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Cross Interrogate Caches in Tightly Coupled Multiprocessor Systems

IP.com Disclosure Number: IPCOM000051222D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 2 page(s) / 40K

Publishing Venue

IBM

Related People

So, K: AUTHOR

Abstract

The scheme described below minimizes the penalty experienced during a serious cross-interrogation of one processor by another.

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Cross Interrogate Caches in Tightly Coupled Multiprocessor Systems

The scheme described below minimizes the penalty experienced during a serious cross-interrogation of one processor by another.

Suppose, in a tightly coupled multiprocessor system, each processor has its own store-in cache keeping the recently referenced data. When one processor requests a data line and finds it in a remote processor's cache, a cross-interrogate (XI) event occurs if the request is exclusive or the line is already held exclusively.

The most serious and concerned event is when the line cannot be shared and has to be invalidated in the remote cache for consistence.

In this case, disregard how close these two caches are; both processors experience the penalty of waiting for the line to be removed from the remote cache before transferring it to the requesting processor. The organization described below minimizes the above penalty on the processor being cross-interrogated.

The figure shows a two-way multiprocessor in Which each processor has a cache as usual and, in addition, an XI cache holding only those lines which have been involved in XI castout; i.e., on an XI castout event, move the line to the requesting processor's XI cache but not the main cache. Each type of cache has its own directory so that concurrent accesses are possible and the total cache size is obviously increased. Now, during an XI castout, the remote processor will no longer be held up if it is not currently accessi...