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Browse Prior Art Database

Early Castout Bit

IP.com Disclosure Number: IPCOM000060546D
Original Publication Date: 1986-Apr-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 1 page(s) / 13K

Publishing Venue

IBM

Related People

Emma, PG: AUTHOR [+5]

Abstract

A method of reducing the vulnerability of data in a store-in cache to parity errors is described below. A cache replacement policy intermediate between store-in and store-through has been described in [*]. In this scheme, each changed line is written back to main memory when it changes replacement status from MRU (most recently used) to non-MRU. This ECO (early castout) replacement policy has the advantage that changed data is resident in the cache for a shorter period of time and is therefore less susceptible to errors and untimely castouts due to cross interrogates; however, it has the disadvantage that the write-back traffic to memory is increased. The benefit obtained by ECO accrues almost entirely to lines that are ECO only once or a few times.

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Early Castout Bit

A method of reducing the vulnerability of data in a store-in cache to parity errors is described below. A cache replacement policy intermediate between store-in and store-through has been described in [*]. In this scheme, each changed line is written back to main memory when it changes replacement status from MRU (most recently used) to non-MRU. This ECO (early castout) replacement policy has the advantage that changed data is resident in the cache for a shorter period of time and is therefore less susceptible to errors and untimely castouts due to cross interrogates; however, it has the disadvantage that the write-back traffic to memory is increased. The benefit obtained by ECO accrues almost entirely to lines that are ECO only once or a few times. Thus, almost the entire benefit ECO can be obtained with reduced write-back traffic by adding a bit to the cache directories that records the fact that a line has been ECO. Then lines that have been subject to ECO would not be ECO again (since they obtain little benefit). This strategy results in a reduction of the average time a line is vulnerable and the increase in total traffic is reduced. On the other hand, since the lines that are ECO many times are lines that are resident in the cache for a long time it is exactly these lines that contain frequently referenced system data, and are the lines for which the vulnerability to soft errors is the most damaging. The ECO bit can also be used to decrease the vulnerability of these frequently used lines as follows. If a non-MRU line has the ECO bit on and there is a request to store in the line, the entire contents of the line can be read from the...