Abstract:
A processor can write its state to an external state cache. Thus, in the event of a processor failure, the stored state can be read and assumed, either by the original processor or another processor. Thus, a process can be resumed from the stored state rather than reconstructed from initial conditions.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to computers and, more particularly, to high-availability computing. In this specification, related art labeled “prior art” is admitted prior art; related art not so labeled is not admitted prior art. 
     High-availability computers are used for applications where the normal amount of downtime suffered by a computer is unacceptable. High-availability computers use redundancy to provide backups for many components such as processors, memory, I/O (input/output) interfaces, power supplies, and disk storage. When one component fails, another similar component is available to take over its function. One approach is to operate identical components in parallel so that if one fails, data is preserved and there is little time lost in switching over from the failed component. Of course, there can be a performance penalty when two components are, in effect, doing the work of one. 
     SUMMARY OF THE INVENTION 
     The present invention, as defined in the claims, provides for external state caching for a processor or set of processors. If a processor fails, its state is preserved so that the state can be resumed by another processor or by the original processor once the problem associated with the failure has been handled. Since the state has been preserved, it is not necessary to return to the beginning of a process to recreate the state. State preservation does not require a second processor, so the waste associated with running two processors in lock-step is avoided. These and other features and advantages of the invention are apparent from the description below with reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of one of many possible systems provided for by the invention. 
         FIG. 2  is a flow chart of one of many possible methods in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A computer system AP 1  in accordance with the present invention comprises a pair of processors  11  and  12 , a core electronics component (CEC)  13 , an external state cache  15 , system memory  17 , and I/O devices  19 , all coupled by a network fabric illustrated in  FIG. 1  as a bus  21 . Connections  23  to bus  21  can be connected to other processor sets and other computer components. System memory  17  holds data  25  and programs  27 , including an operating system. 
     Processors  11  and  12  share external state cache  15 ; they are not run in lock-step. A variation of the illustrated embodiment permits processors sharing an external state cache to run alternatively in lock-step and non-lock-step modes. External state cache  15  is also coupled to CEC  13  for transferring state data to other processor sets. In other embodiments, an external state cache is coupled to only one processor, or to more than two processors. 
     In system AP 1 , external state cache  15  is coupled to both processors  11  and  12  at dedicated state-dump ports  31  and  32  respectively. The use of dedicated state-dump ports  31  and  32  that are independent of respective system interfaces  41  and  42  minimizes the performance impact of state dumps. Each processor writes its state data, e.g., cache contents, registers, program pointer, to a respective section of external state cache  15 . This writing can be periodic as directed by hardware, or in response to program instructions. 
     External state cache  15  has a first port  51  for receiving state dumps from processor  11  via its state-dump port  31 ; port  51  is coupled to state dump ports  31  and  32  of both processors  11  and  12  to provide access thereto to state data written by processor  11 . External state cache  15  has a second port  52  for receiving state dumps from processor  12  via its state-dump port  31 ; port  52  is coupled to state dump ports  31  and  32  of both processors  11  and  12  to provide access thereto to state data written by processor  11 . External state cache  15  has a further system port  53  so that CEC  13  can read from and write states to external state cache  15 . Thus, CEC  13  can transfer state data, e.g., from other processor sets, to either processor  11  or processor  12  via external state cache  15 . In alternative embodiments, a CEC can transfer state data directly to processors; in other embodiments, an external state cache is coupled to the incorporating system through a path not including a CEC. 
     Among the instructions included in programs  27  is a “CPU_dump_state” instruction. Processor  11 , when executing this instruction, writes its state to the respective section of external state cache  15 . Programs  27  also include “CPU_resume_state” and “CPU_assume_state” instructions. Processor  11 , when executing a “CPU_resume_state” instruction, reads and adopts a state stored in the respective section of external state cache  15 ; processor  11 , when executing a “CPU_assume_state” instruction, reads and adopts a state stored in the non-respective section of external state cache  15  (in other words, the processor adopts a state written by the other processor). Others instructions can be used to enable or disable automatic state dumps and set their frequency. 
     In the absence of an explicit instruction, state dumps are controlled by hardware. By default, state dumps occur at regular intervals. The regular interval can be increased or decreased based on a determination, in this case by CEC  13 , of a likelihood of failure, e.g., based on a number of detected correctable and uncorrectable errors, detected voltage rail droops, etc. The regular interval can be cut short upon prediction of an imminent failure. A state dump can also be omitted or delayed based on other demands on the processor. For example, a state dump can be omitted or delayed to avoid synchronization issues. 
     The need for omitting or delaying state dumps is minimized by the use of dedicated state-dump ports  31  and  32  dedicated to external state cache  15 . Since external state ports  31  and  32  are separate from the normal system interface ports  41  and  42 , they allow state dumping to proceed without significant performance issues because normal system bandwidth is not consumed. 
     The frequency of state dumps can be set, for example, as a function of factors relating generally to a tradeoff of need for high availability and performance or power. While using a dedicated state-dump port alleviates most of the performance overhead, there can still be some overhead associated with specific state dump instructions, so fewer state dumps can be called for when performance is critical. There can also be some synchronization overhead associated with a state dump so state dumps can be performed less frequently to ensure synchronicity. In addition, high power consumption can dictate a reduced frequency of state dumps. On the other hand, a processor performing work that requires high reliability can dump state more often. 
     In computer system AP 1 , independent power supplies are used for processor  11 , processor  12 , CEC  13 , and external state cache  15 . If one power supply fails, the respective component fails, but not the other components. In an alternative embodiment, an external state cache includes non-volatile memory so that the state data it holds is not lost even if its power supply fails temporarily. In another embodiment, the CEC and external state cache can be powered by either of the power supplies for the processors, so that if one power supply fails, its processor fails, but the remaining components remain operational. 
     A method M 1  practiced in the context of system AP 1  is flow charted in  FIG. 2 . At step S 11 , processor  11  is executing a process conventionally. A step S 12 , processor  11  writes its state, including on-board (e.g., level  1 ) cache contents, register contents, and pointer values, to a respective section of external state cache  15 . This writing can be in response to an instruction or be in response to a hardware-generated trigger. 
     At method segment S 13 , a failure or a potential imminent failure of processor  11  is detected, e.g., by CEC  13 . A potential imminent failure can be detected when monitored processor health metrics indicate an unacceptable likelihood of a processor failure. 
     If processor  11  can be replaced (hot-swapped) or “repaired”, e.g., reinitialized, at method segment S 14 , CEC  13  can command processor  11  to read the last state it or its predecessor wrote at method segment S 15 , and resume processing at method segment S 16 . Alternatively, method M 1  can proceed to method segment S 24 . At method segment S 24 , CEC  13  determines that processor  12  has completed a process it was executing. At method segment S 25 , CEC  13  directs processor  12  to read the state last written by processor  11 . CEC  13  then causes processor  12  to resume the process processor  11  was executing at the time of failure at method segment S 26 . 
     In an alternative embodiment, there is one external state cache for one processor. The invention also provides for external state caches with more than one section per processor. A processor can write to its sections in alternation so that the presently written state does not overwrite the immediately preceding state. Thus, if a failure occurs during a state dump so that the dumped state data is corrupted, an intact preceding state is available for resuming a process. Alternatively, state cache sections can be filled on a round-robin basis by different processors so that previous states can be preserved without requiring multiple sections per processor. These and other variations upon and modifications to the illustrated embodiment are provided for by the present invention, the scope of which is defined by the following claims.