GB1373027A - Data processing systems - Google Patents

Abstract

1373027 Data processing systems INTERNATIONAL BUSINESS MACHINES CORP 18 Dec 1972 [18 Jan 1972] 58302/72 Heading G4A In a data processing system in which the time required for execution of "overhead" programs (i.e. programs such as tracing, measuring, recording, simulating programs not normally run on the system) is hidden from the "operating system" (i.e. the normal system programs and control functions), start I/O (SIO) instructions are trapped before machine execution, the time at which the requested I/O operation is started is recorded, I/O interrupt responses to each SIO instruction are intercepted, and each I/O interrupt is rescheduled by delaying it by an amount corresponding to the time used by overhead programs subsequent to the corresponding start I/O operation before presenting it to the operating system whereby the system performance evaluation being performed by the overhead programs is not degraded by the presence of the overhead programs themselves. To this end the interrupts are also further delayed in accordance with any time used by the overhead programs after rescheduling of the interrupts. The system hardware clock is reset to the value it had at the start of a burst of overhead program activity when return is made to the operating system, and the rescheduled interrupts are placed on a chain of future events with the time of the first future event to occur being set into the hardware clock to obtain a clock-generated interrupt at the appropriate delayed time. The overhead time hiding functions are incorporated in an interceptor program which maintains two software "clocks" embodied in memory words. One of these clocks is used to measure real time elapsed (i.e. overhead and non-overhead) while the other is used to measure simulated time elapsed (i.e. non-overhead only). Both clocks are used to determine the rescheduling delays for the I/O interrupts. Busy conditions are simulated for real as well as simulated devices to allow for the fact that although a real device is no longer truly busy after issuing its I/O interrupt signifying an end of I/O operation, the operating system may not "see" the interrupt immediately owing to the rescheduling of the interrupt.