Dynamic end to end retransmit apparatus and method

A dynamic end to end retry apparatus and method uses the concept of transaction identification numbers combined with a path number and flow control class to uniquely account for all transactions in a multi-processor computer system. The apparatus and method ensure there are no duplicate transactions through the use of special probe and plunge transactions and their respective responses. The apparatus and method also allow for any number of alternate paths being active simultaneously, such that if one path fails, the remaining alternate paths can continue on the communication (along with the backup alternate path if desired) as usual without any loss of transactions.

TECHNICAL FIELD

The technical field is error detection and correction in multiprocessor computer systems.

BACKGROUND

Path or link errors may exist in multiprocessor computer systems. To tolerate such link errors, computer designers have traditionally made use of error correction code (ECC) or retry mechanisms. ECC handles certain permanent errors such as a wire being disconnected in a link (or interconnect) while other links are working. However, if multiple wires in the link are disconnected, or if the entire link is disconnected, the ECC cannot recover the disconnected link. Retry works well for transient errors. If a packet includes errors that can be detected, but not corrected, then the packet will be sent again from a sending node to a receiving node using the same link. The process of sending the packet may repeat several times. However, retry cannot handle errors such as multiple wires failing in a link or the link being disconnected, or an intermediate routing chip being removed for service.

An end to end retry scheme may be used as a means to tolerate link or immediate route chip failures. The basic approach is that each transaction has a time to live, and as a transaction travels through the multiprocessor computer architecture, the value of the time to live is decremented. A transaction that cannot be delivered to its destination node and has its time to live go from its maximum level to zero is discarded. Request transactions may be retried along a secondary path if some predetermined number of attempts along the primary path failed to generate a response. Response transactions may not be acknowledged. If a response transaction does not reach its destination mode, the failure of the response transaction to reach the destination node will have the same effect as the corresponding request transaction not reaching the destination mode, and as a result the request transaction may be retried.

This end-to-end retry scheme has several disadvantages. First, is that the time-out hierarchy is tied to the retry protocol. If a request transaction is tried four times, for example, (along primary and alternate paths) before the request reaches an error time out, then the next transaction type in the hierarchy has to wait for four times the time out for every lower level transaction, the transaction type can generate. For example, a memory read request may cause several recalls. Thus, the memory read request may be reissued only after allowing all recalls to happen. Thus, the memory read request's reissue time out is the maximum number of recalls times the four times the recall time out, plus the times of flight for the request transaction and the response transaction. As a result, the time out hierarchy keeps increasing exponentially (that is the factor four keeps getting multiplied across the hierarchy).

A second disadvantage is that verifying a time out hierarchy is a challenging design requirement since time outs frequently take place over the period of time measured in seconds, and simulating a large system to the range of seconds of operation is almost impossible. A third disadvantage is that the retry strategy requires participation of all chips in the interconnect (at least to decrement the time out value). Thus, the retry strategy does not work well in a computer architecture that has components, such as a crossbar, that the computer designer is trying to leverage. A fourth disadvantage is that the retry strategy operates in an unordered network, and ordered transactions such as processor input/outputs (PIOs) need an explicit series of sequence numbers to guarantee ordering. In addition, for transactions such as PIO reads that have side effects, a read return cache is needed to ensure the same PIO read is not forwarded to a PCI bus multiple times.

SUMMARY

A dynamic end to end retry apparatus and method uses the concept of transaction identification numbers combined with a path number and flow control class to uniquely account for all transactions in a multi-processor computer system. The apparatus and method ensure there are no duplicate transactions through the use of special probe and plunge transactions and their respective responses. The apparatus and method also allow for any number of alternate paths being active simultaneously, such that if one path fails, the remaining alternate paths can continue on the communication (along with the backup alternate path if desired) as usual without any loss of transactions.

In the multiprocessor computer system with multiple nodes, each node keeps track of transactions the node has sent over time to every other node, as well as every transaction the node has received from every other node along each active path for each flow control class. To accomplish this tracking function, two data structures exist. A send_TID, representing the transaction identification (TID) for the last transaction sent by the sending (or source) node to a given destination node exists along any given active path, and a flow control class. A second structure is a receive_TID, representing the TID of the last transaction that a destination node received and for which the destination node sent an acknowledgement (ACK) back to the source node, for each node, along every active path, and for each flow control class. The send_TID and the receive_TID may be stored in send_TID and receive_TID tables at each node in the multiprocessor computer system.

Each node (destination node for the send_TID or source node for the receive_TID) can receive transactions over multiple paths. All nodes in one flow control class may operate over the same number of paths. For example, the system may have four alternate active paths between any two CPU/memory nodes, but only one active path to or from an I/O hub chip. The system does not require distinct physical paths between any source-destination nodes. For example, the system may comprise four active paths with two active paths sharing a physical path.

Every transaction that is sent from a source node to a destination node is also put into a retransmit buffer. When the transaction results in an acknowledgement from the destination node, the transaction is removed from the retransmit buffer. The acknowledgement can be piggy-backed with an incoming transaction and/or a special transaction. No acknowledgement is necessary for an acknowledgement transaction. If a transaction is not acknowledged within a predetermined time, recovery actions are taken. The destination node may wait to gather several transactions for a given source node before generating an explicit acknowledgement transaction, while trying to ensure that such a delay will not generate any recovery actions at the source node. This delay helps conserve bandwidth by avoiding explicit acknowledgement transactions as much as possible.

When a source node sends a transaction to a destination node, the source node gets the TID number from the send_TID table, checks that no transaction with the source TID number is pending to the same destination node in the same path and the same flow control class, and sends the transaction to the destination node while also sending the transaction to the retransmit buffer. The source node then increments the corresponding TID number in the send_TID table. When the destination node receives the transaction, the destination node queues the transaction in a destination node receive buffer. If the transaction is of a request type, and the destination node can generate a response within a time out period, the destination node sends a response, which acts as in implicit acknowledgement, to the source node. The destination node then checks the receive_TID table to see if the transaction received by the destination node has the correct TID number. If the transaction has the correct TID number, the destination node updates the corresponding entry in the receive_TID table, and sets a flag bit indicating that the destination node needs to send an acknowledgement transaction. If the transaction does not have a correct TID, the transaction is dropped, since an incorrect TID means that earlier transactions have been dropped in the system. If the destination node cannot generate a response (or the transaction is a response transaction) the destination node simply sends an acknowledgement transaction within the timeout period to the source node. In either case, the destination node resets the flag bit in the receive_TID table indicating that the acknowledgement (or response) has been sent. The destination node sends acknowledgement transactions for transactions received from a particular node, path and flow control class, in order.

If a source node does not receive an acknowledgement transaction within a predetermined time, the source node sends a probe request transaction along an alternate path (preferably an alternate physical path). The probe request transaction contains the source node identification, the path number, the flow control class, the TID of the timed-out transaction, and the TID of the last transaction that is pending. The destination node takes the information contained in the probe request transaction and determines if the destination node has already responded to the timed-out transaction. If the destination node has already responded to the timed-out transaction, the destination node indicates so in a probe request response along with the TID of the last transaction that the destination node has received. This probe request response is sent along an alternate path. The probe request transaction, as well as the corresponding probe request response, may then be used for acknowledgement purposes. When the source node receives an acknowledgement to the probe request transaction, the source node resumes retransmission starting with the transaction after the last TID received by the destination node, if any. From this point on, neither the source node nor the destination node use the path where the problem occurred to receive a transaction or to send out an acknowledgement.

DETAILED DESCRIPTION

A dynamic end to end retransmit protocol is implemented by an end to end retransmit apparatus and method employed in a multiprocessor computer system.FIG. 1is a block diagram of a multiprocessor computer system10that employs such an apparatus. InFIG. 1, a send (source) node11is coupled to a receive (destination) node12through alternate paths20. The send node11is coupled to a send_TID (transaction identification) table13and a retransmit buffer15. The destination node12is coupled to a receive_TID table14and a receive buffer16. The designation of the nodes11and12is arbitrary, and for means of illustration. In the system10, both the nodes11and12may send and receive transactions and hence both the nodes11and12may be any source or destination nodes. The nodes11and12may be any nodes in the multiprocessor computer system10, such as CPU or memory nodes or I/O hub chips. The paths20may be distinct physical paths or virtual paths. The send node11sends transactions to the destination node12along one of the paths20and receives responses or acknowledgements from the destination node12along one of the paths20. Transmissions sent from the send node11to the destination node12may be temporarily placed in the retransmit buffer15. Similarly, responses and acknowledgements from the destination node12to the send node11may be temporarily placed in the receive buffer16. The send_TID table13and the receive_TID table14may be used to store information related to the transactions such as the TID number of each transaction, response or acknowledgement, sending node identification (Ni), path identification (Pi), flow control class (FCC), and other information.

FIG. 2is a block diagram of the microprocessor computer system10ofFIG. 1showing additional details of operation of the dynamic end to end retransmit apparatus. The nodes11and12are connected through a number of cross-bar type routing chips. In the illustrated example, the cross-bar chips34,36, and38are used to connect the nodes11and12. However, fewer or more cross-bar chips could be used.

The nodes11and12are connected by two paths, P1and P2. The path P1is designated by links21-24. The path P2is designated by links25-28. Any of the links21-28in the paths P1and P2may fail. For example, the link28(in path P2from the source node11to the destination node12) may fail. Thereafter, any transaction the source node11sends (or has sent) to the destination node12over the link28and the path P2may not arrive at the destination node12, and hence may not be acknowledged by the destination node12. The source node11may eventually time out on the oldest non-acknowledged transaction. The source node11will thenceforth stop using the path P2for sending any subsequent normal transactions. In particular, the source node11may deconfigure the path P2and may stop accepting any acknowledgements that are sent to the source node11over the path P2. However, the source node11may continue to receive normal transactions over the path P2. The source node11may also send a probe request to the destination node12along the path PI, for example, over the links21and23. The probe request will be described in detail later. The destination node12may respond, using the path P1, with the transaction number of the last transaction received by the destination node12from the source node11over the path P2. The destination node12then stops receiving any normal transactions along the path P2. The deconfigured path P2may be indicated by use of a separate status bit, for example.

The source node11may attempt to determine if the failed path P2is still open. For example, an unacknowledged transaction may have been the result of a transient error, in which case the path P2may still be available for sending and receiving transactions, including acknowledgements. After receiving the response to the probe request, the source node11may send a plunge request along the failed path P2and flow control class to the destination node12. The plunge request will be described in detail later. The plunge request indicates the TID of the first transaction the source node11will retransmit if the path P2is re-established. On receiving the plunge request, the destination node12may re-establish the path P2. The destination node12then initiates a response for the plunge request. Since the plunge request itself may be in the response flow control class, the destination node12may use a flag bit in the receive_TID table14to send the plunge request response when space exists in the receive buffer16. Once the source node11receives the response to the plunge request, the source node11can start using the path P2for normal transactions. If the source node11does not receive a response to the plunge request, the source node11does not use the path P2until maintenance software guarantees that the path P2has been re-established. In an embodiment, the source node11may retry the determination of the existence of the path P2by periodically sending plunge requests to the destination node12.

In the multiprocessor computer system10shown inFIGS. 1 and 2, each of the nodes11and12keeps track of transactions the node has sent over time to the other node, as well as every transaction the node has received from the other node, along each active path for each flow control class. To accomplish this tracking function, two data structures exist as shown inFIGS. 3A and 3B. The send_TID table13shown inFIG. 3Amay contain the transaction identification (TID) for transactions sent by the source node11to the destination node12along any given active path and for each flow control class. The send_TID table13may also include valid bits and acknowledge (ACK) received bits for each such transaction. The receive_TID table14shown inFIG. 3Brepresents the TID of the transactions that the destination node12received for each node, along the active path, and for each flow control class. The receive_TID table14may also include valid bits and send ACK bits for each transaction. Each node (destination node12for send_TID or source node11for receive_TID) can operate over multiple paths. All nodes in one flow control class may operate over the same number of paths. For example, the system10may have four alternate active paths between any two CPU/memory nodes, but one active path to or from an I/O hub chip. The system10does not require distinct physical paths between any source-destination nodes. For example, the system10may comprise four active paths with two active paths sharing a physical path.

The flow control class refers to the type of transactions being sent over the paths/links in the system10. For example, a memory read request may be in flow control class A and a write request in flow control class B. A path that is available to one flow control class may not be available to a different flow control class.

Every transaction that is sent from the source node11to the destination node12is also put into the retransmit buffer15. When the transaction gets an acknowledgement from the destination node12, the transaction is removed from the retransmit buffer15. The acknowledgement can be piggy-backed with an incoming transaction and/or a special transaction. No acknowledgement is necessary for an acknowledgement. If a transaction does not get an acknowledgement within a predetermined time, recovery actions may be taken. The destination node12may wait to gather several transactions for a given source node11before generating an explicit acknowledgement transaction, while trying to ensure that such a delay will not generate any recovery actions at the source node11. This delay helps conserve bandwidth by avoiding explicit acknowledgement transactions as much as possible.

When the source node11sends a transaction to a destination node12, the source node11gets the TID from the send_TID table13, checks that the transaction is not pending to the same destination node12, and sends the transaction to the destination node12while placing the transaction in the retransmit buffer15. When the destination node12receives the transaction, the destination node12queues the transaction in the receive buffer16. If the transaction is of a request type, and the destination node12can generate a response within the time out period, the destination node12sends a response to the source node11, which acts as an implicit acknowledgement. The destination node12then checks the receive_TID table14to see if the transaction the destination node12received is not in default. If the transaction has the correct TID, the destination node12adds an entry in the receive_TID table14, and sets the ACK bit to 1 indicating that the destination node12needs to send an acknowledgement transaction. If the transaction does not have a valid TID, the transaction is dropped.

If the source node11does not receive an acknowledgement transaction within a predetermined time, the source node11sends a probe request transaction along an alternate path. The probe request transaction contains the source node identification, the path number, the flow control class, and the TID of the timed-out transaction, and the TID of the last transaction that is pending in the retransmit buffer15. The destination node12takes the information contained in the probe request transaction and determines if the destination node12has already responded to the timed-out transaction. If the destination node12has already responded to the timed-out transaction, the destination node12indicates so in a probe request response along with the TID of the last transaction which the destination node12has received. The probe request response is sent along an alternate path. The probe request transaction, as well as the corresponding probe request response, may then be used for acknowledgement purposes. When the source node receives an acknowledgement to the probe request transaction, the source node resumes retransmission starting with the transaction after the last TID received by the destination node12, if any. From this point on, neither the source node12nor the destination node12use the path where the problem occurred to receive a transaction or to send an acknowledgement.

FIGS. 4A and 4Billustrate state diagrams for the source node11and the destination node12, respectively. Transactions may be sent from the source node11to the destination node12. The destination node12may send a response or an acknowledgement (ACK) back to the source node11. The source node11and the destination node12track all transactions, and for each transaction, the source node11and the destination node12determine if the transaction is valid or invalid. When a transaction is determined to be invalid, either the source node11or the destination node12, or both, may initiate some type of recovery action. A valid transaction may be considered any transaction for which a response or ACK has been received within a specified time limit. The time limit may be set based on an expected “time-of-flight,” which basically relates to the time expected for a transaction to travel from one node to another node. A typical time limit may be set at four times the “time-of-flight.” The source node11and the destination node12, using the send_TID table13and the receive_TID table14, respectively, indicate when a transaction (as an entry in the table) is valid by setting a valid bit for the entry to 1, and indicate when an acknowledgement (ACK) or response has been sent by setting a sent ACK bit to 1, or received by setting an ACK received bit to 1.

InFIG. 4A, a transaction T is sent (transition42) by the source node11to the destination node12, and the source node11makes an entry in the send_TID table13. Because the transaction T is presumptively valid, but an ACK cannot be immediately received from the destination node12, the source node11sets the valid bit to 1 and the ACK received bit to 0, state43. The source node11may then receive an ACK (or a response) from the destination node12(transition44), and the state machine moves to state45, where the valid bit remains set to 1 and the ACK (response) received bit is set at 1. However, when in state43, the destination node12may not be able to receive a transaction because the receive buffer16may be full. In this case, the destination node12may signal a retry to the source node11, and the source node11may indicate receipt of the retry, transition46. Following state45, the state machine can only transition back to the invalid state, transition48, which may occur at a set time, typically about four time the expected time of flight of the transaction from the source node11to the destination node12. This is done to prevent a corner-case scenario in which the source node11refuses a TID that was acknowledged to send a transaction, which gets lost. When the source node11queries the destination node12, the source node11still has the same TID, but for an older transaction, in its receive_TID table. The source node11will indicate that the source node11received the transaction response to the probe request. By waiting, the destination node12is essentially guaranteed to have removed that TID from the receive buffer16.

InFIG. 4B, the state machine begins in state51with an invalid entry in the receive buffer16of the destination node12. The state machine transitions52to the state53upon receipt of the transaction T from the source node11. The valid bit for the corresponding entry in the receive_ID table14is set to 1, and the send ACK bit is set to 0. The state machine then transitions54to the state55, when the destination node12sends an ACK to the source node11, and the entry in the receive_ID table14is updated with the send ACK bit set to 1. After an appropriate wait time, the state machine transitions55back to the invalid state51. The wait time allows the probe to arrive at the destination node12in case the ACK is lost.

FIGS. 5-11are flowcharts illustrating operations of the multiprocess computer system10shown in FIG.1and the dynamic end-to-end retransmit apparatus operating on the computer system10. InFIG. 5, an operation100is illustrated showing a transaction from the source node to the destination node along path Piin flow control class F. The operation100starts in block105. In block110, a check is made to determine if there is an entry in the send_TID table13with TID equal to T, destination node equal to N2, path equal to Pi, flow control class equal to F, and a valid bit set to 1. In block110, if such an entry exists, the operation100moves to block115and either waits, or tries another path P2for the transaction. The operation100then returns to block110. In block110, if there is no entry in the send_TID table13, the operation100moves to block120and a check is made to determine if an entry (i.e., space) is available in the send_TID table13and the retransmit buffer15. If an entry is not available as checked in block120, the operation100moves to block125and waits for a predetermined time before returning to block110. If an entry is available, as checked in block120, the operation100moves to block130and the source node11sends the transaction T to destination node12(N2) along path Piand flow control class F. Next, in block135, the transaction is placed in the retransmit buffer13. Then, in block140, an entry is added to the send_TID table15with destination equal to N2, TID equal T, path equal Pi, flow control class equal to F, with a valid bit set at 1 and acknowledgement received bit set to 0. The operation100then ends, block145.

FIG. 6Aillustrates an operation200in which a transaction in a retransmit buffer times out. Time out typically will occur at either three or four times the maximum time of flight for the given transaction. The operation200begins in block205. In block210, a transaction T in the retransmit buffer15times out. The operation then moves to block215and the source node11sends a probe request to the destination node12with the TID equal to T, the flow control class equal to F, the path equal to Pi, along alternative path Pj. Next, in block220, the source node11checks to see if a probe response has been received. In block220, if a probe response has not been received, the operation200moves to block225, and the source node11determines if a time out condition has occurred. If the time out condition has not occurred according to the check in block225, the operation200returns to block220and the source node11continues to wait for reception of a probe response. In block225, if the time out condition has occurred, the operation200moves to block230and the source node11checks if another alternate path besides the path Pjexists. In block230, if an alternate path is determined to exist, the operation200returns to block215and a subsequent probe request is transmitted. In block230, if another alternate path does not exist, the operation200moves to block235. In block235, a failure condition is noted and the computer system ID “crashes.” The operation200then moves the block265and ends. In block220, if the source node11receives the probe response prior to a time out of the probe request, the operation200moves to block240. In block240, the source node11determines if the original transaction T was received by the destination node12. In block240, if the destination node12received the original transaction T, the operation200moves to block250, and an error is logged that an acknowledgement path from the destination node12(N2) to the source node11(N1) along the path Pimay have a problem. The operation200then moves to block265and ends. In block240, if the destination node12did not receive the transaction T, the operation200moves to block260and the source node11resends the transaction T along the alternate path Pj. The source node11then resets the time out for the transaction T, updates an entry in the send_TID table13to note the new path Pj, and diconfigures the path Pi. The operation200then moves to block265and ends.

FIG. 6Billustrates an optional operation300that may be used if a transaction in a retransmit buffer times out. The operation300commences following completion of the function shown in block260of FIG.6A. In block305, the source node11sends a plunge transaction to the destination node12along alternate path Pj, asking the destination node12to open the path Pi. In block310, the source node11determines if a plunge response has been received. In block310, if a plunge response has been received, the operation300moves to block320and the source node11reconfigures the path Pion. In block310, if the plunge response has not been received, the source node11determines if a time out condition has occurred, block315. If the time out condition has not occurred, the operation300returns to block310, and the source node11continues to wait for reception of a plunge response. In block315, if a time out condition has occurred, the operation300moves to block330and the source node11tries a new plunge transaction along an unused working path Pjand then waits for a response along the path Pi. The operation300then returns to block310. In block330, if an unused working path Pjis not available, the operation300moves to block265(FIG. 6A) and ends.

FIG. 7illustrates an operation400in which the source node11receives an acknowledgement transaction from the destination node12. The operation400starts in block405. In block410, the source node11receives the acknowledgement transaction. The operation400then moves to block415and the source node11removes the transaction corresponding to the acknowledgement from the retransmit buffer15. In block420, the source node11updates the send_TID table entry to acknowledgement received equal 1. In block425, the source node11waits for the N×maximum time of flight. In block430, the source node11invalidates the send_ID table entry. The operation400moves to block435and ends.

FIG. 8illustrates an operation450, in which a node receives a retry transaction. The operation begins in block455. In block460, the node receives the retry transaction. The operation450then moves to block465and the node resends the transaction. In block470, the node resets the time out counter in the retransmit buffer. The operation then moves to475and ends.

FIG. 9illustrates an operation500in which the destination node, such as the node12, receives a regular transaction. The operation500begins in block505. In block510, the destination node12receives the regular transaction. The operation500then moves to block513, and the destination node12determines if the path Piis configured. If the path Piis configured, the operation500moves to block515. Otherwise, the destination node12drops the transaction. In block515, the destination node12determines if space is available in the receive_TID table14and if protocol resources are available. If space is not available, or the protocol resources are not available, the operation500moves to block520and the transaction is retried. In block515, if space is available, the operation500moves to block525and the destination node12determines if ordered transactions and previous TIDs are not in default. If the conditions in block525are met, the operation500moves to block530and the destination node12determines if a previous transaction (TID) is present in a valid entry in the receive_TID table14, for the same source node, path and flow control class. In block530, if the previous transaction is not present, the operation500moves to block535and the transaction is dropped. The operation500then moves to block565and ends. In block535, if the previous transaction is present in a valid entry, the operation500moves to block545. In block525, if the ordered transaction is not in default, the operation500moves to block545. In block545, the destination node12consumes the transaction and adds an entry to the receive_TID table14with the valid bit set to 1 and the sent acknowledgement bit set to 0. The operation500then moves to block550, the destination node12waits, and sends an acknowledgement and sets the acknowledgement bit to 1. In block555, the destination node waits for time periods slightly less then the N×maximum time of flight. The operation500then moves to block560, and the destination node12invalidates the entry in the receive_TID table14. The operation500then moves to block565and ends.

FIG. 10illustrates an operation600in which the destination node12has received a probe request. The operation600starts in block605. In block610, the destination node12receives a probe request. In block615, the destination node12deconfigures path Pifor source S along flow control class F as indicated in the probe request. The operation600then moves to block620and the destination node12determines if an entry exists in the receive_TID table14with the same TID, source, path and flow control class as in probe request. In block620, if the entry exists, the operation600moves to block625and the destination node12sends a response, indicating that the transaction with the TID, equal to T, flow control class equal to F, along path Pi, was received. The operation600then moves to block640and ends. In block620, if the entry does not exist in the receive_TID table14, the operation600moves to block620, and the destination node12sends a probe response indicating that the destination node12never received the transaction with TID equal to T, flow control class equal to F, along path Pifrom the source S. The operation600then moves to block635, and the destination node12deconfigures the path Pi. The operation600then moves to block640and ends.

FIG. 11illustrates an operation650in which the destination node12receives a plunge request from the source11(node N1), along path Pi, in flow control class F. The operation650begins in block655. In block660, the destination node12receives the plunge request. In block665, the destination node12configures path Piin the flow control class F for node N1(the source node11) back on. The operation650then moves to block670and the destination node12sends a plunge response to the source node11. The operation650then moves to block675and ends.