Patent
1994-11-28
1997-03-11
Kim, Kenneth S.
39520003, 39520015, 395464, 395876, G06F 15163
Patent
active
056110499
DESCRIPTION:
BRIEF SUMMARY
This application is the national stage of PCT/US92/04939 filed Jun. 3, 1992 and claims priority therefrom.
TECHNICAL FIELD
The present invention relates generally to the technical field of multi-processor digital computer systems and, more particularly, to multi-processor computer systems in which: processor that manages the storage of and access to the data; that controls the data; access to it.
BACKGROUND ART
Within a digital computer system, processing data stored in a memory; e.g., a Random Access Memory ("RAM") or on a storage device such as a floppy disk drive, a hard disk drive, a tape drive, etc.; requires copying the data from one location to another prior to processing: Thus, for example, prior to processing data stored in a file in a comparatively slow speed storage device such as hard disk, the data is first copied from the computer system's hard disk to its much higher speed RAM. After data has been copied from the hard disk to the RAM, the data is again copied from the RAM to the computer system's processing unit where it is actually processed. Each of these copies of the data, i.e., the copy of the data stored in the RAM and the copy of the data processed by the processing unit, can be considered to be image of the data stored on the hard disk. Each of these images of the data may be referred to as a projection of the data stored on the hard disk.
In a loosely coupled or networked computer system having several processors that operate autonomously, the data needed by one processor may be accessed only by communications passing through one or more of the other processors in the system. For example, in a Local Area Network ("LAN") such as Ethernet one of the processors may be dedicated to operating as a file server that receives data from other processors via the network for storage on its hard disk, and supplies data from its hard disk to the other processors via the network. In such networked computer systems, data may pass through several processors in being transmitted from its source at one processor to the processor requesting it.
In some networked computer systems, images of data are transmitted directly from their source to a requesting processor. One operating characteristic of networked computer systems of this type is that, as the number of requests for access to data increase and/or the amount of data being transmitted in processing each request increases, ultimately the processor controlling access to the data or the data transmission network becomes incapable of responding to requests within an acceptable time interval. Thus, in such networked computer systems, an increasing workload on the processor controlling access to data or on the data transmission network ultimately causes unacceptably long delays between a processor's request to access data and completion of the requested access.
In an attempt to reduce delays in providing access to data in networked computer systems, there presently exist systems that project an image of data from its source into an intermediate storage location in which the data is more accessible than at the source of the data. The intermediate storage location in such systems is frequently referred to as a "cache," and systems that project images of data into a cache are be referred to as "caching" systems.
An important characteristic of caching systems, frequently referred to as "cache consistency" or "cache coherency," is their ability to simultaneously provide all processors in the networked computer system with identical copies of the data. If several processors concurrently request access to the same data, one processor may be updating the data while another processor is in the process of referring to the data being updated. For example, in commercial transactions occurring on a networked computer system one processor may be accessing data to determine if a customer has exceeded their credit limit while another processor is simultaneously posting a charge against that customer's account. If a caching system lacks cache consistency, it is po
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Kim Kenneth S.
Schreiber Donald E.
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