Samsung Ubigate iBG2016 Configuration Manual page 237

The minimum # of buffers required for one class = bbr/100 = 20 buffers
With 20 classes on the bundle,(20 * 20) = 400 buffers would be required for
the bundle. With 14 bundles on a t3 interface, the total buffers requirement
would be(400 * 14) = 5600 buffers! Each buffer occupies about 2000 bytes of
memory, including all the control information, which results in a total memory
requirement of(5600 * 2000) = 11.2 Mbytes! This is the minimum buffer
requirement and does not provide any burst tolerance. Moreover, this memory
requirement increases in direct proportion with the number of additional
classes.
One would tend to think that having a shared pool of buffers with a capacity
of BCR bytes is enough to transmit line rate of traffic. That will work only if
the traffic entering each queue(classes) is proportional to its CR. This is
mostly not the case and which is why we do bandwidth management. It is
possible for traffic entering a class to be at a much higher rate than other
classes, because of which it will gain access to majority of the buffers and
hence most of the bandwidth.
Buffer Requirements Analysis and Buffer Management
Scheme
A more economical scheme can be developed by making a calculated
compromise on the buffer guarantee(availability) for a class. In most cases,
not all classes would simultaneously need to transmit at their BR. Which
means only a few classes, at any given time needs a buffering capacity of bbr
bytes. If all classes are over subscribed and need more than their CR at the
same time, then there is not going to be any unused bandwidth and so each
class can send only bcr bytes in a scheduling interval.(The previous argument
is based on the assumption that the class CRs sum up to the interface
bandwidth). Based on the previous observations, a buffer management scheme
has been devised which is a combination of a 'shared buffer pool' and
'reserved buffer pools' for each class. This provides each class with a
minimum guarantee of buffers at all times(enough to support the CR) with the
option to borrow additional buffers when needed from a common pool.
This results in more efficient use of buffers by allowing sharing of a portion of
the buffers which the classes don't always need.
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