Abstract:
A scheduler for a network processor includes a scheduling queue in which weighted fair queuing is applied. The scheduling queue has a range R. Flows are attached to the scheduling queue at a distance D from a current pointer for the scheduling queue. The distance D is calculated for each flow according to the formula D=((WF×FS)/SF), where WF is a weighting factor applicable to a respective flow; FS is a frame size attributable to the respective flow; and SF is a scaling factor. The scaling factor SF is adjusted depending on a comparison of the distance D to the range R.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]    The present application is related to the following U.S. patent applications Ser. No., each of which is hereby incorporated by reference herein in its entirety:  
         [0002]    U.S. patent application Ser. No. ______, filed ______, titled “WEIGHTED FAIR QUEUE HAVING EXTENDED EFFECTIVE RANGE” (IBM Docket No. ROC920010199US1);  
         [0003]    U.S. patent application Ser. No. ______, filed ______, titled “WEIGHTED FAIR QUEUE SERVING PLURAL OUTPUT PORTS” (IBM Docket No. ROC920010200US1);  
         [0004]    U.S. patent application Ser. No. ______, filed ______, titled “EMPTY INDICATORS FOR WEIGHTED FAIR QUEUES” (IBM Docket No. ROC920010202US1);  
         [0005]    U.S. patent application Ser. No. ______, filed ______, titled “QoS SCHEDULER AND METHOD FOR IMPLEMENTING PEAK SERVICE DISTANCE USING NEXT PEAK SERVICE TIME VIOLATED INDICATION” (IBM Docket No. ROC920010203US1);  
         [0006]    U.S. patent application Ser. No. ______, filed ______, titled “QoS SCHEDULER AND METHOD FOR IMPLEMENTING QUALITY OF SERVICE WITH AGING STAMPS” (IBM Docket No. ROC920010204US1);  
         [0007]    U.S. patent application Ser. No. ______, filed ______, titled “QoS SCHEDULER AND METHOD FOR IMPLEMENTING QUALITY OF SERVICE WITH CACHED STATUS ARRAY” (IBM Docket No. ROC920010205US1); and  
         [0008]    U.S. patent application Ser. No. ______, filed ______, titled “QoS SCHEDULER AND METHOD FOR IMPLEMENTING QUALITY OF SERVICE ANTICIPATING THE END OF A CHAIN OF FLOWS” (IBM Docket No. ROC920010206US1). 
     
    
     
       FIELD OF THE INVENTION  
         [0009]    The present invention is concerned with data and storage communication systems and is more particularly concerned with a scheduler component of a network processor.  
         BACKGROUND OF THE INVENTION  
         [0010]    Data and storage communication networks are in widespread use. In many data and storage communication networks, data packet switching is employed to route data packets or frames from point to point between source and destination, and network processors are employed to handle transmission of data into and out of data switches.  
           [0011]    [0011]FIG. 1 is a block diagram illustration of a conventional network processor in which the present invention may be applied. The network processor, which is generally indicated by reference numeral  10 , may be constituted by a number of components mounted on a card or “blade”. Within a data communication network, a considerable number of blades containing network processors may be interposed between a data switch and a data network.  
           [0012]    The network processor  10  includes data flow chips  12  and  14 . The first data flow chip  12  is connected to a data switch  15  (shown in phantom) via first switch ports  16 , and is connected to a data network  17  (shown in phantom) via first network ports  18 . The first data flow chip  12  is positioned on the ingress side of the switch  15  and handles data frames that are inbound to the switch  15 .  
           [0013]    The second data flow chip  14  is connected to the switch  15  via second switch ports  20  and is connected to the data network  17  via second network ports  22 . The second data flow chip  14  is positioned on the egress side of the switch  15  and handles data frames that are outbound from the switch  15 .  
           [0014]    As shown in FIG. 1, a first data buffer  24  is coupled to the first data flow chip  12 . The first data buffer  24  stores inbound data frames pending transmission of the inbound data frames to the switch  15 . A second data buffer  26  is coupled to the second data flow chip  14 , and stores outbound data frames pending transmission of the outbound data frames to the data network  17 .  
           [0015]    The network processor  10  also includes a first processor chip  28  coupled to the first data flow chip  12 . The first processor chip  28  supervises operation of the first data flow chip  12  and may include multiple processors. A second processor chip  30  is coupled to the second data flow chip  14 , supervises operation of the second data flow chip  14  and may include multiple processors.  
           [0016]    A control signal path  32  couples an output terminal of second data flow chip  14  to an input terminal of first data flow chip  12  (e.g., to allow transmission of data frames therebetween).  
           [0017]    The network processor  10  further includes a first scheduler chip  34  coupled to the first data flow chip  12 . The first scheduler chip  34  manages the sequence in which inbound data frames are transmitted to the switch  15  via first switch ports  16 . A first memory  36  such as a fast SRAM is coupled to the first scheduler chip  34  (e.g., for storing data frame pointers and flow control information as described further below). The first memory  36  may be, for example, a QDR (quad data rate) SRAM.  
           [0018]    A second scheduler chip  38  is coupled to the second data flow chip  14 . The second scheduler chip  38  manages the sequence in which data frames are output from the second network ports  22  of the second data flow chip  14 . Coupled to the second scheduler chip  38  are at least one and possibly two memories (e.g., fast SRAMs  40 ) for storing data frame pointers and flow control information. The memories  40  may, like the first memory  36 , be QDRs. The additional memory  40  on the egress side of the network processor  10  may be needed because of a larger number of flows output through the second network ports  22  than through the first switch ports  16 .  
           [0019]    [0019]FIG. 2 schematically illustrates conventional queuing arrangements that may be provided for a data flow chip/scheduler pair (either the first data flow chip  12  and the first scheduler chip  34  or the second data flow chip  14  and the second scheduler chip  38 ) of the network processor  10  of FIG. 1. In the particular example illustrated in FIG. 2, the first data flow chip  12  and the first scheduler chip  34  are illustrated, but a very similar queuing arrangement may be provided in connection with the second data flow chip  14  and the second scheduler chip  38 . In the queuing arrangement for the first data flow chip  12  and the first scheduler chip  34 , incoming data frames (from data network  17 ) are buffered in the input data buffer  24  associated with the first data flow chip  12  (FIG. 1). Each data frame is associated with a data flow or “flow”. As is familiar to those who are skilled in the art, a “flow” represents a one-way connection between a source and a destination.  
           [0020]    Flows with which the incoming data frames are associated are enqueued in a scheduling queue  42  maintained in the first scheduler chip  34 . The scheduling queue  42  defines a sequence in which the flows enqueued therein are to be serviced. The particular scheduling queue  42  of interest in connection with the present invention is a weighted fair queue which arbitrates among flows entitled to a “best effort” or “available bandwidth” Quality of Service (QoS).  
           [0021]    As shown in FIG. 2, the scheduling queue  42  is associated with a respective output port  44  of the first data flow chip  12 . It is to be understood that the output port  44  is one of the first switch ports  16  illustrated in FIG. 1. (However, if the data flow chip/scheduler pair under discussion were the egress side data flow chip  14  and scheduler chip  38 , then the output port  44  would be one of the network ports  22 .) Although only one scheduling queue  42  and one corresponding output port  44  are shown, it should be understood that in fact there may be plural output ports and corresponding scheduling queues each assigned to a respective port. (However, according to an alternative embodiment, disclosed in co-pending patent application Ser. No. ______, filed ______ (Attorney Docket No. ROC920010200US1), a group of output ports may be associated with each scheduling queue  42 . This co-pending patent application is incorporated herein by reference.)  
           [0022]    Although not indicated in FIG. 2, the first scheduler chip  34  also includes flow scheduling calendars which define output schedules for flows which are entitled to a scheduled QoS with guaranteed bandwidth, thus enjoying higher priority than the flows governed by the scheduling queue  42 .  
           [0023]    The memory  36  associated with the first scheduler chip  34  holds pointers (“frame pointers”) to locations in the first data buffer  24  corresponding to data frames associated with the flows enqueued in the scheduling queue  42 . The memory  36  also stores flow control information, such as information indicative of the QoS to which flows are entitled.  
           [0024]    When the scheduling queue  42  indicates that a particular flow enqueued therein is the next to be serviced, reference is made to the frame pointer in the memory  36  corresponding to the first pending data frame for the flow in question and the corresponding frame data is transferred from the first data buffer  24  to an output queue  46  associated with the output port  44 .  
           [0025]    A more detailed representation of the scheduling queue  42  is shown in FIG. 3. As noted above, the scheduling queue  42  is used for weighted fair queuing of flows serviced on a “best effort” basis. In a particular example of a scheduling queue as illustrated in FIG. 3, the scheduling queue  42  has 512 slots (each slot represented by reference numeral  48 ). Other numbers of slots may be employed. In accordance with conventional practice, flows are enqueued or attached to the scheduling queue  42  based on a formula that takes into account both a length of a data frame associated with a flow to be enqueued and a weight which corresponds to a QoS to which the flow is entitled.  
           [0026]    More specifically, the queue slot in which a flow is placed upon enqueuing is calculated according to the formula CP+((WF×FS)/SF), where CP is a pointer (“current pointer”) that indicates a current position (the slot currently being serviced) in the scheduling queue  42 ; WF is a weighting factor associated with the flow to be enqueued, the weighting factor having been determined on the basis of the QoS to which the flow is entitled; FS is the size of the current frame associated with the flow to be enqueued; and SF is a scaling factor chosen to scale the product (WF×FS) so that the resulting quotient falls within the range defined by the scheduling queue  42 . (In accordance with conventional practice, the scaling factor SF is conveniently defined as a integral power of 2—i.e., SF=2 n , with n being a positive integer—so that scaling the product (WF×FS) is performed by right shifting.) With this known weighted fair queuing technique, the weighting factors assigned to the various flows in accordance with the QoS assigned to each flow govern how close to the current pointer of the queue each flow is enqueued. In addition, flows which exhibit larger frame sizes are enqueued farther from the current pointer of the queue, to prevent such flows from appropriating an undue proportion of the available bandwidth of the queue. Upon enqueuement, data that identifies a flow (the “Flow ID”) is stored in the appropriate queue slot  48 .  
           [0027]    In some applications, there may be a wide range of data frame sizes associated with the flows, perhaps on the order of about 64 bytes to 64 KB, or three orders of magnitude. It may also be desirable to assign a large range of weighting factors to the flows so that bandwidth can be sold with a great deal of flexibility and precision. In practice, however, it is difficult to predict at the time of designing or initializing the scheduler chip  34  what will be the characteristics of the data packets handled by the scheduler chip  34 . Consequently, it is difficult to anticipate over what range of values the product (WF×FS) will fall during operation of the network processor  10 . As a result, the scaling factor SF may be chosen to be a value that is too large or too small. If the value of SF is chosen to be too small, then the enqueuement distance D=((WF×FS)/SF) may overrun the range R of the scheduling queue  42 . If this occurs, an error condition may result, or the enqueuement distance D may be reduced to equal the range R of the scheduling queue  42 , resulting in a failure to properly perform the desired weighted fair queuing.  
           [0028]    If the scaling factor SF is chosen to be too large, then all of the flows to be enqueued may be attached relatively close to the current pointer of the scheduling queue  42 . As a result, the full resources of the range of the scheduling queue  42  may not be used, again possibly resulting in a failure to precisely perform the desired weighted fair queuing.  
           [0029]    It would accordingly be desirable to overcome the potential drawbacks of setting the scaling factor SF either too low or too high.  
         SUMMARY OF THE INVENTION  
         [0030]    According to an aspect of the invention, a scheduler for a network processor is provided. The scheduler includes a scheduling queue in which weighted fair queuing is applied. The scheduling queue has a range R. Flows are attached to the scheduling queue at a distance D from a current pointer for the scheduling queue. The distance D is calculated for each flow according to the formula D=((WF×FS)/SF), where WF is a weighting factor applicable to a respective flow; FS is a frame size attributable to the respective flow; and SF is a scaling factor. The scaling factor SF is adjusted depending on a result of comparing the distance D to the range R.  
           [0031]    In at least one embodiment, the scaling factor SF may be increased if D is greater than R. For example, the scaling factor SF may be increased if D exceeds R in regard to a predetermined number of calculations of D.  
           [0032]    In one or more embodiments, the scaling factor SF may be decreased if D is less than R/2. For example, the scaling factor SF may be decreased if D is less than one-half R in regard to a predetermined number of calculations of D.  
           [0033]    In some embodiments, the scaling factor SF may equal 2 n , where n is a positive integer. For example, n may be incremented to increase SF, or may be decremented to decrease SF.  
           [0034]    According to another aspect of the invention, a method of managing a scheduling queue in a scheduler for a network processor is provided. The scheduling queue has a range R. Flows are attached to the scheduling queue at a distance D from a current pointer for the scheduling queue, the distance D being calculated for each flow according to the formula D=((WF×FS)/SF), where WF is a weighting factor applicable to a respective flow, FS is a frame size attributable to the respective flow, and SF is a scaling factor. The method includes calculating the distance D with respect to a particular flow to be enqueued, comparing the distance D to the range R, and adjusting the scaling factor SF based on a result of the comparing step.  
           [0035]    In a scheduler provided in accordance with the invention, an initial value at which the scaling factor SF is set may be adjusted adaptively during operation of the scheduler to reflect actual experience with data handled by the scheduler, so that the scaling factor SF assumes a value that is suitable for using the range R of the scheduling queue and/or such that the enqueuement distance D does not overrun the range R of the scheduling queue.  
           [0036]    Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of exemplary embodiments, the appended claims and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    [0037]FIG. 1 is a block diagram of a conventional network processor in which the present invention may be applied;  
         [0038]    [0038]FIG. 2 is a block diagram representation of conventional queuing arrangements provided in a data flow chip/scheduler pair included in the network processor of FIG. 1;  
         [0039]    [0039]FIG. 3 is a pictorial representation of a weighted fair queuing scheduling queue provided in accordance with conventional practices;  
         [0040]    [0040]FIG. 4 is a flow chart that illustrates a process provided in accordance with the invention to increase the value of a scaling factor when a range of a scheduling queue is overrun;  
         [0041]    [0041]FIG. 5 is a flow chart that illustrates a process provided in accordance with the invention to decrease the value of a scaling factor when a range of a scheduling queue is underutilized;  
         [0042]    [0042]FIGS. 6A and 6B together form a flow chart that illustrates a process provided in accordance with the invention to increase and/or decrease a scaling factor in response to overrunning or underutilizing a range of a scheduling queue;  
         [0043]    [0043]FIG. 7A is a block diagram of an inventive scheduler for use with the processes of FIGS. 4 and 5; and  
         [0044]    [0044]FIG. 7B is a block diagram of an inventive scheduler for use with the processes of FIGS. 6A and 6B. 
     
    
     DETAILED DESCRIPTION  
       [0045]    Adjustment of a scaling factor SF of a scheduler in accordance with the invention will now be described, initially with reference to FIG. 4 and the conventional network processor  10  of FIGS.  1 - 3 . It will be understood that the present invention may be employed with any suitable conventional network processor.  
         [0046]    [0046]FIG. 4 is a flow chart that illustrates a process provided in accordance with the invention for increasing the value of a scaling factor SF in response to overrunning the range R of the scheduling queue  42  (FIG. 2). In the particular example of the scheduling queue  42  described above, the range R of the scheduling queue  42  corresponds to the number of slots  48 , i.e. R=512. Other ranges may be employed. In accordance with the inventive process of FIG. 4, and as described further below, the schedulers  34  and/or  38  may be provided with a counter C 0  as shown in FIG. 7A. The counter C 0  may comprise any conventional counter, whether hardware or software based.  
         [0047]    Initially in FIG. 4 is block  50 , at which a flow is attached to the scheduling queue  42  using the current value of the scaling factor SF. That is, the enqueuement distance D is calculated according to the conventional formula D=((WF×FS)/SF). Enqueuement can occur in one of two ways. The first way is a “new attach” situation, in which, for a flow having no frames corresponding to it, a new frame arrives, and the flow is attached to the scheduling queue  42  in response to arrival of the new frame. The second way is a “reattach” situation, in which a flow is already enqueued in the scheduling queue  42  and is picked as a winner (because it is closest to the head of the queue and no higher priority service intervenes), a frame is dispatched with respect to the flow, and the flow is rescheduled on the scheduling queue  42  because there is at least one more frame to be dispatched from the flow.  
         [0048]    Following block  50  is decision block  52 . In decision block  52  it is determined whether the enqueuement distance D exceeded (overran) the range R of the scheduling queue  42 . If not, the procedure of FIG. 4 simply returns (block  54 ) so that the scheduling queue  42  may perform conventional queue operations (not described).  
         [0049]    However, if it is determined at decision block  52  that the enqueuement distance D overran the range R of the scheduling queue  42 , then block  56  follows decision block  52 . At block  56 , a value of the counter C 0  (FIG. 7A) is incremented. Any suitable counter may be employed (e.g., a hardware or software based counter).  
         [0050]    Following block  56  is decision block  58 . At decision block  58 , it is determined whether the incremented counter value exceeds a predetermined threshold. This threshold (and other thresholds discussed below) can be set in a variety of ways. For example, the threshold can be determined by software if the software has information concerning the flows/frames to be handled. If so, the scaling factor SF can be set accurately based on the flows/frames that are expected. The software would then set the threshold to handle flows that misbehave. For example, if it is not desired to tolerate an occasional frame that causes the enqueuement distance D to exceed the range R, then the threshold may be set to zero. If system requirements allow some misbehaving flows to be tolerated, then the threshold may be set higher.  
         [0051]    If the software has no information concerning the flows/frames that to be handled, then an arbitrary value for the initial value of the scaling factor SF can be chosen, and the threshold can be set so that the scaling factor SF is increased rapidly if the range R of the scheduling queue  42  is exceeded. (A threshold for decreasing the scaling factor SF, to be discussed below, may be set so that the scaling factor SF is decreased slowly if the flows are all being scheduled in the lower part of the scheduling queue  42 .) These threshold values would allow the system to quickly adapt to unknown input.  
         [0052]    If a positive determination is made at decision block  58 , the procedure returns (block  54 ). However, if it is determined at decision block  58  that the predetermined threshold is exceeded by the incremented counter value, then block  60  follows decision block  58 .  
         [0053]    At block  60  the value of the scaling factor SF is increased. This may be done in a number of ways. For example, if the scaling factor SF is expressed as an integral power of 2 (i.e., 2 n ), then the scaling factor SF may be doubled by incrementing the value of n (e.g., via a left shifting operation as previously described, such as left shifting a register (not shown) in which the scaling factor is stored). It is contemplated, alternatively, to increase SF by a factor other than two.  
         [0054]    Following block  60  is block  62  at which the counter C 0  is reset. The procedure of FIG. 4 then returns (block  54 ) so that the scheduling queue  42  may perform conventional queue operations (not described).  
         [0055]    It will be appreciated that the procedure of FIG. 4 operates so that when the range of the scheduling queue  42  is overrun a certain number of times (e.g., as set by the predetermined threshold), the value of the scaling factor SF is increased, to reduce the likelihood of overrunning the range of the scheduling queue  42  in the future. Thus the initial value of the scaling factor SF can be set at a low value, and the scheduler  34  (FIG. 2) can be allowed, in operation, to increase the value of the scaling factor SF to adapt to the actual characteristics of the data traffic, so that, after an initial period, overrunning of the range of the scheduling queue  42  does not occur.  
         [0056]    [0056]FIG. 5 is a flow chart that represents a procedure provided in accordance with the invention for decreasing the value of the scaling factor SF of the scheduler  34  of FIG. 2 in response to underutilization of the range of the scheduling queue  42 . As with the procedure of FIG. 4, the procedure of FIG. 5 may be employed with other schedulers and/or scheduling queues, and employs the counter C 0  (FIG. 7A).  
         [0057]    The procedure of FIG. 5 begins with block  70  which is like block  50  of FIG. 4 (e.g., a flow is attached to the scheduling queue  42  using the current value of the scaling factor SF during calculation of the enqueuement distance D). Following block  70  is a decision block  72 . At decision block  72  it is determined whether the enqueuement distance D calculated in block  70  is less than one-half the range R of the scheduling queue  42 . If the enqueuement distance D is found to be less than one-half the range R of the scheduling queue  42 , then block  74  follows decision block  72 . (If the scaling factor SF is decreased by a factor other than two, then the enqueuement distance D is advantageously to be compared to something other than one-half of the range R. For example, if the scaling factor SF is to be decreased by a factor of 4, then the enqueuement distance D may be compared to one-fourth of the range R.)  
         [0058]    At block  74  a value of the counter C 0  is incremented. Following block  74  is decision block  76  at which it is determined whether the incremented counter value is greater than a predetermined threshold. If not, the procedure of FIG. 5 returns (block  78 ). However, if it is found at decision block  76  that the incremented counter value exceeds the predetermined threshold, then block  80  follows decision block  76 .  
         [0059]    At block  80  the value of the scaling factor SF is decreased. The decreasing of the value of the scaling factor SF may occur in a number of ways. For example, if the scaling factor SF is expressed as a power of 2 (i.e., 2 n ) then the scaling factor SF may be halved by decrementing n (e.g., by right shifting a register (not shown) in which the scaling factor is stored). It is contemplated, alternatively, to decrease the scaling factor SF by a factor other than two.  
         [0060]    Following block  80  is block  82 , at which the counter C 0  is reset. The procedure of FIG. 5 then returns (block  78 ).  
         [0061]    Considering again decision block  72 , if it is determined at that decision block that the enqueuement distance D is not less than one-half the range R of the scheduling queue  42 , then block  84  follows decision block  72 . At block  84  the counter C 0  is reset, and the procedure of FIG. 5 then returns (block  78 ). The counter C 0  is reset because, if the upper part of the scheduling queue  42  is ever used, then the scaling factor SF will not be too large.  
         [0062]    With the procedure of FIG. 5, the value of the scaling factor SF can be set to a high value, in anticipation of a wide range of enqueuement distances that may be encountered during processing of data frames. In the event that the high value of the scaling factor leads to underutilization of the range of the scheduling queue, the procedure of FIG. 5 will adaptively decrease the value of the scaling factor to a value that is well suited to the actual characteristics of the data that is being processed.  
         [0063]    [0063]FIGS. 6A and 6B together form a flow chart that illustrates a procedure provided in accordance with the invention and by which the value of the scaling factor SF of the scheduler  42  of FIG. 2 can be either increased or decreased to adapt to characteristics of the data handled by the network processor  10 . In accordance with the inventive process of FIGS. 6A and 6B, and as described further below, the schedulers  34  and/or  38  may be provided with a first counter C 1  and a second counter C 2  as shown in FIG. 7B. The counters C 1  and C 2  may comprise any conventional counters, whether hardware or software based.  
         [0064]    Initially in the procedure of FIGS. 6A and 6B is a block  90 , which entails the same activity as block  50  of FIG. 4 (e.g., a flow is attached to the scheduling queue  42  using the current value of the scaling factor SF during calculation of the enqueuement distance D). Following block  90  is a decision block  92  at which it is determined whether the enqueuement distance D is greater than the range R of the scheduling queue  42 . If it is determined at decision block  92  that the enqueuement distance D exceeded the range R of the scheduling queue  42 , then a value of the first counter C 1  (FIG. 7B) is incremented (block  94 ). Following block  94  is a decision block  96 . At decision block  96  it is determined whether the value of the first counter C 1  is greater than a first threshold. If not, then the procedure returns (block  98 ). However, if it is determined at decision block  96  that the value of the first counter C 1  exceeds the first threshold, then the value of the scaling factor SF is increased (block  100 ). This may be done, for example, by incrementing the value of n, where SF is expressed as 2 n , or by any other technique.  
         [0065]    Following block  100  is block  102 . At block  102  the first counter C 1  is reset. The second counter C 2  (FIG. 7B) also is reset. (As will be seen, the second counter C 2  is involved with determining whether to decrease the value of the scaling factor SF in response to underutilization of the range R of the scheduling queue  42 .) Following block  102  the procedure of FIGS. 6A and 6B returns (block  98 ).  
         [0066]    Considering again decision block  92 , if it is determined at decision block  92  that the enqueuement distance D is not greater than the range R of the scheduling queue  42 , then decision block  104  (FIG. 6B) follows decision block  92 . At decision block  104  it is determined whether the enqueuement distance D is less than one-half of the range R of the scheduling queue  42 . If the enqueuement distance D is less than one-half the range R, then block  106  follows decision block  104 .  
         [0067]    At block  106 , the value of the second counter C 2  is incremented. Following block  106  is decision block  108 , at which it is determined whether the value of the second counter C 2  is greater than a second threshold. If not, the procedure returns (block  98 ). However, if it is determined at decision block  108  that the value of the second counter C 2  is greater than the second threshold, then block  110  follows decision block  108 . At decision block  110  the value of the scaling factor SF is decreased. This may be done, for example, by decrementing n where SF is expressed as 2 n , or by any other technique.  
         [0068]    Following block  110  is block  112 . At block  112  the first and second counters C 1 , C 2  are reset. The procedure then returns (block  98 ).  
         [0069]    Considering again decision block  104 , if it is determined at decision block  104  that the enqueuement distance D is not less than one-half the range R of the scheduling queue  42 , then block  114  follows decision block  104 . At block  114  the second counter C 2  is reset. The procedure of FIGS. 6A and 6B then returns (block  98 ).  
         [0070]    In one embodiment of the procedure of FIGS. 6A and 6B, the scaling factor SF may initially be set at 2 7  (i.e.,  128 ). The first threshold may be set to be 0 (i.e., the scaling factor SF is increased each time the range R is overrun), and the second threshold may be set to be 8 (i.e. 9 consecutive enqueuements in the lower half of the scheduling queue  42  result in decreasing the scaling factor SF).  
         [0071]    In the procedure of FIGS. 6A and 6B, the scaling factor SF may be set at an intermediate value or an arbitrary value, and the scheduler  34  (when configured in accordance with the present invention) then operates to adapt the scaling factor SF, by either increasing or decreasing the value of the scaling factor SF, as required in response to characteristics of the data being processed. This aspect of the invention also makes it unnecessary to attempt to predict the characteristics of the data to be processed upon initially setting the value of the scaling factor.  
         [0072]    A scheduler configured in accordance with the present invention can also adapt to changes in a stream of data by increasing or decreasing the scaling factor SF as the situation requires. Thus the scheduler may, for example, increase the scaling factor SF during an initial period of operation, then may decrease the scaling factor SF in response to a change in the pattern of data traffic, and further may increase the scaling factor SF again in response to another change in the pattern of data traffic.  
         [0073]    Noting again that plural scheduling queues (e.g., 64) may be maintained in the inventive scheduler, it should be understood that respective scaling factors SF of the scheduling queues are advantageously to be adjusted independently of one another. Consequently, in a typical situation in accordance with the invention, different values of scaling factors are applicable to different scheduling queues at any given time.  
         [0074]    The processes of FIG. 4- 6 B may be implemented in hardware, software or a combination thereof. In at least one embodiment of the invention, the processes of FIGS.  4 - 6 B are implemented in hardware employing a suitable combination of conventional logic circuitry such as adders, comparators, selectors, etc. Such hardware may be located, for example, within the scheduler  34  and/or the scheduler  38  (FIG. 2). A person of ordinary skill in the art may develop logic circuitry capable of performing the inventive processes described with reference to FIGS.  4 - 6 B. In a software embodiment of the invention, the processes of FIGS.  4 - 6 B may comprise one or more computer program products. Each inventive computer program product may be carried by a medium readable by a computer (e.g., a carrier wave signal, a floppy disk, a hard drive, a random access memory, etc.).  
         [0075]    The foregoing description discloses only exemplary embodiments of the invention; modifications of the above disclosed apparatus and methods which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. According to one alternative embodiment, a scheduling queue may have plural subqueues of different ranges and resolutions, according to an invention disclosed in co-pending patent application Ser. No. ______, filed ______ (Attorney Docket No. ROC920010199US1). This co-pending patent application is incorporated herein by reference.  
         [0076]    Moreover, in the above description, the invention has been implemented in a separate scheduler chip associated with a network processor. However, it is also contemplated to implement the invention in a scheduler circuit that is implemented as part of a data flow chip or as part of a processor chip.  
         [0077]    Furthermore, in accordance with above-disclosed embodiments of the invention, reduction of the scaling factor SF has been triggered by underutilization of the range of the scheduling queue, where underutilization has been effectively defined as attaching flows repeatedly in the lower half of the scheduling queue. It is alternatively contemplated, however, to define underutilization of the range of the scheduling queue in other ways. For example, underutilization may be deemed to have occurred upon repeated attachment of flows in the lower third or lower quarter of the scheduling queue.  
         [0078]    Accordingly, while the present invention has been disclosed in connection with exemplary embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.