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IXP1200 Network Processor Family ATM OC-3/12/Ethernet IP Router Example Design
Application Note 45
Modified on: 3/20/02,
4.7 Buffer Descriptor Queues - bdq.uc
This design uses a generic buffer descriptor queuing subsystem to pass data between microengines.
This section describes the facility so that it will be clear when it is applied throughout the design.
Buffer Descriptor Queues (BDQs) are analogous to packet queues, as defined in packetq.uc and
tx.uc. BDQs support cached dequeues, and are therefore more efficient when a microengine
dequeues from a small number of queues.
4.7.1 BDQ Management Macros
Buffer descriptor queue management macros are used for queueing SRAM buffer descriptors
between microengines.
4.7.1.1 Features
4.7.1.2 Limitations
For the dequeue front of queue to be cached by the dequeuing microengine, a single microengine
must be assigned to dequeue from each queue, and must have three available absolute registers.
... ; process the message, threads may get out of order.
move(message, $xfer)
sequence_wait(MY_SEQUENCE) ; wait until it is my turn to send
msgq_send(message, $xfer, MY_MSGQ, ctx_swap)
.if (message != 0)
counter_inc(OUTPUT_MSGQ_IS_FULL) ; record failure
buf_push(message, ...)
; if message is descriptor, return it...
.endif
sequence_exit(MY_SEQUENCE)
; allow next thread through sequence_wait()
Feature Description
Arbitrary queue capacity
Queues are implemented via a linked list of buffer descriptors in SRAM.
These lists can grow to any size up to a configurable water mark, or the
enqueuing microengine exhausts its supply of available buffers.
High water marks (HWMs)
and low water marks (LWMs)
The queue handle has settings for LWMs and HWMs to manage queue
length. bdq_enqueue() will reject all enqueues when the queue size is above
the HWM. bdq_enqueue() will reject a handle-specified ratio of the enqueues
when queue length is between LWM and HWM.
Non-blocking simultaneous
enqueue and dequeue
If the queue has more than 1 entry, then the dequeuing thread can perform a
"cached deqeueue" where it not only doesn’t contend for the lock on the
queue header, it doesn’t read the queue header at all
Empty queue notification
The dequeuing threads have the option of sleeping on an inter-thread signal if
the queue is empty.