Abstract:
A service processor for a server system includes an event log that, once full, stores recent events by overwriting events of intermediate age so that the information required to diagnose both cascade errors and hangs are preserved. This contrasts with bottom-up buffers that discard recent events when full and with circular buffers that discard the oldest events when full. The event log can be reset by moving an exception region, that is, a region that is not overwritten by recent events. Alternatively, a partial reset can initialize an exception region (e.g., a bottom-up sublog), while a circular region or sublog continues to operate without being reset.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to computers and, more particularly, to computers that use event logs for diagnostic purposes. The present invention provides an improved event log that better uses limited capacity to retain events of greatest interest. 
   Much of modern progress is associated with the increasing prevalence of computer and network technology in society. Due to the complexity of computers and networks, errors are inevitable. Accordingly, diagnostic tools are continuously developed to address these errors. In the case of personal computers, an encountered error can be reported to a user when it occurs. For example, a message can be presented to the user, perhaps suggesting corrective action. 
   On the other hand, computers used as network and Internet servers are not typically attended by users. Accordingly, detected errors are typically compiled by a service processor that logs the errors (and other significant events) in an event log. When the computer is serviced, e.g., after some problem or failure, the event log can be examined to help determine the cause of the problem or failure. 
   One standard form of event log stores events until the log capacity is filled. Once full, it stops accepting events for logging. In other words, it favors old events over recent events for retention. Such an event log works well for error events that trigger a cascade of other error events. For example, an error event associated with the failure of a network port would be followed by a large number of detections of a failed network port. Error events that are reported after the log is full are discarded. However, the error events associated with the original failure are retained. 
   However, it is possible that a log would be full before the original error of interest occurs, in which case, that error would not be retained and would not be available for diagnoses. Also, in many cases, late reported errors are of interest. For example, in the event of a computer failure (e.g., “hang”), the last error or last few errors would be of greatest interest. So a system that makes these the least likely to be retained would not be optimal. 
   The likelihood of overflows can be reduced by using event logs with greater capacity. However, integrated-circuit real estate is limited so that it is not practical to use an event log that is large enough to store all possible events of interest. Also, no matter how great the log capacity, the problem of overflow must be addressed. One seemingly cost-effective approach to increasing effective log capacity is archive of logged events prior to a reset of the event log. Thus, as the event log approaches its capacity limits, its contents are written to hard disk; then the log can be emptied, ready to accept new events. However, archiving normally requires software running on a user processor—and there is no guarantee that it will be installed or properly maintained by the user. Another approach is to reset without archiving—but there is always a risk that the reset will delete events of interest. 
   In a prior-art approach implemented in a HP9000 K-Class design by Hewlett-Packard Company in some of its servers, a “circular” event log is used. Once the event log is full, new events overwrite the oldest events in the log. This works well for errors that hang a computer, but not so well for events that trigger a cascade of error events. If there are enough follow-on events, the trigger event(s) could be discarded and unavailable for diagnoses. To make it less likely that critical events will be discarded, a second circular log that only retains “severe” errors is used. This can be wasteful as many errors are entered into both logs. Furthermore, there is still the possibility of the severe event log being filled when a cascade of errors occur. What is needed is an improved event log system. 
   SUMMARY OF THE INVENTION 
   The present invention provides for an event log that discards events of intermediate age in favor of older or newer events. In other words, the invention provides a circular log that “excepts” a series of oldest events from overwriting when the log is full. Instead, the oldest un-excepted events are overwritten when the log is full. 
   Preferably, the invention is implemented as a unified log so that no events are subjected to (wasteful) double entry. In this case, the log can be effectively partially reset by moving the excepted region (but not its contents) of the log. Alternatively, the invention can be implemented using a combination of a bottom-up sublog and a circular sublog. In this case, a partial reset can be achieved by initializing the bottom-up sublog. 
   The invention further provides for a log manager that dedicates an “oversize” address space to a circular log (or sublog). When the circular log is first filled, log locations are overwritten cyclically, but each time a log location is overwritten it is assigned a different address. If system-management software attempts to access an address that is not currently assigned to a log location, a “not available” indication is returned. This can serve as a warning to the system-management software that previously-logged events have been overwritten. 
   The invention addresses both the cascade-type and the hang-type errors. The cascade-type errors are addressed since the oldest events are retained in the log. The hang-type errors are addressed as the most-recent errors are also retained. Partial resets reduce the risk of the lost of critical entries by retaining recently stored events. Of course, the invention can be further optimized by archiving logged events before they are overwritten. 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 diagram of a computer system in accordance with the present invention. 
       FIG. 2  is a flow chart of an event-log method implemented using the system of  FIG. 1 . 
       FIG. 3  is a progression diagram showing an approach to partially resetting an event log of  FIG. 1 . 
       FIG. 4  is a schematic diagram of an alternate event log in accordance with the present invention. 
       FIG. 5  is a schematic diagram of sublogs of the event log of  FIG. 4  illustrating their differential selectivity. 
       FIG. 6  is a schematic diagram of sublogs of the event log of  FIG. 4  illustrating a “hang” case. 
       FIG. 7  is a schematic diagram of sublogs of the event log of  FIG. 4  illustrating a “first-error” case. 
       FIG. 8  is a schematic diagram of an address space that a log manager of the log of  FIG. 4  can assigned to locations of a circular sublog. 
   

   DETAILED DESCRIPTION 
   A computer network server API comprises data processors DP 1  and DP 2 , memory MEM, and a service processor SP 1 , all connected via a system bus BUS, as shown in  FIG. 1 . Service processor SP 1  gathers event reports generated by the other components of server AP 1  and records them. To this end, service processor SP 1  includes an event log EL 1  and a log manager LM 1 . Event log EL 1  has capacity for 1024 events; it is illustrated in  FIG. 1  as having four sections S 11 –S 14  for storing  256  events each. 
   Service processor SP 1  implements a log method M 1 , flow charted in  FIG. 2 . At step ST 1 , the first events are stored bottom up in an excepted region R 11 , which is coextensive with section S 11  in  FIG. 1 . New events are added in succession to excepted region R 11  until it is full. 
   Step ST 2  begins once excepted region R 1  is full. After that time, further events are added bottom up to an unexcepted region R 12 , which is coextensive with sections S 12 –S 14  in  FIG. 1 . Events continue to be written to unexcepted region R 12  until it is full. Note that step ST 1  and ST 2  are collectively represented by an upward arrow A 11  in  FIG. 1 . 
   Once unexcepted region R 12  is full, further events cause unexcepted region R 12  to be overwritten bottom up. The transition from step ST 2  to ST 3  is indicated by a return arrow A 12 . During step ST 3 , unexcepted region RI 2  functions as a circular event log. 
   Step ST 3  continues until there is some form of reset. Log manager LM 1  is designed to trigger a reset as a function of time and severity of events. Depending on programming, the reset can take various forms. 
   In the illustrated variant of method M 1 , the reset takes the form of a move of the excepted region at step ST 4 .  FIG. 3  illustrates a move of excepted region R 11  from section S 11  to section S 12 . For example, if an event of special interest is written to section S 12  during step ST 3 , log manager LM 1  can move excepted region from section S 11  to section S 12 . Then when the log entries cycle from section S 14 , they cycle to section S 11  instead of section S 12 . Once the log entries cycle through section S 11 , they skip excepted region S 11  (now coextensive with section S 12 ) and continue with section S 13 . More generally, excepted region R 11  can move from any section S 11 –S 14  to any other section as dictated by log manager LM 1 . 
   An advantage of resetting by moving the excepted region is that, once step ST 3  is reached, event log EL 1  is always full of event information, like a circular buffer. Yet, selected events can be preserved for during multiple cycles through event log EL 1 . Furthermore, reset timing is not critical, as the excepted region can be moved any time the location being written to is not in either the source or destination section for the excepted region. In an alternative variant, depending on programming, log manager LM 1  can reinitialize event log EL 1  so that method M 1  returns to step ST 1 . 
   A second service processor SP 2  incorporating an event log EL 2  in accordance with the invention is shown in  FIG. 4 . Event log EL 2  includes two sublogs, a bottom-up sublog ELA and a circular sublog ELB. A log manager LM 2  causes events can be entered into both sublogs ELA and ELB until bottom-up sublog ELA is full. Alternatively, log manager LM 2  causes bottom-up event sublog ELA to be filled first, then circular sublog ELB is filled, overwriting as required to accept new events. Once sublog ELA is full, events are written to sublog ELB only until a reset. In this embodiment, bottom-up sublog ELA serves as a fixed excepted region, while circular sublog ELB serves as a fixed unexcepted region. 
   Preferably, bottom-up sublog ELA is more selective than circular sublog ELB. Thus, as indicated in  FIG. 5 , circular sublog ELA stores all events (e.g., error events S 1 –S 6  and Event A) that are stored in bottom-up sublog ELA plus additional events, e.g., forward-progress events F 19 –F 23 . When important event A occurs, it is logged into bottom-up sublog ELA and then copied by log manager LM 2  to circular sublog ELB. (In  FIGS. 5–8 , reduced-size logs are used for explanatory purposes.) Note that the selectivity procedures can also be applied to the single log with moving excepted region of  FIG. 1 . 
     FIG. 6  illustrates the “hang” case for service processor SP 2 . When event A occurs, bottom-up sublog ELA is already full. With bottom-up sublog ELA full, the hang condition is still viewable in forward-progress sublog ELB. If bottom-up sublog ELA is not full, then the hang condition is viewable in both sublogs. 
     FIG. 7  illustrates the “first-error” condition when a lot of logs are received. In this case, Event A is the important event that is followed by eight additional events S 1 –S 8 . The additional events fill bottom-up sublog ELA up to event S 5 . Events S 6 –S 8  are not stored in sublog ELA. However, they are stored in forward-progress sublog ELB; resulting in events A, S 1  and S 2  being discarded from sublog ELB. In this case, the first error (event A) can be found in bottom-up sublog ELA. 
   Log manager LM 2 , which incorporates a baseboard management controller or “BMC”, has an address space, part of which is assigned to each sublog. The part of the address space assigned to circular sublog ELB is much larger than the number of event locations in circular sublog ELB. In the illustrated case, the address space assigned to circular sublog ELB is four times the capacity of circular sublog ELB. 
     FIG. 8  shows the relationship between the space  801  occupied at a given moment by circular sublog ELB and the entire address space  802  dedicated to circular sublog ELB. For expository purposes, the sizes are shown as  4  and  16 , respectively. Records N-3 to N are addressable and available. Thus, if system management software SMS tries to access events stored at addresses N-3 to N, an event will be retrieved as expected. On the other hand, older events assigned to addresses N-13 to N-4 are addressable but not accessible (because they are no longer stored in circular sublog ELB. Thus, if system management software SMS tries to access events at addresses N-13 to N-4, a “not available” indication is returned. 
   System management software SMS can use the “not available” indications to determine when event information has been lost. For example, software SMS may be trying to transfer log information to hard disk by accessing addresses in order. A “not available” indication can be interpreted that events were logged faster than they could be transferred. Software SMS can then note that events were lost and start reading available addresses. 
   Log manager LM 2  provides for partial and total resets. In a total reset, both sublogs ELA and ELB are initialized. In a partial reset, only bottom-up sublog ELA is initialized. In the latter case, events occurring just before a partial reset are preserved in circular sublog ELB for about one cycle through sublog ELB. Such recent events can be useful in indicating the cause of a hang, for example. 
   In service processor SP 2 , each event sublog is  512  events deep. Alternative event logs can have one event log bigger than the other. More generally, event logs and sublogs can be of various sizes. Also, the invention provides for sublogs or log sections with different degrees of selectivity. In other words, one sublog can store all reported events, while another stores only the most severe events. 
   Different reset strategies are provided for. An event log can be reinitialized, or reset on a per-section basis. The invention provides embodiments in which there can be more than one excepted region and in which the excepted regions can vary in size. While the invention applies generally to all systems that log events, the invention has particular applicability to systems that use the Intelligent Platform Management Interface (IPMI) standard for event logging. These and other variations upon and modifications to the described embodiments are provided for by the present invention, the scope of which is defined by the following claims.