Patent Publication Number: US-6665811-B1

Title: Method and apparatus for checking communicative connectivity between processor units of a distributed system

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to fault-tolerant distributed or clustered multiprocessor systems. More particularly, the invention relates to methods, and apparatus implementing those methods, for improving the resilience of a multiprocessor system in partial and total communication failure scenarios, and in the face of failure of periodic or timed events on a constituent processor. Thereby, system fault tolerance is improved. 
     One inventive method of achieving fault-tolerant capability in distributed multiprocessor architectures is to detect processor failures quickly with an “I&#39;mAlive” protocol (described in U.S. Pat. No. 4,817,091). Briefly, the I&#39;mAlive protocol involves each processor of the system periodically sending or otherwise broadcasting I&#39;mAlive message packets to each of the other processors in the system. Each processor determines whether another processor is operational by timing I&#39;mAlive packets from it. When a processor sees that the time interval passes without receipt of an I&#39;mAlive packet from a given processor, the first processor decides that the silent processor might have failed. 
     The I&#39;mAlive protocol message scheme is often combined with a “process pair” mechanism, comprising a primary process installed and running on one processor of the multiple processor system with a copy of that process operating as a backup process on another processor of the system. Periodic updates are sent from the primary process to the backup process so that should the primary process, or the processor it runs on, fail, the backup can take over for the primary process with minimal interruption. An example of the use of process pairs can be found in the above-identified &#39;091 patent. 
     Unfortunately, there are situations in which a processor will be late in broadcasting its required I&#39;mAlive message, in turn causing one or more of the other processors to assume that the tardy processor has failed. These situations, in turn, can give rise to such actions (to name a few) as: (1) both of the processes of a process pair (running on different processors) regarding themselves as the primary, destroying the ability to perform backup functions and possibly corrupting files; or (2) all system processors becoming trapped in infinite loops, contending for common resources; or (3) corrupting various system tables. Although such situations are rare, they are possible and have been observed in systems developed prior to implementation of a “Regroup” technique (described below). For fault tolerant systems, such situations must be made practically non-existent. 
     To supplement the I&#39;mAlive protocol, and to avoid the problems referred to above, a technique referred to as “Regroup” was developed. Triggered when a processor fails to see an I&#39;mAlive message within a prescribed check period, Regroup begins with messages being exchanged between all processors of the system in order to enlist them in the Regroup operation. Regroup then employs a consensus algorithm to determine the true state of each processor in the system by having each volunteer its record of the state of all other processors. Each processor compares its own record with records from other processors and updates its record accordingly. When the consensus is complete, all processors have the same record of the system&#39;s state. The processors will have coordinated among themselves to reintegrate functional but previously isolated processors and to correctly identify and isolate nonfunctional processors. 
     Later developments have refined the Regroup technique, allowing its use to determine membership even when physical communication among processors is lost. See, for example, U.S. Pat. No. 5,884,018 for “Method and Apparatus for Distributed Agreement on Processor Membership in a Multi-Processor System.” To the extent necessary, said U.S. Pat. No. 5,884,018 is incorporated by reference as if fully set forth herein. 
     As indicated above, a missing or tardy I&#39;mAlive message can cause each of the two processes of a process pair to operate as if it is the primary process. This, in turn, gives rise to the possibility of data corruption caused by both processors of the pair trying to write (and/or overwrite) portions of a disk drive (or other I/O controller) managed by the two processes. To avoid this situation, Regroup is structured to require each processor, at the start of a Regroup event, to invoke a “Hold I/O” state in which all input/output transmissions, except those necessary to the Regroup operation, are suspended. The Hold I/O state is turned off for those processors determined to be fit to continue operation. Any processor(s) determined to be faulty (either on its own, or by the Regroup operation) will continue the Hold I/O state until it halts. In addition, to preclude premature takeover of a process&#39; operations by its backup process, the first stage of Regroup is “stalled” (i.e., extended) long enough to ensure that all processors have entered the Regroup operation and invoked the Hold I/O state. It has been determined that the stall period is at least two I&#39;mAlive checking periods (2T check ), plus a safety margin to cover message transit time and to account for small differences in the processor clocks in the multiple processors of the system; one extra T tick  more than covers these small effects. (T tick , approximately 0.3 seconds, is a basic time unit in the I&#39;mAlive messaging operation as well as the Regroup algorithm. In addition to its use in “pacing” the sending of I&#39;mAlive messages every two T tick  intervals and checking every four T tick  intervals, the Regroup operation, when activated, proceeds in multiple rounds, the interval between which is T tick . That is, when Regroup is active, on every T tick  interval, every processor sends its Regroup messages to all other processors.) 
     The stall period described in the previous paragraph is known as stage  1  of the Regroup operation. Under certain special circumstances, a processor participating in a Regroup operation may determine that the duration of the stall period (or stage  1 ) must be extended beyond 2T check +T tick . This is referred to as a “cautious mode” Regroup operation. While in cautious mode, the processor invokes Hold I/O and waits longer for stage  1  to complete. The situations leading to a processor entering cautious mode are: 
     1. The processor detects that two or more peer processors are missing in the current Regroup operation. Note that this situation is possible if the processor is isolated, or if the system is subject to multiple processor and/or communication path failures. 
     2. The Regroup operation is restarted due to processor or communication failures that occur during the Regroup operation itself. 
     3. The Regroup operation is started because the multiprocessor system is recovering from a power outage. Because the recovery speed of individual processor units may vary, processor units must wait longer in stage  1  of the Regroup operation. 
     The I&#39;mAlive protocol is most often employed in a prioritized interrupt scheme in which the work done by the processor in order to construct and send I&#39;mAlive messages is assigned a low priority level. Message construction allows the I&#39;mAlive message to provide a fair indication of the state of well-being or health of the sending processor. It informs the other processors that the sending processor is capable of doing more than just sending I&#39;mAlive messages. However, higher priority tasks can delay the I&#39;mAlive interrupt, and the send of an I&#39;mAlive message, long enough so that another of the processors of the system, failing to see the I&#39;mAlive message within the requisite time, will initiate the Regroup operation. Regroup operations that are triggered by late sending of I&#39;mAlive messages (as opposed to a processor failure or communications path failure) can be considered a “false alarm.” Regroup will run, and discover no real problem. This may happen repeatedly, even frequently, under certain types of heavy processor loads such as a high rate of I/O interrupts requiring substantial processing. These false alarm Regroup operations not only use resources unnecessarily, they are somewhat risky in possibly halting a tardy but otherwise healthy processor, and they obscure diagnosis of what is really happening in the system. A candidate solution to this problem is to increase the frequency with which the I&#39;mAlive messages are sent. However, there is an upper bound to increasing the frequency of I&#39;mAlive message broadcasts where they begin to significantly impact upon the efficiency of processor operations. 
     Another candidate solution would be to decrease the frequency of I&#39;mAlive checking (but not necessarily the frequency of sending). This would almost certainly decrease the probability of false alarm Regroup operations, but at the significant cost of increasing the failure detection latency and also extending the duration of the Hold I/O time period. A important goal of the present invention is to both decrease the failure detection latency and to reduce the duration of the Hold I/O (described below) period. 
     Thus, it can be seen that there exists a need to reduce both the time for detecting a problem and entering Regroup and the time needed for the Hold I/O state. 
     SUMMARY OF THE INVENTION 
     Broadly, the present invention supplements the I&#39;mAlive method with periodic path probe messages that function to check all communicative paths (and, to some extent, the operability of the destination processor unit) between every processor unit of a distributed system and all other processor units of that system. Path probe messages are structured to be sent at greater frequencies than I&#39;mAlive messages, resulting in quicker detection of problems and a much faster initiation of the Regroup operation. In addition, the present invention produces a significant reduction of the “stall” period for ensuring that all processor units have entered the Regroup operation and invoked the Hold I/O state. 
     The invention is preferably used in a multiprocessor system in which multiple processor units are communicatively intercoupled to form a clustered system. According to the invention, each of the processor units will periodically send to all other processor units, on all available interconnecting paths, a path probe message. All processor units will respond to receipt of all received path probe messages with an acknowledgment (ACK) or a negative acknowledgment (NACK). If a sending processor unit fails to receive an ACK from any of the processor units to which a path probe message was sent within a prescribed amount of time, or receives a NACK response, a Regroup operation will be initiated to determine processor unit membership in the system, and the connectivity of those processor units. According to one embodiment of the invention, every processor unit entering the Regroup operation will invoke a “Hold I/O” to suspend input/output activity (other than Regroup messages). 
     The Hold I/O ensures, in the case of a full disconnect between two different processors, that a takeover won&#39;t happen until it is guaranteed that both primary and backup processes of an I/O process pair won&#39;t both act as primary (owner) of a particular disk drive (or other I/O controller). All processor units will have the I/O transfers on hold, and will wait until the Regroup operation completes before resuming such transfers. 
     In another embodiment of the invention, processor units will not invoke Hold I/O upon entering the Regroup operation. Rather, when Regroup establishes that certain of the processor units, if any, must halt (e.g., because they no longer have the required connectivity for communicating with the other processor units of the system), those processor units will then invoke Hold I/O. Again, regroup operation is extended (“stalled”) a predetermined amount of time to ensure that all halting processor units have invoked Hold I/O. 
     The present invention offers a number of advantages, which are achieved by both of its embodiments. First, since the path probe messages of the invention are used primarily to check the health of the communication between the sending processor unit and all other processor units of the distributed system, a higher priority interrupt handler can be used—assuming the processor unit has a prioritized interrupt structure permitting path probe messages to be sent with greater frequency to detect communication problems much quicker. In fact, the sending of path probe messages would preferably use an interrupt handler with a priority higher than that of other interrupt handlers in order to avoid having path probe transmissions potentially starved by other (higher priority) interrupt handlers. 
     Further, path probes are meant to primarily to check interprocessor communication by eliciting a response from the target processor unit. As such, path probe message construction can be minimal and, therefore, does not have the overhead of I&#39;mAlive message transmission. Thus, path probe operation does not have more than a minimal impact on the performance of a processor unit. 
     The use of path probe transmissions can check the connectivity to a destination processor unit and the destination processor unit will not even know that it is being “probed.” This is possible if proper hardware support is provided. The destination processor unit will not be interrupted by a path probe message. An ACK or a NACK will be returned by the hardware to the sender processor unit, depending upon the outcome of certain permission checks (described below) accomplished on the path probe message by the destination processor unit. 
     Further, in distinction to the I&#39;mAlive method without the supplemental path probing taught in the present invention in which it is the receiver that is checking for a lack of I&#39;mAlive messages, the present invention employs the sending processor unit to check for a timely response to transmitted path probe messages. There are expected latencies attendant with having a receiver of a message initiate time-based checking. In contrast, the present invention can significantly accelerate detection of a failed processor or communications path by using the message sender as the checking entity of the paths. 
    
    
     These and other advantages and features of the present invention will become apparent to those skilled in this art upon a reading of the following description of the invention which should be taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a multiprocessing system in which the processor units of the system are communicatively intercoupled for incorporating use of the present invention; 
     FIG. 2 is a flow diagram broadly illustrating the major steps taken to implement the “I&#39;mAlive” method of fault-tolerant checking supplemented by an embodiment of the present invention; 
     FIG. 3 is a time-line diagram illustrating the maximum time after a processor unit halt before a Regroup operation will start as a result of the methods of the present invention; and 
     FIG. 4 is a flow diagram illustrating an alternate embodiment of the present invention. 
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
     Turning now to FIG. 1, there is illustrated a distributed processing system, designated generally with the reference numeral  10 , comprising processor units  12   a ,  12   b , . . . ,  12   n . The actual number of processor units  12  could be anything more than one for effective use of the present invention, although the multiprocessor systems for which the present invention has been designed are often limited to sixteen processor units for reasons not relevant to the present invention. It will be evident to those skilled in this art, however, that the present invention does not limit the actual number of processor units that can be used. 
     The processing system  10  is preferably constructed to have an architecture similar to that taught in U.S. Pat. No. 5,751,932 (issued May 12, 1998; hereinafter, the ′932 patent″) and, therefore, to the extent necessary is incorporated herein by reference. 
     The processors  12  are interconnected by X and Y system area networks (SANs)  14  ( 14   x ,  14   y ) and connection links  16  ( 16   x ,  16   y )that provide the processors  12  with interprocessor communication. The X and Y SANs  14 , which may include routing apparatus (not shown) as described in the referenced &#39;932 patent, are constructed to form a joint I/O system that provides each of the processors  12  with at least two communication paths for both interprocessor communication to each of the other processors  12 , and to each of the various I/O units (e.g., printers, secondary storage, and the like)—here, represented generally by the disk system  20 . 
     Each of the processor units  12  implements the I&#39;mAlive protocol for fault tolerance and, as is typical, will employ a prioritized interrupt system in which the I&#39;mAlive message sending is relegated to a low priority. Also, the processor units  12  will use software constructs to implement the Regroup operation described in referenced U.S. Pat. No. 5,884,018. Accordingly, each processor unit  12  sends an I&#39;mAlive message to each of the other processor units  12  approximately every T seconds (e.g., 0.6 seconds or two T tick  intervals), and checks for receipt of I&#39;m live messages from the other processors  12  every 2T seconds (e.g., 1.2 seconds or one T check  (four T tick ) intervals). When a check indicates missing messages from a processor unit, the checking processor will initiate the Regroup operation to determine what processor units  12  continue to operate, and which may have lost communicative connectivity with the majority of the operating processor units. For a more complete description of this process, see the referenced U.S. Pat. No. 5,884,018. 
     Initiation of a Regroup operation by a processor unit  12  will include transmission of Regroup messages by that processor unit to the other processor units  12 , causing them to also enter the Regroup operation. 
     One way to check the communication paths between the processors  12  is to use the I&#39;mAlive method of fault-tolerant checking. FIG. 2 illustrates, in part, the I&#39;mAlive technique at steps  32 ,  34 ,  35  of the flow diagram  30 . Typically, I&#39;mAlive messages are sent by a processor unit  12  with a frequency twice that of the checking frequency. So, for every two I&#39;mAlive messages sent, each processor unit will initiate a check (at step  32 ) to see if it has received I&#39;mAlive messages from itself and its siblings. If an I&#39;mAlive message has been received from every processor, the processor unit  12  will do nothing and will return to step  32  after a delay of duration T check , expressly indicated at  35  (T check  is the interval between two consecutive checks for I&#39;mAlive messages performed by the processor unit  12 ). If, on the other hand, an I&#39;mAlive message has not been received from every processor since the last check (step  34 ), the processor unit  12  will proceed to step  36  to initiate a Regroup operation—the steps of which (steps  36 - 40 ) are also shown in FIG.  2 . 
     Note, as FIG. 2 illustrates, that the steps ( 36 - 40 ) of Regroup may also be entered by receiving (step  33 ) a Regroup message sent by a processor unit  12  that reached step  37  of the Regroup operation. 
     When a processor unit  12  enters Regroup, it will invoke a “Hold I/O” procedure in which all outgoing input/output (I/O) traffic is suspended except for those messages required for the Regroup operation. The processor unit  12  will then begin distributing Regroup messages to the other processor units to alert them that Regroup has been initiated and provide all processors entering Regroup with a view of the system  10  according to the processor unit(s) from which Regroup messages are received. Thus, step  37  sees an iterative exchange of Regroup messages in a series of steps (not shown) amongst the processor units  12  of the system  10 , to ultimately construct a view of the system, including processor unit membership in the system. That view may or may not be the same as the view held by the processor units before Regroup was entered. If a processor unit  12  has failed, and has not yet come back on line, the system view will, of course, be different by that one processor unit whose membership is no longer included. Also, certain processor units may be found, during the Regroup operation, to no longer have direct communication with other processor units of the system. These processor units will, at step  38 , be told to halt, excluding them from the system. (See the referenced U.S. Pat. No. 5,884,018 for further details on this aspect of Regroup operation.) 
     The iterative exchange (steps  37 - 39 ) will last a predetermined number of iterations, although it could be extended (in the case of a cautious mode Regroup operation), at the end of which the surviving processors will turn off Hold I/O (step  40 ). The operation will end at step  41  with the failed processor units, or processor units that are unable to otherwise participate in the Regroup operation, ostracized from system membership. 
     The I&#39;mAlive method, therefore, provides the processors  12  with a way to ensure that all have at least one communication path to all the other processor units  12  of the system  10 . For example, should the processor  12   a  (FIG. 1) lose both links  16  ( 16   x ,  16   y ), the I&#39;mAlive messages it sends will no longer be received by the other processors  12  ( 12   b , . . .  12   n −1,  12   n ). The processor unit  12   a  will also not receive I&#39;mAlive messages from those other processor units  12 . Thus, the processor units  12  will initiate a Regroup operation. The processor unit  12   a  will discover that it is isolated from the other processor units  12  of the system  10  and will halt operation. The remaining processor units  12  ( 12   b , . . .  12   n ) will establish a new membership view for the system  10  that excludes processor unit  12   a  and proceed. 
     There still remains the problem of a tardy processor unit, i.e., one that is delayed by higher priority tasks, from issuing its I&#39;mAlive broadcast on time, causing the other processor units  12  to initiate a Regroup operation. The tardy processor unit will participate in the Regroup, and Regroup will end with the same complement of processor units (unless one has actually failed or lost communication with the others). As indicated above, such false alarms can not only unnecessarily use system resources, but create a risk of halting an otherwise healthy processor. 
     The present invention is provided to overcome this problem, and additionally provides a way to verify the communication paths between the processor units  12  in a manner that does not unduly burden the efficiency of the processor units  12 . The invention is limited to checking communication paths, and the major steps ( 52 ,  54 ) of the invention are also illustrated in the flow diagram  30  of FIG.  2 . The invention uses path probe messages which have a major distinction from the I&#39;mAlive operation: it is the path probe message sender that is responsible for checking for a response to a message sent to a destination—as opposed to the intended receiver of an I&#39;mAlive message. 
     Regroup can also be entered according to the present invention, when in addition to the two ways of entering a Regroup operation discussed above (from either of steps  33  or  34 ), a processor unit  12  fails to receive at least one of the expected responses to path probe messages it sent. As mentioned, each of the processor units  12  will periodically send path probe messages on all communication paths available from it to all other processor units  12  (step  52 , FIG.  2 ). The path probe messages are very short, requiring only an acknowledgment (ACK) from the destination of the message. Also, path probe messages are assigned, in the interrupt hierarchy, a higher priority than that used for I&#39;mAlive messaging. With the transmission of each path probe message, a timer is set. If, as step  54  illustrates, the sender processor unit  12  fails to receive the required acknowledgment (ACK) from the destination of any path probe message it sent, the timer will timeout to issue an interrupt to that processor unit, alerting it to the possibility of a bad communication path. Alternatively, the destination of the path probe message may return a negative acknowledgment (NACK) which initiates the same interrupt as the timeout. The processor unit  12   a , when it takes this interrupt, will move from step  54  to the Regroup operation of steps  36 - 40 . Receipt of an ACK from every target of a path probe message (and absence of a NACK; see below) will return the operation to step  52 . 
     Note that path probe messages between processor units  12  use all available paths. That is, since each processor unit  12  has at least two paths provided by the links  16   x  and  16   y  and the corresponding SANs  14   x  and  14   y , a path probe message transmitted on both paths could produce different results. If a NACK is received on either of the connection links  16 , a Regroup operation will be triggered. However, a timeout on both connection links (in response to path probe messages sent to a particular destination processor unit) is needed to trigger a Regroup operation. A time-out on one connection link and an ACK on the other, in response to path probe messages sent to the same destination processor unit, will preferably not produce a Regroup operation. 
     Prior systems employing the I&#39;mAlive method in a distributed architecture have lacked the ability to quickly detect such occurrences as a processor unit halt or a loss of a communication path between one of the processor units and any other. Certainly, such occurrences would ultimately be detected, but checks for missing I&#39;mAlive transmissions were typically made approximately every 1.2 seconds. If a processor unit halts, or is otherwise disconnected from the system, a sibling processor unit sending I&#39;mAlive messages would not know until it checked and found that it had not received an I&#39;mAlive message from the affected processor unit. 
     The present invention preferably has the sending processor unit expecting some response to each path probe message within some predetermined period of time. Preferably, the architecture of each of the processor units  12  will include hardware (e.g., state machines and associated logic—not shown) that will respond to received path probe messages with the ACK and NACK replies, taking the burden off the heart of the processor unit  12  (e.g., typically a microprocessor). In addition, the processor units preferably also use the access validation and translation table (AVrr) taught in the referenced &#39;932 patent that is located in the memory of each processor unit. The AVTT maintains “permissions” information as to what other elements of the system  10  may send messages, such as processor memory read and write operations, to that processor unit. A message containing a request for a memory read operation for instance, from an element having such permission will be responded to by processing the memory read operation and providing the requested data from the processor unit&#39;s memory. A message from an element not having such permission will be responded to with a NACK. When a processor unit  12  halts (by a detected software fault) the memory location of the AVRT will be changed, in effect revoking all permissions. Thereafter, all messages sent to that processor unit will be responded to with a NACK—resulting in initiation of the Regroup operation. If a processor unit  12  halts due to a detected hardware error, the AVTT is not moved, but the processor unit will not respond to any subsequently received messages—in which case path probe messages to this halted processor unit will timeout, causing a Regroup operation to be initiated. 
     Thus, the sender of a path probe message knows that it can expect (1) an ACK if the recipient processor unit  12  is operating properly and a communication path exists between the sender and receiver, or (2) a NACK if the recipient processor unit has experienced a software halt, or (3) a timeout if the recipient processor unit has experienced certain hardware errors preventing it from returning a response to the message or if that particular communication path between the sender processor unit and the recipient processor unit is lost. Since path probe messages involve much less overhead than what is normally involved in an I&#39;mAlive transmission, and checking them are the responsibility of the sender processor unit, they are much more efficient than prior I&#39;mAlive implementations. 
     The present invention allows the Hold I/O action to be minimized. Prior fault-tolerant implementations of the I&#39;mAlive protocol using the Regroup method would conservatively see the processor units taking at least two I&#39;mAlive check periods before initiating a Regroup operation in response to absence of an I&#39;mAlive message from one of the processor units. Further, all processor units entering Regroup were required to enter a Hold I/O state, and the stage  1  of Regroup (see referenced Patent No. 5,884,018) would not be left until it was certain that all processor units had entered the Hold I/O state. This was something on the order of 2.7 seconds. With Regroup now being triggered by the path probe messages, this time can be reduced to approximately 0.6 seconds. 
     FIG. 3 illustrates the latest time that a Regroup operation can start after a processor unit halts due to a detected software fault. As FIG. 3 shows, if the processor unit is halted at time T 0 , when the software fault is detected a “cooldown” period occurs before the AVT table is moved. Thus, the latest Regroup is entered, after detection of a fault, is the interval between sending path probe messages (“TProbe” in FIG. 3) plus the cool-down period. FIG. 3 shows a processor unit  12  halting at time T 0  and a path probe message (Probe 2 ) being sent to the halted processor just before the cool-down period ends. The next path probe message (Probe 3 ) is sent T probe  seconds later, resulting in invalid permissions due to movement of the AVTT, and a NACK of the path probe message which will initiate a Regroup operation. 
     In the case of a hardware detected error causing the processor unit to halt, or a loss of communication paths to the processor unit, the maximum time taken to initiate a Regroup operation is a path probe period, T probe , since there is no cool-down period. 
     Preferably, path probe messages are sent every T tick  time period. (Thus, in FIG. 3, T probe  is equal to the T tick  time period.) This allows the stall period of stage  1  of Regroup to be shortened to T probe  +T tick  or approximately 0.6 seconds. This is a significant improvement over the prior stall period of 2*T check T   tick  or approximately 2.7 seconds. 
     The embodiment of the invention discussed above uses a Regroup operation in which all processor units entering the Regroup operation invoke a Hold I/O state (step  36 , FIG.  2 ). In a further embodiment of the invention illustrated in FIG. 4, the Regroup operation will have the Hold I/O state invoked by only those processor units that are going to halt or enter a cautious mode Regroup. This allows surviving processor units to continue with I/O operations, and never invoking Hold I/O in “false alarm” cases such as when an I&#39;mAlive message is missed because a processor unit is involved in other, higher priority work when an I&#39;mAlive message is due to be sent, and therefore delayed. 
     This further embodiment extends Stage  1  of the Regroup operation a sufficient time to permit a determination of whether or not there are processor units that will halt. However, as indicated above, this stall need not be longer than T probe  +T tick  +T stagel . T stagel  is the time spent before an isolated processor unit will decide to enter cautious mode; this will be approximately three T tick  intervals (i.e., approximately 0.9 seconds). 
     As FIG. 4 shows, entry into the Regroup operation remains the same, i.e., by detecting a missing I&#39;mAlive message (steps  32 ,  34 ,  35 ), or by lack of an appropriate response to a transmitted path probe message (steps  52 ,  54 ), or by receiving a Regroup message sent by a processor that has entered the Regroup operation (step  33 ). However entered, the processors will initiate the Regroup operation at step  70  in the same manner as described above, except that the Hold I/O state will not be invoked at this time. Step  72  has the processor(s) entering Regroup engaging in an iterative exchange of Regroup messages. Those processor units that enter the cautious mode (described above), or that find they will halt, invoke the Hold I/O state. Step  76  extends the Regroup operation to provide additional time for permitting the processor units to determine which are to halt and which will continue. Those processor units  12  determined by the Regroup operation as needing to halt are instructed to halt in step  78 . 
     The iterative exchange of messages will last a predetermined number of rounds; the total duration of the iterative exchange must be sufficiently long to accommodate any time extensions added to the Regroup operation at step  76 . If the iterative exchange is to continue, step  79  will return the procedure to step  72 . Ultimately, however, it will be determined that the iteration process should end, and step  79  is excited in favor of step  82 , where those processors that are to halt will halt (step  84 ), and those processors that will not halt will go on to step  86 . At step  86 , those surviving processors that may have turned on Hold I/O (because they entered cautious mode) will turn off Hold I/O, and the procedure will be exited at step  90 .