Samsung Ubigate iBG3026 Configuration Manual page 232

CHAPTER 19. QoS
buffers1 = (2 * bbr) / average_packet_size; /* 2*bbr */
buffers2 = (4 * bcr) / average_packet_size; /* 4*bcr */
buffers3 = (6 * bcr) / average_packet_size; /* 6*bcr */
reserved_buffers_for_class = MAX(MIN(buffers3, MAX(buffers1, buffers2)),
QOS_MIN_RES_CLASS_Q_SIZE);
Where, QOS_MIN_RES_CLASS_Q_SIZE is 5.
As explained above, a maximum of(6 * BCR / average_packet_size) buffers
will be used up, in total, for the reserved buffer pools of all the classes on the
interface. The remaining amount of(4 * BCR / average_packet_size) is used to
create the shared buffer pool. On the other hand the minimum guaranteed
number of reserved buffers is not less than(4 * BCR) for all classes on the
interface. The remaining amount of(6 * BCR / average_packet_size) is used to
create the shared buffer pool.
A single class by itself should not be allowed to borrow all of the available
common pool buffers. So we need to enforce a maximum limit on the buffer
consumption of each class. Since we have allocated(10 * BCR
average_packet_size) buffers for the entire interface, we should, at least, allow
each class to use its fair share of(10 * bcr / average_packet_size) buffers(out
of which [6 * bcr / average_packet_size] have been reserved for it). However,
the buffer allocation for a class should also take into consideration the Burst
Rate(BR) configured for the class. If classes are configured with a very small
CR but BR = interface bandwidth, the above amount of buffers might not be
enough for the classes. Therefore, we allow a class to use up to a maximum
of(3 * bbr / average_packet_size) buffers by borrowing from the common
pool. In addition to supporting the BR, this will accommodate a burst of(2 *
scheduler interval) 10 ms at a rate of BR kbps.(For packet sizes larger than
average_packet_size bytes, the burst duration
will be even longer). In this case the maximum buffers that a traffic class can
loan from the common pool is as shown below.
Max loaned buffers = (3*bbr / average_packet_size) – reserved_buffers
For a class with BR = interface BR and CR~0,
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