AM79C970 datasheet(162/168 Pages) AMD

162 / 168 page

AMD

1-1029

Am79C970

An average increase in performance can be achieved if

the general guidelines of buffer sizes in figure 2 is fol-

lowed. However, as was noted earlier, the correct sizing

for buffers will depend upon the expected message size.

There are two problems with relating expected message

size with the correct buffer sizing:

1. Message sizes cannot always be accurately pre-

dicted, since a single application may expect differ-

ent message sizes at different times, therefore, the

buffer sizes chosen will not always maximize

throughput.

2. Within a single application, message sizes might

be somewhat predictable, but when the same

driver is to be shared with multiple applications,

there may not be a common predictable message

size.

Additional problems occur when trying to define the cor-

rect sizing because the correct size also depends upon

the interrupt latency, which may vary from system to

system, depending upon both the hardware and the

software installed in each system.

In order to deal with the unpredictable nature of the mes-

sage size, the driver can implement a self tuning mecha-

nism that examines the amount of time spent in tasks S5

and S7 as such: while the driver is polling for each de-

scriptor, it could count the number of poll operations per-

formed 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 the 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 adjusted,

buffer sizes for buffer number 2 and buffer 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 ar-

rives under a certain size limit, the driver should not ad-

just the buffer sizes in response to the short frame.

An Alternative LAPP Flow – the 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 look-

ing for the SRP interrupt as was 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.

Note that 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 descrip-

tors, then there is a net loss in performance with this

method. Therefore, the LAPP method implemented

should be carefully chosen.

AM79C970 датащи(PDF) 162 Page - Advanced Micro Devices

AM79C970 датащи(HTML) 162 Page - Advanced Micro Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL