Users Guide

The following table illustrates the buer usage statistics when shared headroom is not used and each queue is allocated with a xed
headroom buer space:
Table 20. Buer usage statistics when shared headroom is not used
Parameter Description
PFC Shared Buer Size 208 KB
Buer-size parameter of dcb-buer-threshold per
lossless queue
94KB
Pause-threshold parameter of dcb-buer-threshold
per lossless queue
18KB
The Headroom value reserved per lossless queue 94 – 18 = 76KB
NOTE: 76KB is the headroom space that is required per PG [or a lossless
queue] on a 40 Gigabit port in a worst case scenario to guarantee
lossless behavior.
Total buer required for 2 lossless queues on 12
ports
12*2*94 + 208 = 2464KB
NOTE: Out of the 2464KB, 1824KB [12*2*76KB] is reserved only for the
headroom, which might never be utilized in most of the cases. An ideal
approach is to increase the PFC shared buer and reduce the buer
reserved for headroom. However, this approach subsequently reduces
the time to trigger PFC, thereby, reducing the eectiveness of PFC.
In the shared headroom feature, the main assumption is that not every PG uses the headroom buer at the same time. This approach
enables the system to save the headroom buer space that is reserved for every PG to guarantee lossless delivery during trac bursts. For
each PG, you can assign a lower value for headroom buer. This headroom buer is sucient enough to guarantee lossless behavior as this
buer is global and is shared among all the lossless queues.
The following table depicts the buer usage statistics when shared headroom is used:
Table 21.
Buer usage statistics when shared headroom is used
Parameter Description
PFC Shared buer size 208 KB
Buer-size parameter of dcb-buer-threshold per
lossless queue
18KB
Pause-threshold parameter of dcb-buer-threshold
per lossless queue
18KB
Average Headroom value reserved per lossless
queue
35KB [Considering 76KB as worst case value]
Total Headroom buer to be congured in Shared
Headroom Pool
12*2*35 = 840KB
Total buer required for 2 lossless queues on 12
ports
12*2*18 + 208 + 840 = 1480KB
Shared PG headroom buer approach saves 984KB from the traditional dedicated PG headroom approach for the same number of lossless
queues. The excess buer can either be assigned to PFC shared buer size (to reduce the PFC aggressiveness) or to the lossy pool to
eectively manage lossy trac.
Data Center Bridging (DCB)
281