Average response time

Average response time is the average amount of time a requester must wait before a request for a global resource can be granted. It depends on cycle time — the amount of time that the RSA-message requires to make a complete cycle around the ring. In tuning ring performance, the primary goal is to ensure that the average response time is as close as possible to the acceptable response time at your installation.

One of the factors used to calculate average response time is the transmission delay for a global resource serialization signal. Table 1 shows the transmission delays for the global resource serialization signalling paths.

Table 1. Transmission Delays by Signalling Path and Device
Signalling Path	Device	Transmission Delay (in milliseconds)
Global resource serialization	ESCON® channel in basic mode	0.5
XCF	ESCON channel in CTC or basic mode	0.5

You can use the following formulas to calculate average response time and predict the effect of changes you might make. The formulas you need depend on whether your ring is using ring acceleration.

If the ring is using ring acceleration, then n is the number of systems, and a is the ACCELSYS value. The calculations for cycle time and average response time are:

cycle time = (RESMIL * n) + (transmission delay * n)
response time = cycle time/2 + (RESMIL * (a - 1)) + (transmission delay * a)

Because the configuration includes a 3088, the transmission delay is about 1 millisecond for the RSA-message and 1 millisecond for the ring acceleration signal. Assuming ACCELSYS(2), the average response time is about 7 milliseconds, as follows:

cycle time = (1 millisecond * 4) + (1 millisecond * 4)
response time = 8 milliseconds/2 + 1 millisecond + 2 milliseconds

If the ring is not using ring acceleration, n is the number of systems, and the calculations for cycle time and average response time are:

cycle time = (RESMIL * n) + (transmission delay * n)
response time = cycle time * 1.5

In this case, the average response time is about 12 milliseconds, as follows:

cycle time = (1 millisecond * 4) + (1 millisecond * 4)
response time = 8 milliseconds * 1.5

Experience has shown that the cycle time is a stable value; it does not normally vary significantly during any given time period. Resource contention, however, can add to the response time. That is, average response time measures only the time a global resource requester waits for ring processing to complete. It does not reflect the time spent waiting until a requested resource becomes available (this delay is not directly related to ring processing).