are based on point-to-point communication operations. Similar to a snow-ball principle, the root sends to multiple nodes, who in return, send each to multiple nodes, until the leaves are reached. In a tree-based method, the number of steps to reach leaf nodes increases with the total number nodes n typically in a logarithmic manner O(log n). 
     More particularly, as shown in  FIG. 1C , contrary to the scheme in  FIG. 1B , after the multicast by the root sender, to avoid the serial ACK hitting the sender all at the same time (a condition known as “ACK implosion”), a hierarchical structure is used for ACK collection and thus distributes the load to a number of nodes. In a tree based structure to collect ACKs, all nodes form a tree structure with the root node being the root of the tree. Intermediate nodes are responsible for collecting ACKs for their children. A variation is called the co-root scheme where, in addition to the root node, a subset of other nodes are selected as co-roots which receive the data in a reliable manner. The remaining nodes are called leaf nodes. Each of the root and the co-roots is responsible for a group of leaf nodes and performs as described above. These types of methods are using O(log n) steps for n number of nodes. 
     Although the tree-based scheme improves throughput, delays still increase as the number of nodes increase (although by a slower logarithmic factor of log n). 
     And there are sources for additional delays even in a tree-based implementation. For example, broadcasting is typically implemented as a blocking operation to ensure that the operation does not return at the root node until the communication buffer can be reused. Accordingly, if a message gets lost in the communication network, the reliability protocol will re-send the message and the communication buffer has to keep the original message until the last node has confirmed receipt. For a receiving node, the operation returns only after the broadcast data has been delivered to the respective receive buffer. This blocking requirement can therefore introduce significant delays, especially as the number of nodes increase and the chances of communication interruptions increase correspondingly. 
     Another disadvantage of tree-based methods is that if intermediate nodes that are expected to forward broadcast data are busy, they delay the forwarding which has an adverse impact on the execution time of an application. 
     Accordingly, it would be desirable if there were a reliable method that needed only a fixed number of steps for distributive/collective communication functions in parallel computing systems and would allow the use of simple commodity based cluster computers to achieve similar performance compared to a custom-built supercomputer. 
     SUMMARY 
     A reliable and scalable system and method of broadcasting information to other computer nodes in a communication network in O(2) time steps is disclosed. According to one aspect, after broadcasting data in O(1) steps to all nodes in the network, the system and method provide a distributed reliability protocol to ensure its delivery which only requires an additional O(1) steps. Therefore, unlike in approaches where the root or co-root is responsible for the reliable data delivery, each node in the network takes on responsibility to deliver the message to a partner/neighbor node. The broadcasting method and system can be used as building block for most collective/distributive operations, and provides a significant performance advantage to all known parallel computer systems that have hardware supported multicast/broadcast capabilities. 
     According to one embodiment, a method of sharing data among a plurality of nodes comprises sending the data to all of the nodes and, at each of the nodes that received the data, sending an acknowledgment corresponding to the sent data to a respective designated sender node among other ones of the nodes. 
     According to another embodiment, an apparatus for sharing data among a plurality of nodes comprises means for sending the data to all of the nodes and, at each of the nodes that received the data, means for sending an acknowledgment corresponding to the sent data to a respective designated sender node among other ones of the nodes. 
     According to another embodiment, a system for sharing data among a plurality of nodes comprises, at each of the nodes, a designated sending node ID, a designated receiving node ID, a network interface for receiving a message containing the shared data, and a reliability protocol that causes a first acknowledgment to be sent using the designated sending node ID in response to the message being received. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  are block diagrams illustrating conventional approaches to reliable data broadcasting; 
         FIG. 2  is a block diagram illustrating an example system implementing a broadcasting method and system; 
         FIG. 3  is a flowchart illustrating in more detail an example broadcasting method; and 
         FIGS. 4A to 4E  are diagrams further illustrating an example application of the broadcasting system and method. 
     
    
    
     DETAILED DESCRIPTION 
     The system and method will now be described in detail with reference to the drawings, which are provided as illustrative examples so as to enable those skilled in the art to practice the system and method. Notably, the figures and examples below are not meant to limit the scope of the claimed invention to any specific embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the system and method can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the claimed invention will be described, and detailed descriptions of other portions of such known components will be omitted. In the present specification, an embodiment showing a singular component should not necessarily be considered limiting; rather, other embodiments including a plurality of the same component may be possible, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the claimed invention encompasses present and future known equivalents to the known components referred to herein by way of illustration. 
     Generally, the disclosed system and method provides a fundamentally different approach to broadcasting data to all processes in a parallel computing system. Whereas a known systems and methods require the root sending process to wait until acknowledgements are collected from all processes (either directly or through a hierarchical structure), the disclosed system and method distributes the acknowledgement collection function among all the nodes in the system. 
     According to one embodiment, each node in the system has a designated send partner/neighbor node and a designated receive partner/neighbor node. Once a node receives a message from its designated receive partner/neighbor, it behaves as though it itself was the original multicast sender. It waits for an ACK from its designated send partner/neighbor, and re-sends the message if a time-out occurs. 
     Although the data broadcasting techniques disclosed herein may find particularly useful application in a parallel computing system, the principles of the claimed invention are not limited to this application, and other applications will become apparent to those skilled in the art after being taught by the present disclosure. 
       FIG. 2  is a block diagram of a system having data broadcasting capabilities. As shown in  FIG. 2 , a system  200  includes a plurality of nodes or processors  202 - 1  to  202 - n  that communicate with each other via a network  204 . 
     The data broadcasting capabilities can have many applications, and the diagram in  FIG. 2  is intended to be illustrative rather than limiting. The system  200  can implement a parallel computing system such as a computer cluster, a Symmetric Multiprocessor (SMP) system or other architecture, and can also implement shared and distributed memory systems such as network attached storage (NAS) caches, virtual memory disks, coherent memory pools, and further can implement schemes such as SAN synchronization and file-locking and software clusters. Processors  202  can be any type of computer or processor using any type of operating system such as Windows, Linux, Solaris, Unix etc. Network  204  can be any type of high speed or other network such as Ethernet, Infiniband, proprietary interconnects, switch fabrics, buses or System Area Networks (SANs). 
       FIG. 2  illustrates an example implementation of processor  202  in more detail. As shown in  FIG. 2 , processor  202  includes a reliability protocol functionality  220  and a network interface  222  that includes a send buffer and a receive buffer. Processor  202  also includes a local memory or other storage  224  that includes a send partner ID, a receive partner ID and a multicast ID for use by the reliability protocol functionality  220 . It should be noted that processor  202  may include other hardware and software components whose descriptions are not necessary for an understanding of the concepts disclosed herein, and so their explanation will be omitted here for clarity. 
     Reliability protocol functionality  220  is preferably implemented as software in accordance with the particular operating system and computing environment of processor  202  and provides the distributed message broadcast acknowledgment collection methodology, as will be described in more detail below, in conjunction with the ID&#39;s maintained in storage  224 . The protocol functionality is further preferably implemented in accordance with the type of network protocol being used such as IP, UDP or Infiniband, and the system and the method are amenable to application with any packet-based protocol, and preferably having broadcast or multicast capabilities, and may be designed to, for example, to operate at Level 5 of the OSI stack. 
     Network interface  222  is implemented in accordance with the requirements of the particular network  204  in which the system operates, as well as the operating system and CPU capabilities of the processor  202 . For example, if the network is an Ethernet LAN, the network interface  222  is an Ethernet-compatible interface. Those skilled in the art will be able to understand and implement the particular network interface appropriate for a given implementation. The ID&#39;s maintained in storage  224  are also implemented in accordance with the particular network  204  in which the system operates. For example in some embodiments, the ID&#39;s can be IP addresses, and the multicast ID can be a subnet. In other embodiments, the ID&#39;s can be maintained as lists of proprietary, machine level or other types of addresses. 
     It should be noted that it is not necessary for all processors  202  in system  200  to be configured the same. However, the reliability protocol functionality  220  is preferably present at each node in the system in accordance with the computing capabilities of that node. 
       FIG. 3  is a flowchart illustrating an example method of broadcasting data in a parallel computing system. Those skilled in the art will understand how to implement this methodology in various processor systems and software environments after being taught by this example. 
     As shown in  FIG. 3 , processing begins in step S 302  by identifying all the nodes in the system and informing each node who their designated send partner/neighbor and designated receive partner/neighbor nodes are. This can be done many ways and can depend on the type of network and protocol that is used. However, preferably the identification step ensures that all nodes in the system are identified as both a designated receiver of another node, and a designated sender of a different node. For example, each node in the system can be given an identifying sequential number, and its send partner/neighbor can be the next highest number node, and its receive partner/neighbor can be the next lowest number node, with a modulus of the total number of nodes. Alternatively, a distance (or time delay) between each node can be measured, and a determination can be made to assign partner/neighbor nodes based on a minimum distance (or time delay) among nodes. In other embodiments, each node can also obtain ID&#39;s for all other nodes in the system, whether an explicit list of a group identifier such as an IP multicast group, subnet mask, broadcast lists, etc. After learning the IDs, these are stored in each respective node&#39;s storage  224 . 
     In step S 304 , during processing, the system allows any node to broadcast data at any time. When a node is ready to send data, processing advances to step S 306  where the data is multicast to all the nodes in the system, preferably using the broadcast or multicasting techniques of the particular network. 
     If no message is to be sent, processing proceeds to step S 308 , where it is determined whether a new broadcast message has been received that is associated with the system. If no message is to be sent, or to be received, by the node, processing returns to step S 304 . 
     If a new broadcast message for the system has been received, unless the node itself sent the message in step S 306 , the node sends an acknowledgement (ACK) to its designated receive partner/neighbor in step S 310 . Further, in step S 312 , both the node that sent the message, and every node that receives the message, stores the message locally in a send buffer associated with its designated send partner/neighbor node. 
     In step S 314 , after buffering the message, the node waits for an acknowledgement (ACK) from its designated send partner/neighbor (whether or not it actually sent the message in its send buffer). If the node receives an ACK from the send partner/neighbor, it deletes the message from the send buffer because the ACK provides confirmation that the message was delivered. 
     If no ACK is received from the designated send neighbor/partner node after a timeout period of time (determined in step S 314 ), the node re-sends the message to its designated send neighbor/partner node in step S 316  and wait again for an ACK in step S 314 . This behavior ensures that the message will be delivered to the partner/neighbor that did not receive the message with the first multicast. 
       FIGS. 4A to 4E  further illustrate an example implementation of the broadcasting method. 
     As shown in  FIG. 4A , similarly to the conventional method, the sender node s sends data over the network using the broadcast/multicast capability that the network hardware provides. After the message is delivered, each node r stores the message locally in a send buffer. Differently from the conventional method, however, each node that receives the message also pretends that it sent the message itself to a designated send partner/neighbor node, and that it received the message from a designated receive partner/neighbor. 
     As shown in  FIG. 4B , in the case of being the pretend sender, each node waits for an acknowledgement (ACK) from its designated send partner/neighbor. If the node receives an acknowledgement from the partner/neighbor, it deletes the message from the send buffer since it received confirmation that the message was delivered. In the case of being the receiver, each node sends an acknowledgment (ACK) back to its designated receive partner/neighbor node. 
     If no message is lost, all nodes send the acknowledgement (ACK) at approximately the same time, thus taking up one single time step O(1). In this case, all nodes receive an ACK, delete the message from the send buffer and continue the program. 
     An important aspect of the system and method is that it provides the ability to automatically recover from a situation where data is lost or dropped and one or more nodes do not receive the multicast data. In such a case, the node which pretended to be the sender will run into a timeout since it did not receive an ACK. 
     More particularly, as shown in  FIG. 4C , nodes  402  and  404  did not receive an ACK from their designated send neighbor/partner nodes  406  and  408 , respectively. The pretending senders  402  and  404  will thus re-send the message to neighbor/partner nodes  406  and  408  and wait again for an ACK. This behavior ensures that the message will be delivered to the partner/neighbor that did not receive the message with the first multicast. 
     The disclosed system and method may overcome an extreme situation where more than one node in the sequence of partners/neighbors does not receive the original multicast message. In this situation, the reliability method will still work and close the gap. 
     More particularly, referring back to  FIG. 4C , node  410  never received the original message, and its designated send partner/neighbor node  408  did not receive the original message either. However, as explained above, and as illustrated in  FIG. 4D , node  408  eventually receives and acknowledges the re-sent message from node  404 . Upon receiving the message from node  404 , node  408  begins its timer for receiving an acknowledgement of its own, and times out after not receiving an acknowledgement from its own send partner/neighbor node  410 . It then “resends” the message (even though it never sent one originally) to node  410 . Accordingly, as shown in  FIG. 4E , node  410  eventually acknowledges receipt of the last message, and then completes the loop by performing identical processing as described above in connection with its own designated send partner/neighbor node  412 . 
     Although the claimed invention has been particularly described with reference to embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention. Certain aspects of one embodiment may also be interchanged with the other embodiments. It is intended that the appended claims encompass such changes, modifications and interchanges.