The 7210 SAS allows tuning the calculation of the Shared Buffer Average Utilization (SBAU) after assigning buffers for a packet entering a queue as used by the RED slopes to calculate a packet’s drop probability. It implements a time average factor (TAF) parameter in the buffer policy which determines the contribution of the historical shared buffer utilization and the instantaneous Shared Buffer Utilization (SBU) in calculating the SBAU. The TAF defines a weighting exponent used to determine the portion of the shared buffer instantaneous utilization and the previous shared buffer average utilization used to calculate the new shared buffer average utilization. To derive the new shared buffer average utilization, the buffer pool takes a portion of the previous shared buffer average and adds it to the inverse portion of the instantaneous shared buffer utilization (SBU). The formula used to calculated the average shared buffer utilization is shown in the following figure.
where:
SBAUn = Shared buffer average utilization for event n
SBAUn-1 = Shared buffer average utilization for event (n-1)
SBU = The instantaneous shared buffer utilization
TAF = The time average factor
The following table describes the effect the allowed values of TAF have on the relative weighting of the instantaneous SBU and the previous SBAU (SBAUn-1) has on the calculating the current SBAU (SBAUn).
| TAF | 2TAF | Equates to | Shared buffer instantaneous utilization portion | Shared buffer average utilization portion |
|---|---|---|---|---|
0 |
20 |
1 |
1/1 (1) |
0 (0) |
1 |
21 |
2 |
1/2 (0.5) |
1/2 (0.5) |
2 |
22 |
4 |
1/4 (0.25) |
3/4 (0.75) |
3 |
23 |
8 |
1/8 (0.125) |
7/8 (0.875) |
4 |
24 |
16 |
1/16 (0.0625) |
15/16 (0.9375) |
5 |
25 |
32 |
1/32 (0.03125) |
31/32 (0.96875) |
6 |
26 |
64 |
1/64 (0.015625) |
63/64 (0.984375) |
7 |
27 |
128 |
1/128 (0.0078125) |
127/128 (0.9921875) |
8 |
28 |
256 |
1/256 (0.00390625) |
255/256 (0.99609375) |
9 |
29 |
512 |
1/512 (0.001953125) |
511/512 (0.998046875) |
10 |
210 |
1024 |
1/1024 (0.0009765625) |
1023/2024 (0.9990234375) |
11 |
211 |
2048 |
1/2048 (0.00048828125) |
2047/2048 (0.99951171875) |
12 |
212 |
4096 |
1/4096 (0.000244140625) |
4095/4096 (0.999755859375) |
13 |
213 |
8192 |
1/8192 (0.0001220703125) |
8191/8192 (0.9998779296875) |
14 |
214 |
16384 |
1/16384 (0.00006103515625) |
16383/16384 (0.99993896484375) |
15 |
215 |
32768 |
1/32768 (0.000030517578125) |
32767/32768 (0.999969482421875) |
The value specified for the TAF affects the speed at which the shared buffer average utilization tracks the instantaneous shared buffer utilization. A low value weights the new shared buffer average utilization calculation more to the shared buffer instantaneous utilization. When TAF is zero, the shared buffer average utilization is equal to the instantaneous shared buffer utilization. A high value weights the new shared buffer average utilization calculation more to the previous shared buffer average utilization value. The TAF value applies to all high and low priority RED slopes for ingress and egress buffer pools controlled by the buffer policy.