Simulation of a Scheduling Algorithm Based on LFVC (Leap Forward Virtual Clock) Algorithm

Transcription

1 Simulation of a Scheduling Algorithm Based on LFVC (Leap Forward Virtual Clock) Algorithm CHAN-SOO YOON*, YOUNG-CHOONG PARK*, KWANG-MO JUNG*, WE-DUKE CHO** *Ubiquitous Computing Research Center, ** Electronic System R&D Division Korea Electronic Technology Institute 7 th fl., Good friend Bank B/D 270-2, Seohyun, Pundang, Sungnam, Kyunggi, Korea Abstract: - In this paper, we modified some operations of LFVC algorithm to implemant the LFVC based scheduling algorithm in the bufferd switch system. We analyzed traffic charateristics of our algorithm when we changed input rates and guaranteed rates of each packet flow. Our algorithm shows that it satisfy guaranteed rate of packet flow on various source traffic rates, and congested traffic also doesn t affect on other traffic s rate. In addition, delay of congested traffic increased linearly, but, it does not affect on delay of other traffic as well. Key-Words: - Leap, Forward, Virtual, Clock, Scheduling, Algorithm, Simulation, QoS 1 Introduction To use fair share of limited network resources and to enable seamless transfer of real time multimedia data, QoS guarantee of data traffic is the most important one. To guarantee such a QoS, an efficient traffic management technique is needed in network devices. There are many techniques in traffic management area such as, congestion control and congestion avoidance, admission control, scheduling, resource reservation, and so on. And, scheduling algorithm is one of the most important parts of traffic management. Until now, many papers of scheduling algorithm have been proposed [1-6]. Generally, scheduling algorithm has some desirable features such as isolation of flow, bounded delay, fairness, simplicity of implementation, etc. The LFVC algorithm is regareded as an efficient algorithm that satisfies many desirable features of scheduling algorithms [1]. The LFVC algorithm is work conserving scheduling algorithm based on virtual clock scheduling algorithm which assigns tag value and services each packet with non decreasing order of that tag value [2]. The tag value represents the clock value of system at which the system must transmit the packet. The LFVC algorithm also needs assignment of guaranteed rate of each packet flow by means of prior call setup. Also, the buffered switch is the popular system architecture in switch system which handles input traffic under 5 Gbps [7]. It has some disadvantages that it needs fast switching block and high-speed memory operations. But, it also has useful advantages such as, high throughput, bounded queue length, packet to the different don t interfere each other, may provide QoS guarantee, etc. In this paper, we modified some operations of LFVC algorithm to implemant the LFVC based scheduling algorithm in the bufferd switch system. We analysed traffic charateristics of our algorithm when we changed input rates and guaranteed rates of each packet flow. This paper is organized as follows. In chap. 2, a pseudo code of LFVC algorithm was presented. In chap. 3 & 4, our applied system & details of modified LFVC algorithm was presented. In chap. 5, simulation and the results was presented. And finally, conclude in chap. 5 2 LFVC algorithm Pseudo code for LFVC algorithm and the related parameter are shown in table 1 and 2. Due to lack of space, we omit detailed explanation of LFVC algorithm. Please refer to [1] for details. B l(p) t s t f rate of server packet length of the packet p current server time current tag of flow f

2 r f f τ ρ Q f guaranteed rate of flow f the time needed to send the largest packet of a flow at its guaranteed rate (= l max f / r f ) the time to transmit a largest packet M in server at the server rate (= M / B) rounded tag FIFO queue for flow f Table 1 Related parameters port and buffers are placed in each port. According to packet s destination, incoming packets, which are switched by switch, are stored in each buffer. As shown in Fig. 2, each buffer is composed of 2 FIFOs and 2 priority queues. H_buffer stands for high priority queue and L_buffer stands for low priority queue respectively. Input Switch Output buffer Output ProcessHead(Q f ) if (Q f is empty) return; p head(q f ); t f T(p) max(t s, t f ) + l(p)/r f ; if (t f t s + f + τ + ρ) then Insert(H, p, t f ); else Insert(L, p, t f f); End Enqueue(*A new Packet P f arrives*) AddToTail(Q f ) if (Q f was empty) then ProcessHead(Q f ) Dequeue(*The Server is idle and there is a packet in the system.*) kmin MinKey(L) Let f be the flow corresponding the key k min. if (H is empty) then ts = max(t s, k min - ρ) (*Leap Forward*) while (kmin < t s + τ + ρ) p f ExtractMin(L); Insert(H, p f, T(t f )); (* Transfer*) kmin MinKey(L); end pf ExtractMin(H); t s t s + l(p f )/B; (*Start Service) RemovefromTail(Q f ) ProcessHead(Q f ); Transmit p f ; (*Real time elapses*) Table 2 LFVC algorithm pseudo code[1] 3 Output Buffered Switch System Our algorithm will be applied to the buffer switch system shown in Fig. 1. It displays one-way direction only. A switch is located right behind input In Classify Fig. 1 Output buffered switch system High Priority FIFO Low Priority FIFO Tagging H_buffer L_buffer scheduler Fig. 2 Output buffer Mux In buffer, incoming packets are classified by packet classifier according to packet s priority and then stored in high priority FIFO and low priority FIFO respectively. High priority FIFO, which stores high priority packets, stores time critical data such as alarm signals. Low priority FIFO stores low priority packets. Low priority packet is general data traffic and it also includes real-time multimedia traffic. The low priority packet, which passes through low priority FIFO, passes scheduler, and then, is transmitted to port. On the other hands, high priority packets bypass the scheduler. 4 Our Algorithm Figure 3, 4 show the flow chart of our algorithm. In this section, dequeue operation of the existing LFVC algorithm is modified in order to apply the LFVC scheduler to the buffered switch system in chap. 3. As shown in table 2, the dequeue operation of the existing LFVC scheduler is executed when the system is idle and packet is on the system. Also, the existing LFVC scheduler have one ts, which stands for Out

3 current server time, and updates the ts according to dequeue operation rate. In that case, the ts is increased with agregated rate of total packet flows the scheduler received, not with each packet flow s guaranteed rates. So, the existing scheduler do not know exactly how much each packet flow violates it s guaranteed rate. Also, the dequeue operation of the existing LFVC scheduler includes both ts update part and packet transmission part. In that case, if packet transmission rate are changed, for examle, due to temporal link congestion, ts update rate also are changed. Therefore, it also makes do not know exactly how much each packet flow violates it s guaranteed rate and change of the conditions, packet transfer to L_buffer and packet transfer from H_buffer. However, in our algorithm, each flow has it s own ts respectively, and the ts is updated according to each packet flow s assigned guaranteed rate. Also, dequeue operation is divided into two parts, ts_update and p_. In this paper, rounded tag value, ρ, for minimizing computational complexity when actually implementing the algorithm and compensation value, τ, for discrete server clock are not considered for simplicity[1]. The related parameters for the flowcharts are shown in Table 3. rf delta_f ts tf guaranteed rate of flow f the time needed to send a packet of flow f at its guaranteed rate (= packet length / r f ) current server time current tag of flow f Table 3 The related parameters 4.1 Enqueue flow chart As shown in Figure 3, enqueue operation is executed whenever the new packet is passed through low priority FIFO of the buffer and comes into the scheduler. Whenever a packet enters, tf, which stands for virtual clock tag of the packet, is increased with the value of delta_f. Therefore, it increases in proportion to incoming packet rate. Also, when the system is on, ts is initialized to 0, and increases its value with delta_f at the guaranteed rate of each packet flow. The expression max(ts,tf) shown in Figure 3 is to avoid credit accumulation in virtual clock algorithm[2]. We see that virtual clock tag, tf is driven either by incoming packets or by the ts, the current server time, whichever is faster. start packet receive tf = max(ts, tf) + delta_f tf ts + delta_f? Enqueue in H_buffer Fig. 3 Enqueue Enqueue in L_buffer By sudden increase of packets corresponded to a packet flow f, tf, virtual clock tag of a packet flow f, becomes larger than ts, which increases in proportion to guaranteed rate rf of flow f. It means that the corresponding packet flow f violated the contract, which is supposed to send to guaranteed rate rf. In this case, all packets corresponding packet flow f are transfered to L_buffer. On the other hand, if the packets corresponding flow f are decreasing, therefore, the rate of flow f is equal or less than guaranteed rate, then subsequent packets of flow 1 are transfered to H_buffer. 4.2 Dequeue flow chart Dequeue operation is divided into two parts: ts_update and p_. The ts_update operates according to guaranteed rate of packet flow and p_ operates according to rate of port. First of all, ts_update checks whether a packet exists in L_buffer or not. If there is no packet in L_buffer, it will search for a packet in H_buffer. If there are more than one packet in H_buffer, the packet which has minimum tag value among other packet in H_buffer. If there are packets in L_buffer, the procedure will pass through block-(1) in fig. 4. The block-(1) will be explained later. After block-(1), it searches for a packet to transfer from L_buffer to H_buffer. For this procedure, key value is required. Key value is the minimum value of virtual clock tag of packets within L_buffer subtracted by delta_f. This key value is compared with the ts. If the value of key is greater or equal to the ts, then, it means that the time currently remains are as much as current delta_f time to transmit the corresponding packet to link with it s guaranteed rate. In this

4 case, the corresponding packet are transmitted from L_buffer to H_buffer. start L_buffer is empty? key = min(tag of packets in L_buffer - delta_f) H_buffer is empty? ts_update runs in high rate key < ts? transfer the packet from L_buffer to H_buffer ts = ts + delta_f key = min(tag of packets in L_buffer - delta_f) Fig. 4 ts_update ts_update runs in normal rate block-(1) For simulation, 10-8 sec system clock was used. And constant rate flows and a single ON-OFF flow are applied to our algorithm [8-9]. The maximum of rate is 10 Mbps and packet length is fixed size of 100 bytes. The length of H_buffer and L_buffer of priority queues are not limited. And processing delay of enqueue and dequeue operation are not considered. The high rate operation clock of block-(1) are set up by 10-7 sec. 5.2 Simulation results Constant rate input Input Source: 2 constant rate source. case 1: flow1: Mbps, flow2: Mbps case 2: flow1: Mbps, flow2: 3.34 Mbps case 3: flow1: 3.34 Mbps, flow2: 2.50 Mbps start A packet exist in H_buffer? the packet in H_buffer which has minimum tag value Fig. 5 p_ block-(1) in ts_update is designed to guarantee the work conserving property of scheduling algorithm. It changes the operation frequency of ts_update. If H_buffer empty, then, a lot of packets in L_buffer are moved to H_buffer by increasing the operation frequency of ts_update. If H_buffer is not empty, ts_update operates at the packet flow s guaranteed rate. 5 Simulation 5.1 Simulation environment For simulation environment, we assumed that two packet flows are connected to single input port, and these packet flows have the same destination, pass through the scheduler, and to single port. We examined rate, delay of each packet flow, which is generated from port via scheduling algorithm, and the effect of congested packet flow, when setting up the guaranteed rate rf and changing the input rate of each packet flow. When we change the guaranteed rate rf of packet flow 1 and 2 in each case, the rates of each packet flow are shown in Table 4. From the result of 1~11 in Table 4, rate *, which stands for non-work conserving rate, it is without block-(1), is bounded to rf. Output rate +, which stands for work conserving rate, it is with block-(1), shows that more higher rates. In rate +, if there are packets in L_buffer when H-buffer is empty, then these packets are sent to port through H_buffer. Case 1 is congested with both input traffic flow 1 and 2. Case 2 is congested with traffic flow 1 only. Output rate * and rate + of each flow in case 1 and case 2 show fairness that the complying traffic does not affected by any congetsed traffic. case rf 6.60, 3, , , , 2.00 rate * 6.64, , , , 1.99 rate , , , , 4.64 case rf 6.60, 3, , , , 2.00 rate * 6.64, , , , 1.99 rate , , , , 3.34 * rate without block-(1) in ts_update + rate with block-(1) in ts_update

5 case rf , , , 2.00 rate * , , ,1.99 rate , , ,2.49 Table 4 The result of constant rate input Delay characteristics Fig. 7, 8 shows the mean delay of packets when we fix the input rate of flow 2 with 2 Mbps, and increase the input rate of flow 1. Input conditions and rates are shown in Table 5. Fig. 7 does not include block-(1), but, fig. 8 includes block-(1). Delay in this figures are the same as D(p) of Worst-case Fairness in Bennet & Zhang[1]. It is calculated by the arrival time to scheduler subtracted from the departure time of scheduler. The packet delays of each input rate are averaged on 10-4 sec interval. flow 1 flow2 input rate from 3.34 to 9.10 Mbps 2.00 Mbps rf 4.00 Mbps 2.00 Mbps rate * 3.94 Mbps 1.99 Mbps rate Mbps 1.99 Mbps Table 5 Input conditions and rates sample times and rate(peak rate) is 300 sample times. One sample time is 10-8 sec. Fig. 7 Delay characteristics Fig. 8 Delay characteristics Fig. 7 shows that the delay of flow 1 increases rapidly, when the input rate of flow 1 exceedes 4 Mbps, which corespond guranted rate rf of flow 1. But it does not effect on the delay of flow 2. In fig. 8, the delay of flow 1 rather decresed in comparision with fig. 7, because of block-(1) effects. Also does not effect on the delay of flow ON-OFF source input Input Source: single ON-OFF source mean rate: 4.67 Mbps, peak rate: 10 Mbps rf: 5 Mbps Fig. 9 ON-OFF source input & In order to analyze characteristics of our algorithm, when we apply the burst traffic, ON-OFF source is used for input traffic. Without block-(1), rate(mean rate) is plotted vs. time in fig. 9 and rate(peak rate) in fig. 10. Observation interval of rate(mean rate) is 2000

6 It will be also needed that scheduling algorithm is implemented as a form of software stack or chipset under real network environment and verification and comparison of simulation result with actual result. Fig. 10 ON-OFF source input & As shown in fig. 9, 10, both rate(mean rate) and rate(peak rate) are bounded within 5 Mbps, which is guaranteed rate rf. 5. Conclusion So far, we analyzed the rates, the delays and the effects of congested traffic, of our algorithm when we applied various rates of input traffic. Our algorithm shows that it satisfy guaranteed rate of each packet flow on various input traffic rates, and congested traffic also doesn t affect on other traffic s rates. In addition, the delay of congested traffic increased linearly with input rate, but, it does not affect on the delay of other traffic as well. In real implementation, however, the congested traffic will make the increase of system processing time, which including, both enqueue and dequeue time of L_buffer. Therefore, it is expected that the delay characteristics of other complied traffic will be degraded. If the delay is on real-time interactive traffic, for example, MoIP(Multimedia over IP), the effect on the delay could be serious. Considering this situation, if traffic congestion is happening, and the delay of MoIP traffic beyonds certain one-way delay bound, then, the methods such as congestion control and congestion avoidance are recommended to prevent congested flows from entering in system and with this, we can guarantee fairness of other flows, and maintain precious system resources. Consequently, as a future work, it will be needed to analyze the characteristics of scheduler regarding various input rates and types with the methods of congestion control and congestion avoidance. References [1] S. Suri, G. Varghese, G. Chandranmenon, Leap Forward Virtual Clock: A New Fair Queuing Scheme With Guaranteed Delays and Throughput Fairness, in Proc. of IEEE INFOCOM, [2] L. Zhang, VirtualClock: A New Traffic Control Algorith for Packet Switching Networks, ACM Transactions on Computer Systems, Vol. 9, [3] A. K. Parekh, R. G. Gallager, A Generized Processor Sharing Approach to Flow Control: The Single de Case, in Proc. of IEEE INFOCOM, [4] S. J. Golestani, A Self Clocked Fair Queueing Schem for High Speed Applications, in Proc. of IEEE INFOCOM, [5] A. Demers, S. Keshav, S. Shenker, Analysis and Simulation of a Fair Queuing Algorithm, in Proc. of Sigcomm 89, [6] J. Bennett, H. Zhang, Worst-case Fair Weighted Fair Queueing, in Proc. of IEEE INFOCOM, 1996 [7] L. Levendovszky, P. Boros, A. Molnar, Development of Home Station on a Chipest Congestion Control Algorithms, Technical Report of Budapest University of Technology & Economics, [8] A. Bhadra, M. N. O. Sadiku, Simulation of an ATM Network Using an ON-OFF Model, Southeastcon in Proc. of the IEEE, [9] Z. Zhifei, Q. Zhengding, A vel Approach For Real Time Equivalent Bandwidth Estimation, ICCT(International Conference on Communication Technologies) at 16th IFIP World Computer Congress, 2000.