System and method for bounding response time jitter for high priority

Abstract

Response time jitter for high priority commands in a multimedia datastreaming system is bounded by issuing only non-blocking I/Os and by limiting the number of concurrent SCSI bus commands. A device driver in the system bounds the maximum time a VCR command must wait after it is released from a VCR queue by limiting the number of I/Os on the bus to a predetermined number functionally related to the magnitude of jitter permitted in the deterministic response requirement, and the time required for each data I/O to be issued to the decoder card. The number is such that the I/Os may issue within the jitter time with enough time remaining for issuance of a high priority VCR I/O. By limiting the I/Os to a predetermined number, the I/Os issued are constrained to be non-blocking to avoid hanging the I/O scheduler when a deterministic hhigh priority command arrives. A consequence of allowing only a limited predetermined number of I/Os is that all I/Os issued must be non-blocking to avoid the scheduler hanging at a time when a deterministic command arrives. Accordingly, the driver tracks the state of each decoder card and, in particular, the number of free buffers which can accept input data. The driver thereby ensures that the decoder card will not block before issuing a write I/O.