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|United States Patent Application
Rosenbluth, Mark B.
;   et al.
June 19, 2003
Congestion management for high speed queuing
A device such a network device implemented as a hardware-based
multi-threaded processor is described. The processor can execute a
computer instruction that is a write queue descriptor count instruction
that causes the instruction sequence to write a single word containing a
queue count for each of a number of queue entries in a queue array cache.
Rosenbluth, Mark B.; (Uxbridge, MA)
; Bernstein, Debra; (Sudbury, MA)
; Wolrich, Gilbert; (Framingham, MA)
FISH & RICHARDSON, PC
4350 LA JOLLA VILLAGE DRIVE
December 17, 2001|
|Current U.S. Class:
||711/154; 711/108; 711/160; 712/E9.032; 712/E9.053 |
|Class at Publication:
||711/154; 711/108; 711/160 |
What is claimed is:
1. A computer instruction comprises: a write queue descriptor count
instruction that causes a processor to write a single word containing a
queue count for each of a plurality of queue entries in a queue array
2. The instruction of claim 1 further comprising: an address field that
specifies a location in memory of a queue descriptor.
3. The instruction of claim 1 further comprising: an entry field that
specifies a location of a queue descriptor in the queue array cache.
4. A method comprising: in a processor, maintaining a count field for
queue descriptors of active output queues current in a memory.
5. The method of claim 4 in which the count field is a word.
6. The method of claim 4 further comprising: writing the count field
subsequent to incrementing a count of buffers for a selected queue.
7. The method of claim 4 further comprising: writing the count field
subsequent to decrementing a count of buffers for a select queue.
8. The method of claim 4 in which the count fields for queues descriptors
are stored in a queue array cache.
9. Apparatus comprising: a memory containing queue descriptors
representing output queues, a queue manager programming engine and a
content addressable memory (CAM); a processor connected to the memory,
the processor containing a memory controller, the memory controller
having a cache containing a queue descriptor array for storing a subset
of the queue descriptors; and an array in memory for storing a count of
queue descriptors in the subset.
10. The apparatus of claim 9 further comprising: a plurality of
11. A computer program product residing on a computer readable medium
having instructions stored thereon which, when executed by the processor,
cause the processor to: maintain a count field for queue descriptors of
active output queues current in a memory.
12. The computer program product of claim 11 in which the count field is a
13. The computer program product of claim 11 further comprising
instructions to: write the count field subsequent to incrementing a count
of buffers for a selected queue.
14. The computer program product of claim 11 further comprising
instructions to: write the count field subsequent to decrementing a count
of buffers for a selected queue.
 This invention relates to congestion management for high speed
 Some network devices such as routers and switches have line speeds
that can be faster than 10 Gigabits. For maximum efficiency the network
devices should be able to process data packets, including storing them to
and retrieving them from memory at a rate at least equal to the line
rate. Network devices implement congestion avoidance algorithms such as
Weighted Random Early Discard (WRED) to preserve chip resources and to
regulate packet flow by probabilistically dropping packets as output
queue lengths increase beyond predefined limits. The count of packets or
buffers for each queue should be observable for all output queues.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a block diagram of a network system.
 FIG. 2 is a block diagram of a network device used in the system of
 FIG. 3 is a block diagram of an output queue.
 FIG. 4 is a block diagram of a datapath in a processor.
 FIG. 5 is a block diagram of entries in a CAM device to track queue
 FIG. 6 is a flow diagram of a queue description update process.
 Referring to FIG. 1, a network system 10 for processing data
packets includes a source of data packets 12 coupled to a network device
14 and a destination for data packets 16 coupled to the network device
14. The network device 14 includes a processor 18 and a memory 20 having
memory data structures 22 configured to receive, store and forward the
data packets to a specified destination. Example network devices 14 are
network switches, network routers and other network devices. The source
of data packets 12 can include, for example, other network devices (not
shown) connected over a communications path (not shown) operating at high
data packet transfer line speeds. Examples of such communications paths
include as an example, an optical carrier (OC)-192 line or a 10-Gigabit
Ethernet line. The destination of data packets 16 may also include other
network devices as well as a similar network connection.
 Referring to FIG. 2, the network device 14 includes memory 20
coupled to the processor 18. The memory 20 provides output queues 22 and
their corresponding queue descriptors 24 in a queue array 26. The memory
20 includes a queue manager programming engine 27 and Content Addressable
Memory (CAM) 28.
 Upon receiving a data packet from the source 12 (of FIG. 1), the
processor 16 performs enqueue and dequeue operations to process the
packet. An enqueue operation adds information that has arrived in a data
packet to one of the output queues 22 and updates its corresponding queue
descriptor 24. A dequeue operation removes information from one of the
output queues 22 and updates the corresponding queue descriptor 24,
allowing the network device 14 to transmit the information to the
appropriate destination 16.
 Enqueue and dequeue operations for a large number of output queues
22 in memory 20 at high bandwidth line rates can be accomplished by
storing some of the queue descriptors 24 in a cache 42 at the processor's
memory controller 44. Commands to perform enqueue or dequeue operations
check whether queue descriptors 24 corresponding to the enqueue or
dequeue commands are stored in the cache 42. When an enqueue or a dequeue
operation is required with respect to a queue descriptor 24 that is not
in the cache 42 (a cache miss), the processor 18 issues commands to the
memory controller 44 to move a queue descriptor 24 from the cache 42 to
the memory 20 and to fetch a new queue descriptor 24 from memory 20 for
storage in the cache 42. In this manner, modifications to a queue
descriptor 24 made by enqueue and dequeue operations occur in the cache
42 and are copied to the corresponding queue descriptor 24 in memory 20
upon removal of that queue descriptor 24 from the cache 42.
 A sixteen entry CAM 28 with a Least Recently Used (LRU) replacement
policy is used to track sixteen queue descriptors 24 that are cached in a
queue array 46 of the memory controller 44.
 Using a network device 14 implemented as hardware-based
multi-threaded processor having multiple microengines (not shown), each
CAM entry stores a 32 bit value. Microengines each maintain a plurality
of program counters in hardware and states associated with the program
counters. Effectively, a corresponding plurality of sets of threads can
be simultaneously active on each of the microengines while only one is
actually operating at any one time. During a lookup operation CAM entries
are compared against a source operand. All entries are compared in
parallel, and the result of the lookup is a 6-bit value. The 6-bit result
includes a 2-bit code concatenated with 4-bit entry number. Possible
results of the lookup are three fold. A first result is a miss where the
lookup value is not in the CAM 28 and the entry number is the Least
Recently Used (LRU) entry which can be used as a suggested entry to
replace. The second result can be a hit where the lookup value is in the
CAM 28 and state bit is clear, and the entry number is an entry which has
matched. In addition, a locked result may occur where the lookup value is
in the CAM 28, the state bit is set and the entry number is an entry. The
state bit is a bit of data associated with the entry, used typically by
software. There is no implication of ownership of the entry by any
 Referring to FIG. 3, an example of an output queue 22 and its
corresponding queue descriptor 24 is shown. The output queue 22 includes
a linked list of elements each of which has a pointer 32 to the next
element 30 in the output queue 22. Each element in the linked list 30
includes an address 34 of information stored in memory 20 that the linked
list element represents. The queue descriptor 24 includes a head pointer
36, a tail pointer 38 and a count 40. The head pointer 36 points to the
first linked list element 30 of the queue 22, and the tail pointer 38
points to the last linked list element 30 of the output queue 22. The
count 40 identifies a number (N) of linked list elements 30 in the output
 Referring to FIG. 4, details of an arrangement of the CAM 28 in a
datapath 70 of the network device 10 implemented as a processor are
shown. A General Purpose Register (GPR) file 72 stores data for
processing elements 74. The CAM receives operands as any other processing
element 74 would. Operational code (Opcode) bits in an instruction select
which processing element 74 is to perform the operation specified by the
instruction. In addition, each of the processing elements 74, including
the CAM 28, can return a result value from the operation specified by the
instruction back to the GPR file 72.
 Referring to FIG. 5, a CAM 28 includes an array 76 of tags having a
width the same as the width of the GPR file 72. Associated with each of
the tags in the array are state bits 78. During a CAM lookup operation, a
value presented from the GPR file 72 is compared, in parallel, to each of
the tags in the array 76 with a resulting match signal 80 per tag. The
values in each tag were previously loaded by a CAM load operation. During
the CAM load operation the values from the GPR file 72 specify which of
the tags in the array 76 to load and a value to load. Also during the CAM
load operation the state information to load is part of the operand.
 The result of the CAM lookup is written to a destination GPR file
82 and includes three fields. A hit/miss indication field 84, an entry
number field 86 and a state information field 88. If a "hit" occurs, the
entry number field 86 is matched. In a "miss," the entry number field 86
is the Least-Recently-Used (LRU) entry.
 The following instructions are one example of instructions used to
manage and use the CAM 28:
 Load (Entry_Number, Tag_Value, State Value)
 Lookup (Lookup_Value, Destination)
 Set_State (Entry_Number, State_Value)
 Read_Tag (Entry_Number, Destination)
 Read_State (Entry_Number, Destination)
 The LRU Logic 90 maintains a time-ordered list of the CAM 28 entry
usage. When an entry is loaded or matches on a lookup, it is marked as
MRU (Most Recently Used). A lookup that misses does not modify the LRU
 If a queue descriptor 24 required for either an enqueue or dequeue
is not in the queue array 46, the queue manager programming engine 26
issues a write-back to memory of the LRU entry, followed by a fetch to
the same entry, before issuing the enqueue or dequeue command. If the CAM
28 lookup indicates that the needed queue descriptor 24 is already in the
queue array 46, then the enqueue or dequeue command is issued without
replacing an entry.
 Each enqueue command increments the count 40 of packets or buffers
for a particular output queue 22. A dequeue command decrements the count
40 of packets or buffers when a pointer to the buffer descriptor 24 at
the head of the output queue 22 is updated.
 The microengine (in a processor containing multiple microengines)
tasked with congestion avoidance reads the queue descriptors 24 from
memory 20 to determine the length (count word 40) of each output queue
22. The queue descriptors 24 for highly used output queues 22 can remain
in the queue array 46 of the memory controller 44 for an infinitely long
time period. A Write_Q_Descriptor_Count Command is issued by the queue
manager programming engine 26 after the enqueue or dequeue command, when
the entry used "hits" the CAM 28. the format of the command is:
 Write_Q_Descriptor_Count (address, entry).
 The command uses two parameters, i.e., address and entry, and keeps
the countfield 40 for all queue descriptors 24 current in memory 20 for
the microengine implementing congestion avoidance. The write of a single
word containing the queue count information for entries that hit in the
query array 46 in the cache 42 replaces a write-back of two or three
words when a new entry needs to be fetched.
 Referring to FIG. 6, a write queue descriptor process 100 includes
receiving (102) an address and a queue subsequent to an enqueue or
dequeue command. The process 100 maintains (104) a count field for all
queue descriptors current in memory for the microengine implementing
congestion avoidance. The process 100 writes (106) a single word
containing the queue count information for the queue entry that hits the
queue array in the cache.
 It is to be understood that while the invention has been described
in conjunction with the detailed description thereof, the foregoing
description is intended to illustrate and not limit the scope of the
invention, which is defined by the scope of the appended claims. Other
aspects, advantages, and modifications are within the scope of the
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