Duplicated-memory synchronization arrangement

Abstract: A shared-buffer-memory-based ATM switch module (1) is duplicated (2) and operates in active-standby mode for fault-tolerance. Following failure and repair of a module, buffer memory (12) contents of the two modules are resynchronized as follows. At the time of commencing the synchronizing, contents of the memory of the standby, repaired, module are cleared, all writes to the active module's memory also commence to be made to the standby module's memory, and track begins to be kept of overwriting of the contents of the active module's memory that existed at the time of commencing the synchronizing. This is done by a function which sets a flag (64) in a queue-length-counter monitor (60) for every active-module queue-length counter (200) that reaches a count of zero to indicate that its corresponding buffer-memory queue (100) has been emptied. When all monitor flags have been set in response to each queue-length counter having reached a count of zero, it indicates that the original contents of the active module's memory have been overwritten and the memories again have identical contents, and consequently that resynchronization of the two memories has been achieved and fault-tolerance has been re-established.

Description: This invention relates to arrangements and methods for determining that the contents of two memories whose contents are normally synchronised and frequently change have become resynchronised after occurrence of a fault in a duplicated-memory apparatus. A common way of achieving fault-tolerance in computer and telecommunications systems is to duplicate the fault-prone unit and operate the system with one of the duplicate units active and performing system tasks, while the other duplicate unit is either inactive or operating in lock-step with the active unit and standing by to take over the performance of system tasks upon the failure the active unit. For example, in telephony switching systems, it is common to duplicate the switching fabric and/or the system control computer (including the computer memory and its contents) and to operate the duplicate units in active-standby mode. In the case of duplicate, active-standby, memories, proper system operation requires that the contents of the memories be identical. Initially, this is achieved by loading the same contents into both duplicate memories at initialization. During system operation, synchronization of the duplicate memory contents is maintained by performing all write operations on both memories, so that the duplicate contents of both memories change identically. During system operation, following a failure of the active memory, substitution therefor of the standby memory, and repair of the formerly-active memory, it is necessary to bring the repaired, now standby, memory into synchronism with the now active memory in order to again achieve fault-tolerance. This requires the contents of the two memories to again become, and to remain, identical. This means that the repaired memory must be populated with the contents of the active memory. In the prior art, population of the standby memory with contents of the active memory is achieved by serially reading the contents out of the active memory and writing the read-out contents into the standby memory, and in the meantime also writing into the standby memory any changes being made to the contents of the active memory that have already been copied into the standby memory. An illustrative system of this type is disclosed in U.S. Patent No. 3,864,670. This scheme for achieving synchronism between duplicate memories has disadvantages, however. On the one hand, if the system processor is used to copy the contents of the active memory into the standby memory, processing power for this job is taken away from system tasks, and system performance is adversely affected. On the other hand, if a separate controller is provided to perform the copying, or if the memories perform the copying operation autonomously, between system-processor accesses of the active memory, it may take a long time to achieve memory synchronization, particularly in systems that are performing memory-intensive tasks. But fault-tolerance is lost during all of the time before synchronization is achieve, making it imperative to achieve synchronization as soon as possible. According to one aspect of this invention there is provided an arrangement as claimed in claim 1. According to another aspect of this invention there is provided a method as claimed in claim 7. In an embodiment of the invention, at the time of commencing the synchronizing of an active and a standby memory, all writes to the active memory also commence to be made to the standby memory (which is, illustratively, empty), and track begins to be kept of overwriting of the contents of the active memory that existed at the time of commencing the synchronizing. Illustratively, in a memory that serves to buffer data in queues, a queue-length counter is associated with each queue of the active memory, and the system begins to keep track of the counters reaching a count of zero. Once it has been found that all of the original contents of the active memory have been overwritten--illustratively, when each queue-length counter has reached a count of zero at least once--it is an indication that synchronization of the two memories has been achieved and their contents are again identical. At that point, the standby memory can again be substituted for the active memory to achieve fault-tolerance. In a variation of this embodiment, a single queue-length counter is associated with the plurality of queues of the active memory and keeps track of the total contents of the queues taken together. At commencing of the synchronization, this counter begins to keep track of the queue contents--illustratively of only the contents then present. Once this counter reaches a count of zero, it is an indication that synchronization has been achieved. The invention has numerous advantages over the prior art. It does not take processing power of the system processor away from system taks for memory-copying purposes, and hence it does not adversely impact system performance. Nor does it require the expense of having a separate processor for memory-synchronization purposes. Rather, it accomplishes synchronization of the active and standby memories merely through normal use of the memory for its intended application. Yet it accomplishes memory synchronization--and hence return to fault-tolerance - - rapidly in memory-intensive applications where turnover of the memory contents is frequent. Such applications include: buffer-memory-based switching-fabric architectures, which are gaining prominence in Asynchronous Transfer Mode (ATM) systems for providing broadband ISDN (BISDN) services; duplicated FIFOs, which can be monitored for when the active FIFO empties; and processor stacks, which can be monitored for when the active processor's stack depth becomes zero. These and other advantages and features of the invention will become more apparent from the following description of an illustrative embodiment of the invention taken together with the drawing. Brief Description of the Drawing FIG. 1 is a block diagram of a communications switching arrangement that includes an illustrative embodiment of the invention; FIG. 2 is a flow diagram of an INITialization function of the administrative processor of the arrangement of FIG. 1; FIG. 3 is a flow diagram of a switch module failure-handling function of the administrative processor of the arrangement of FIG. 1; and FIG. 4 is a flow diagram of a failed switch module reinitialization function of the administrative processor of the arrangement of FIG. 1. Detailed Description FIG. 1 shows in block diagram form a communications switching arrangement that includes an illustrative embodiment of the invention. The arrangement shown in FIG. 1 is a part of an Asynchronous Transfer Mode (ATM) switching and transmission system. It constitutes either a stand-alone switch of that system or a portion of a switch made up of a plurality of such portions. The arrangement constitutes a duplicate pair of ATM switch modules 1 and 2, which are arranged to operate in active-standby mode for fault-tolerance purposes. Modules 1 and 2 are connected in parallel to separate output ports 41 and 42, respectively, of a demultiplexer/distributor (DEMUX/DISTR) 21, and to separate input ports 51 and 52, respectively, of a multiplexer (MUX) 22. Both devices 21 and 22 are of conventional design and function. DEMUX/DISTR 21 has an input port 40 connected to one or more input links 17. It connects its input port 40, and thereby connects input links 17, either to output port 41 or 42 to provide a demultiplexing selection function or to both output ports 41 and 42 to provide a distribution (i.e., a broadcast) function. MUX 22 has an output port 50 connected to a plurality of output links 19. It connects its output port 50, and thereby connects output links 19, to either input port 51 or 52 to provide a multiplexing selection function. DEMUX/DISTR 21 and MUX 22 operate under control of an administrative processor 20. Switch administrative processors are well known in the art. Processor 20 controls the input-port-to-output-port interconnections of DEMUX/DISTR 21 and MUX 22 via control links 31 and 32, respectively. Additionally, processor 20 is in communication with ATM switch modules 1 and 2 through control links 33 and 34, respectively. FIG. 1 also shows in block form the internal construction of a shared-buffer-memory-based ATM switch module 1; module 2 is constructed identically. Such switches are well known in the art. An illustrative example thereof is disclosed in WO 91/04624. Module 1 comprises one or more input ports 7 over which module 1 receives incoming ATM cells. Input ports 7 are connected to input and output circuitry (I/O) 10, which couples input ports 7 to a control 14, a buffer memory 12, and to output ports 9. I/O 10 illustratively comprises phase-alignment circuitry, serial-to-parallel and parallel-to-serial shift registers, and multiplexers and demultiplexers. Copies of header portions of incoming ATM cells received from input ports 7 are sent by I/O 10 to control 14, and the cells are sent by I/O 10 to buffer memory 12 for storage. Buffer memory 12 comprises one or more RAMs that illustratively implement one or more queues 100 for each output port. For every individual one of the output ports 9, buffer memory 12 provides a separate queue 100 for each one of the ATM cell priorities. Control 14 controls storage of incoming ATM cells in buffer memory 12 and transmission of stored ATM cells at output ports 9. Control 14 illustratively comprises a queue processor, a pointer RAM, an output-port counter, and queue-length counters 200. Counters 200 include one counter 200 for each queue 100. Based on the header of an incoming ATM cell, received from I/O 10, control 14 determines the priority and the destination output port of that cell, causes the cell to be stored in the corresponding queue 100 in buffer memory 12, and increments that queue's corresponding counter 200. Control 14 also periodically accesses the highest-priority non-empty queue 100 of the one of the output ports 9 that is identified by the present count of the output-port counter, retrieves therefrom an ATM cell that is at the head of the queue, decrements that queue's corresponding counter 200, and causes I/O 10 to transmit the retrieved cell on the corresponding one of the output ports 9. Administrative processor 20 includes a queue-length-counter monitor 60, and a plurality of fault-tolerance-related routines or functions 61-63. Monitor 60 is illustratively nothing more than a register or a memory word that has a flag bit 64 for each queue-length counter 200 of a switch module. Its use is explained further below. Functions 61-63 are illustratively implemented as instructions stored in a read-only memory 59 which processor 20 executes when called upon to do so. Alternatively, functions 61-63 can be implemented as hard-wired circuits. The operational features of functions 61-63 are diagramed in flowchart form in FIGS. 2-4, respectively. Alternatively, an individual counter 200 keeps count of the sum of the contents of a plurality of queues 100. For example, a single counter 200 may be used to keep track of the contents of all queues 100. In this latter case, monitor 60 becomes superfluous. Turning to FIG. 2, upon initialization of the arrangement of FIG. 1, INIT function 61 is invoked at step 210, and executed. In response, it causes DEMUX/DISTR 21 to connect its input port 40 to both output ports 41 and 42, at step 212, thereby to perform a distribution function and to send any ATM cells received on any input links 17 to corresponding input ports 7 of both ATM switch modules 1 and 2. Function 61 also causes MUX 22 to connect its output port 50 to the one of input ports 51 or 52 which is connected to the one of modules 1 and 2 which is designated as the active module, at step 214. In consequence, ATM cells appearing at output ports 9 of only the active module 1 or 2 will be transmitted on the corresponding ones of output links 19. Illustratively, processor 20 obtains the information on which one of modules 1 and 2 is designated as the active module as input from an administrator's terminal (not shown). Function 61 then continues with the performance of conventional system initialization tasks, at step 216. These tasks include either resetting (i.e., zeroing out) the contents of both memories 12 or loading both memories 12 with identical initial contents. When the arrangement of FIG. 1 is active, both modules 1 and 2 perform identical operations. Due to the activities performed by INIT function 61, both modules 1 and 2 receive the identical inputs, and use them to keep the contents of their buffer memories 12 synchronized. When one of modules 1 and 2 fails, it either reports that fact autonomously to processor 20, or processor 20 discovers that fact through periodic maintenance activities (e.g., tests) that it performs on modules 1 and 2. When processor 20 detects a failure of one of the modules 1 or 2, function 62 of FIG. 3 is invoked, at step 300. Function 62 first determines whether the failed module is the active or the standby module, at step 302. If the active module failed, function 62 causes MUX 22 to connect its output port 50 to the one of input ports 51 and 52 which is connected to the standby module, at step 304, thereby disconnecting the failed module from output links 19 and isolating the failed module. Function 62 also causes DEMUX/DISTR 21 to disconnect its input port 40 from to the one of output ports 41 and 42 which is connected to the failed module, at step 306, thereby disconnecting the failed module from input links 17. Function 62 then designates the standby module as the active module; and designates the failed module as being out-of-service (OOS), at step 308. Function 62 then continues with the performance of conventional fault-handling activities, at step 314. Returning to step 302, if the failed module is the standby module, function 62 causes DEMUX/DISTR 21 to disconnect its input port 40 from the one of output ports 41 and 42 which is connected to the failed module, at step 310. Function 62 then designates the failed standby module as being out-of-service, at step 312, and then continues with the performance of conventional fault-handling activities, at step 314. When the faulty one of modules 1 and 2 has been repaired (i.e., the fault has been identified and removed), processor 20 is informed, for example again by input from an administrator's terminal. In response, function 63 of FIG. 4 is invoked, at step 400. In response, function 63 resets (i.e., zeroes out) the contents of buffer memory 12 of the repaired module, at step 401. Function 63 then causes DEMUX/DISTR 21 to again connect its input port 40 to both output ports 41 and 42, at step 402, thereby causing both modules 1 and 2 to commence receiving the same input. However, the contents of buffer memory 12 of the repaired module are at this time not the same as the contents of buffer memory 12 of the active module - - buffer memory 12 of the repaired module is empty. But the identical set of activities henceforth performed by both modules 1 and 2, combined with the identical changes being made to contents of both buffer memories 12 as a consequence of step 402, will eventually synchronize the contents of both buffer memories 12. To determine when synchronization has occurred, function 63 uses queue-length counters 200 of the active one of the modules 1 and 2 and queue-length-counter monitor 60. Function 63 initializes monitor 60, illustratively by zeroing its contents, at step 404. Function 63 then goes to sleep to await either the count of a counter 200 of the active module reaching zero or a buffer-overflow condition that results in deletion of a cell from buffer memory 12 of the active one of the modules 1 and 2, at step 406. Various schemes are known for controlling overflow of buffer memory 12 or individual ones of its queues 100. The typical result of these schemes is the discarding either of the newly-arriving cell that results in the overflow or of a cell that is already stored in a queue 100 to make room for the newly-arriving cell. Because buffer memory 12 of the active module typically starts out at step 401 with more contents than buffer memory 12 of the standby module, prior to buffer memory synchronization being achieved the overflow condition and resultant cell deletion typically occurs only at the active module and not at the standby module. The cell deletion therefore typically destroys the synchronization requirement that identical changes be made to the contents of buffer memories 12 of both modules 1 and 2. Therefore, upon the deletion of a cell from buffer memory 12 of the active module, control 14 of the active module notifies processor 20. This notification causes function 63 to awaken, at step 409, and to return to step 400 to start anew the process of memory content synchronization. Considering momentarily FIG. 1, when a queue 100 of the active module becomes empty, the count of its corresponding counter 200 reaches 0, and control 14 notifies processor 20. Returning to FIG. 4, this causes function 63 to awaken, at step 410. Function 63 determines which one of counters 200 of the active module was the cause of its awakening, at step 412, and then updates monitor 60 with that information, at step 414, by setting that counter's corresponding flag 64 in monitor 60. Function 63 then checks whether any flags 64 in monitor 60 remain cleared, at step 416. If any flags 64 do remain cleared, it means that their corresponding counters 200 of the active module have not reached a count of zero, and consequently that those counter's corresponding queues 100 have not become empty since monitor 60 was initialized at step 404. Function 63 therefore returns to step 406. If however, all flags 64 within monitor 60 are found at step 416 to have been set, it means that all contents of buffer memory 12 of the active module have been cleared out of buffer memory 12 since step 404, and hence the contents of buffer memories 12 are now synchronized. This means that the repaired module is ready to assume a standby role to the active module. Function 63 therefore changes the status designation of the repaired module from out-of-service to standby, at step 418. Function 63 then continues with the performance of conventional administrative activities, at step 420. Of course, it should be understood that various changes and modifications to the illustrative embodiment described above will be apparent to those skilled in the art. For example, in an environment where all ATM cells are of the same priority, a buffer memory may implement only one queue per output port. Also, the queued items need not be ATM cells, but may be other communications, including other packet types. Furthermore, track of memory-content clearing need not be kept on a per-queue basis, but may be done on the basis of any other memory sub-entity, such as a memory sector, block, word, or even byte. Additionally, in systems that utilize "idle" communications (e.g., idle code or idle packets) when no "real" communications are available, contents of a memory sub-entity that consist entirely of these "idle" communications are the equivalent of the memory sub-entity being empty, and hence likewise serve as an indication that the sub-entity's (real) contents have all been replaced.

Claims: An arrangement for determining that the contents of two memories (1:12,2:12) whose contents are normally synchronized and frequently change have become resynchronized after occurrence of a fault in a duplicated-memory apparatus that comprises the two memories and a memory-update arrangement (14) which responds to receipt, resulting from normal use of the apparatus for its intended application, by the duplicated-memory apparatus of multiple items of information over a time by writing the received information into both of the memories as a part of the normal use of the apparatus, comprising selectively activatable means (20:400-416) for monitoring, upon activation of said monitoring means during the normal use of the apparatus after the occurrence and a mending of the fault, the supplanting of the contents, that exist at the time of the activation of said monitoring means in an active one of the two memories, by the received information which is being written into both of the memories as part of the normal use of the apparatus, to determine when the contents that existed in the active memory at the time when the monitoring means were activated have all been supplanted by the received information; and means (20:416-418) cooperative with the monitoring means, for indicating that the contents that existed in the active memory at the time when the monitoring means were activated have all been supplanted by the received information, thereby signalling that the contents of the two memories have become resynchronized by the writing over a time of the two memories resulting from the normal use of the apparatus. An arrangement as claimed in claim 1 wherein the monitoring means comprises means (20:400-401) for clearing the contents of the other one of the two memories upon activation of the monitoring means. An arrangement as claimed in claim 1 wherein the receipt of the information by the duplicated-memory apparatus results from normal use of the active memory for its intended application. An arrangement as claimed in claim 1 comprising means (20:409, 400. 401) for reactivating the monitoring means in response to deletion of information, that occurs while the monitoring means are active, due to overflow of data, in an active one of the two memories. An arrangement as claimed in claim I for a duplicated-memory apparatus wherein each memory implements at least one queue (100) for buffering the received information and the memory-update arrangement responds to the receipt of information, destined for an individual queue, by the duplicated-memory apparatus, by writing the received information into the individual queue in each memory, the arrangement comprising means (9,10,14) for removing, from individual queues of both memories, information that has been written into the individual queues; and wherein the monitoring means comprises at least one queue length indicator (200), each corresponding to a different at least one queue in the active memory, for indicating an amount of information stored in the corresponding at least one queue, and selectively activatable means (64, 20:400-416) for monitoring, upon activation of said monitoring means, individual ones of the queue-length indicators for an indication that the corresponding at least one queue has been emptied since the time when the monitoring means were activated; and the indicating means indicate when every queue-length indicator has indicated that its corresponding at least one queue has been emptied since the time when the monitoring means were activated, thereby signalling that the contents of the two memories are synchronized. An arrangement as claimed in claim 5 wherein the monitoring means monitor queue-length indicators for an indication that the corresponding at least one queue has been emptied of one of either (a) all information or (b) information that was stored therein at the time of activation of the monitoring means, since the time when the monitoring means were activated. A method of determining that the contents of two memories (1:12, 2:12) whose contents are normally synchronized and frequently change have been resynchronized after occurrence of a fault in a duplicated-memory apparatus that contains the two memories and a memory update arrangement (14), comprising the steps of in response to receipt, resulting from normal use of the apparatus for its intended application, over a time of multiple items of information by the apparatus, writing (212 and 216) the information into both memories as a part of the normal use of the apparatus, activating (400-404) monitoring, during the normal use of the apparatus after the occurrence and a mending of the fault, of the supplanting of the contents, that exist at the time of the activation of said monitoring in an active one of the two memories, by the received information which is being written into both of the memories as a part of the normal use of the apparatus, to determine (406-416) when the contents that existed in the active memory at the time when the monitoring was activated have all been supplanted by the received information; and in response to the monitoring, indicating (418-420) that the contents of the active memory, that existed when the monitoring was activated, have all been supplanted by the received information, to signal that both memories now have the same contents and thus have become resynchronized by the writing over a time of the two memories resulting from the normal use of the apparatus. A method as claimed in claim 7 wherein the step of activating monitoring comprises clearing (401) the contents of a standby one of the memories. A method as claimed in claim 7 wherein the receipt of the information by the apparatus results from normal use of the active memory for its intended application. A method as claimed in claim 7 comprising the steps of: deleting (406-409) information, in response to overflow of the active memory; and reactivating (400-404) the monitoring of the replacement, in response to the deletion. A method as claimed in claim 7 for a duplicated-memory apparatus wherein each memory implements at least one queue (100) for buffering the received information and the apparatus comprises at least one queue-length indicator (200) each corresponding to a different at least one queue in the active memory and indicating an amount of information stored in the corresponding at least one queue. wherein the step of writing comprises the step of, in response to each receipt by the apparatus of information, destined for an individual queue. writing the received information into the individual queue in each memory and incrementing the corresponding queue-length indicator, the method comprises the step of removing, from individual queues of both memories, information that has been written into the individual queues and decrementing the corresponding queue-length indicators, the step of activating monitoring comprises the step of commencing (404) to monitor individual ones of the queue-length indicators for an indication (416) that the corresponding at least one queue has been emptied; and the step of indicating comprises the step of in response to the monitoring, indicating (418) that every queue-length indicator has indicated that its corresponding at least one queue has been emptied since the time when the monitoring was commenced, to signal that the contents of the two memories are synchronized. A method as claimed in claim 11 wherein the step of activating monitoring comprises the further step of ceasing to increment the queue-length indicators.

Inventors: PASHAN MARK ALLEN, SPANKE RONALD ANTHONY, PASHAN, MARK ALLEN, SPANKE, RONALD ANTHONY
Applicants: AT & T CORP, AT&T CORP.
Application year: 1992
Main classification: G06F11
IPC codes: G06F11