Alcatel-Lucent 7750 SR OS Interface Configuration Manual page 461

buffer-allocation
Syntax buffer-allocation min percentage max percentage
no buffer-allocation
Context config>card>fp>egress>max-wred-control
Description
The buffer-allocation command defines the amount of IOM3-XP buffers that will be set aside for
WRED queue buffer pools. Note that the min percentage and max percentage parameters must be set
to the same value. The IOM3-XP protects against cross application buffer starvation by implementing
a hierarchy of buffer pools. At the top of the hierarchy are mega-pools. Mega-pools are used to
manage buffers at a system application level. Two mega-pools are currently used by the system. The
first (default) mega-pool services all non-WRED type queues and when WRED queues are not
enabled will contain all available forwarding plane queue buffers. When WRED queuing is enabled,
the second mega-pool (the WRED mega-pool) is given buffers from the default mega-pool based on
the buffer-allocation command and the size if further fine-tuned by the forwarding class
oversubscription factors.
The mega-pools provide buffers to the second tier buffer pools. The default mega-pool services all
default pools and explicitly created named pools. As the name implies, the WRED mega-pool
services all the WRED buffer pools created for the WRED queues. The WRED mega-pool allows
each WRED queue pool to be configured to an appropriate size while allowing the sum of the WRED
queue pool sizes to oversubscribe the total amount set aside for WRED queue buffering without
affecting the queues using the default or named pools. Further oversubscription controls are described
within the resv-cbs command later in this document.
The WRED mega-pool is allowed to expand between the min and max percent of total forwarding
plane buffers based on the sum of the WRED queue sizes and the WRED oversubscription factors. As
the WRED mega-pool grows, the number of buffers available to the default mega-pool will shrink. If
the WRED mega-pool shrinks, the default mega-pool will grow accordingly. When min and max are
defined as the same value, the WRED mega-pool size will not fluctuate and the oversubscription
factors will have no effect.
No buffers are allocated to the WRED mega-pool until the wred-queue-control shutdown command is
set to no shutdown. When the shutdown command is executed, all buffers allocated to the WRED
mega-pool are returned to the default mega-pool and all WRED queues are returned either to their
default buffer pool or their specified named buffer pool.
FC MBS Oversubscription Factors and WRED Mega-Pool Sizing
Each WRED queue in a SAP egress QoS policy is created on an egress IOM3-XP when the policy is
applied to an egress SAP on the IOM and at least one forwarding class is mapped to the queue. For
WRED queue buffer management purposes, each forwarding class is configured with an MBS
oversubscription factor (OSF) on the IOM using the osf command. The MBS oversubscription factor
is used by the system as a provisioning parameter that defines the acceptable level of oversubscription
between the sum of the maximum buffer sizes (mbs) of the WRED queues for a given class and the
number of buffers for that class in the WRED mega-pool. Since multiple forwarding classes may be
mapped to the same queue, the oversubscription factor associated with the highest forwarding class
mapped is used for dynamically sizing the WRED mega-pool.
As an example, when a WRED queue is configured with the following attributes:
7750 SR OS Interface Configuration Guide
• MBS equal to 10Kbytes
Interface Configuration
Page 461