Browse Prior Art Database

Channel Delays due to CPU Cache Back Ups

IP.com Disclosure Number: IPCOM000052592D
Original Publication Date: 1981-Jun-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 39K

Publishing Venue

IBM

Related People

Court, JF: AUTHOR [+2]

Abstract

In certain systems data currently resident in a CPU cache buffer is not transferred into main storage until the storage controls execute a page "cast-out" operation. If an I/O channel issues a fetch request to main storage for data which is currently resident in such a cache buffer, e.g., for a CCW (Channel Command Word), the request is placed in a queue, pending later completion of an associated cast-out operation. The storage controls must later re-issue the request. Since the period for completion of such cast-out operation is necessarily of indefinite duration, the channel is susceptible of experiencing operational overrun during this period (with resulting degradation of channel and system performance).

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Channel Delays due to CPU Cache Back Ups

In certain systems data currently resident in a CPU cache buffer is not transferred into main storage until the storage controls execute a page "cast-out" operation. If an I/O channel issues a fetch request to main storage for data which is currently resident in such a cache buffer, e.g., for a CCW (Channel Command Word), the request is placed in a queue, pending later completion of an associated cast-out operation. The storage controls must later re-issue the request. Since the period for completion of such cast-out operation is necessarily of indefinite duration, the channel is susceptible of experiencing operational overrun during this period (with resulting degradation of channel and system performance).

To reduce the time required for such "un-cast-out" data to reach a requesting channel, system storage controls may be adapted to route such data to the requesting channel "on the fly", as it is being written into main storage, during a page cast-out operation. Consequently, when the the requested data is a CCW, the requesting channel may proceed to execute the operation specified by that CCW, while the cast-out operation is still in process relative to the remainder of the page containing that CCW. If such a CCW indicates chaining to a "next" CCW located in the same page, the next CCW will usually be resident in main storage by the time the channel reaches the point in its operating sequence at which it issues its fetc...