AM79C978VCSLASHW datasheet(248/261 Pages) AMD

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B-8

Am79C978

mechanism that examines the amount of time spent in

tasks S5 and S7. As such, while the driver is polling for

each descriptor, it could count the number of poll oper-

ations performed and then adjust the number 1 buffer

size to a larger value, by adding “t” bytes to the buffer

count, if the number of poll operations was greater than

”x.” If fewer than “x” poll operations were needed for

each of S5 and S7, then software should adjust the

buffer size to a smaller value by subtracting “y” bytes

from the buffer count. Experiments with such a tuning

mechanism must be performed to determine the best

values for “x” and “y.”

Note: Whenever the size of buffer number 1 is ad-

justed, buffer sizes for buffer number 2 and buffer num-

ber 3 should also be adjusted.

In some systems, the typical mix of receive frames on

a network for a client application consists mostly of

large data frames, with very few small frames. In this

case, for maximum efficiency of buffer sizing, when a

frame arrives under a certain size limit, the driver

should not adjust the buffer sizes in response to the

short frame.

An Alternative LAPP Flow: Two-Interrupt

Method

An alternative to the above suggested flow is to use two

interrupts, one at the start of the receive frame and the

other at the end of the receive frame, instead of just

looking for the SRP interrupt as described above. This

alternative attempts to reduce the amount of time that

the software wastes while polling for descriptor own

bits. This time would then be available for other CPU

tasks. It also minimizes the amount of time the CPU

needs for data copying. This savings can be applied to

other CPU tasks.

The time from the end of frame arrival on the wire to de-

livery of the frame to the application is labeled as frame

latency. For the one-interrupt method, frame latency is

minimized, while CPU utilization increases. For the

two-interrupt method, frame latency becomes greater,

while CPU utilization decreases. See Figure B-3.

Note: Some of the CPU time that can be applied to

non-Ethernet tasks is used for task switching in the

CPU. One task switch is required to swap a non-Ether-

net task into the CPU (after S7A) and a second task

switch is needed to swap the Ethernet driver back in

again (at S8A). If the time needed to perform these task

switches exceeds the time saved by not polling de-

scriptors, then there is a net loss in performance with

this method. Therefore, the LAPP method imple-

mented should be carefully chosen.

Figure B-4 shows the buffer sizing for the two-interrupt

method. Note that the second buffer size will be about

the same for each method.

There is another alternative which is a marriage of the

two previous methods. This third possibility would use

the buffer sizes set by the two-interrupt method, but

would use the polling method of determining frame

end. This will give good frame latency but at the price

of very high CPU utilization. And still, there are even

more compromise positions that use various fixed

buffer sizes and, effectively, the flow of the one-inter-

rupt method. All of these compromises will reduce the

complexity of the one-interrupt method by removing the

heuristic buffer sizing code, but they all become less ef-

ficient than heuristic code would allow.

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