A Cost-Effective Technique

8/8/2019 A Cost-Effective Technique

1/17

A Cost-Effective Technique to

Reduce HOL Blocking in Single-

Stage andMultistage Switch Fabrics

NITHIN KUNJAPPAN

S7 CSE,No:32


2/17

CONTENTS

INTRODUCTION

PACKET DESTINATION LOCALITY

DESTINATION-BASED BUFFER MANAGEMENT

SHARED-QUEUE BUFFER MANAGEMENT

PERFORMANCE EVALUATION

CONCLUSION

REFERENCES


3/17

INTRODUCTION

Head-of-line (HOL) blocking is one of the main problemsarising in high-speed switches, due to the use of FIFOqueues, leading to severe throughput degradation.

Virtual Output Queing(VOQ)-using as many queues ineach IA as output ports in the switch, one for each port. Cost is high. Destination Based Buffer Management(DBBM)-using a

relatively small set of buffers at each IA together with anew buffer management strategy.

Cost effective and efficient. IP routers.


4/17

HOL BLOCKING


5/17

Traffic across current IP routers is not uniformlydistributed over time and output ports.

Mainly 2 types of localities1.Temporal->correlation between packet destinations.

2.Spatial->most packets are destined to a few ports.

Temporal and spatial locality in packet destination lead

to an inefficient use of VOQs.

PACKET DESTINATION LOCALITY


6/17

DESTINATION-BASED BUFFER

MANAGEMENT(DBBM) Main idea is to use a small number of queues in each IA

(a fraction of the number of ports in the switch fabric),together with a suitable mapping scheme that promotes aproper utilization of those queues and exploits locality.

In DBBM, packets are stored in queues sorted bydestination.

It allows queue sharing Family of Buffer Management Strategies. Parameters

1. Queue Sharing2. Mapping function.3. Replacement4. Restoration.


7/17

1. Queue Sharing

indicates whether packets with different destinations can

be concurrently stored in the same queue or not. If yeseach buffer behaves as FIFO otherwise behaves ascaches. Overflow buffer is also used.

2. Mapping function

computes the queue where an incoming packet will bestored.

3. Replacement

indicates whether packets can be removed from a queuewhen an incoming packet requests that queue.

4. Restoration.

This parameter indicates whether packets the overflowbuffers can move back to a normal queue when itbecomes free.


8/17

Family of Buffer Management Strategies


9/17

SHARED-QUEUE BUFFER

MANAGEMENT

Works on queue sharing.

Each queue can only store packets for a subset ofdestination ports.

Can be viewed as if the physical output ports of therouter were grouped into a smaller set of logical outputports and a queue was used at each IA to allocate onlypackets destined to a particular logical output port.

The total number of queues and the number ofdestinations mapped per queue (the size of the logical

port) reduce the HOL blocking effect to the minimum.


10/17

When a packet arrives, a mapping algorithm computesthe logical output port and, therefore, the queue that willbe used to store that packet, as a function of itsdestination.

It can be done simply by removing some bits from theoutput port address in the packet header.

Mapping may be cyclic or block mapping.


11/17

PERFORMANCE EVALUATION

Simulation Model

The simulator models the Input Adapters (IA) and theswitch fabric of a router.The IA is modeled with Nqueues,a mapping module that implements the mapping

function. Prior to entering the IA, messages are packetized in 64-

byte packets. Each incoming packet is mapped to a queue using the

shared-queue buffer management technique.

The queue is computed using the following expression.Queue= OP MOD N,

OP =output port of the router requested by the packet,N =number of queues at each IA,MOD = modulo function.


12/17

IAs with 1, 8, 64, and 512 queues have been evaluatedwith a 512-port switch fabric.

Here also (BT) Base time compression factor representshigher traffic demands. Here 2*BT and 8*BT are used.

VOQ and Random Mapping Evaluation

In this section we evaluate the performance obtained bythe VOQ scheme in a 512-port router with 512 IAs.

Each IA has 512 queues and the VOQ technique isused.

The random mapping function with 1, 8 and 64 queuesat each IAs is evaluated.

Here the maximum and minimum throughput obtained bythe router is calculated.


13/17

DBBM Evaluation

In this section we evaluate the DBBM technique for

different base time compression factors. When base time compression is 8*BT DBBM with 64

queues achieves roughly the same throughput as theVOQ scheme.

Even with 8 queues, DBBM achieves much better results

than when using random mapping. Here efficient link bandwidth is achieved. A reduced number of queues together with the random

mapping function achieves roughly the same throughputachieved by the VOQ scheme. Only a small fraction of

queues (one eighth in one case) are required. Here may be the existence of HOL blocking at somequeues in each IA but it does not significantly degradeperformance.


14/17

Throughput achieved by the different schemes


15/17

CONCLUSION

HOL blocking is one of the main problems arising ininput-buffered switch fabrics. Although VOQs are able toeliminate HOL blocking, this solution is not scalable andrepresents a very high cost and locality issues.

In DBBM a reduced number of queues together with the

random mapping function achieves roughly the samethroughput achieved by the VOQ scheme.

The simplest scheme in the DBBM family consists ofcyclically mapping packets to queues based on theirdestination (modulo mapping).

In particular,with only 64 queues(1/8) the throughput isroughly the same as with VOQ in a 512-port router.


16/17

REFERENCES

T. Anderson, S. Owicki, J. Saxe, C. Thacker, HighSpeed Switch scheduling for local area networks, ACMTransactions on Computer Systems,Nov.1993.

C. Minkenberg and T. Engbersen, A Combined Input and

Output Queued Packet-Switched System Based onPRIZMA Switch-on-a-Chip Technology, IEEECommunications Magazine, December 2000.

N. McKeown, Scheduling algorithms for input-queuedcell switches, Ph.D. Thesis, University of California at

Berkeley, 1995. M. Karol and M. Hluchyj, Queuing in highperformancepacket-switching, IEEE J. Select. Areas. Commun. vol. 6,pp. 1587-1597, Dec. 1998.


17/17

THANK YOU!!!

Documents

A Cost-Effective Technique