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Communication Networks. Recitation 5 Input Queuing Scheduling & Combined Switches. Output-queued switches. Best delay and throughput performance. Main problem. Requires high fabric speedup (S = N). Unsuitable for high-speed switching. Input-queued switches. Big advantage.
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Communication Networks Recitation 5 Input Queuing Scheduling& Combined Switches Comnet 2010
Output-queued switches Best delay and throughput performance Main problem • Requires high fabric speedup (S = N) Unsuitable for high-speed switching Comnet 2010
Input-queued switches Big advantage • Speedup of one is sufficient Main problem • Can’t guarantee delay due to input contention Overcoming input contention: use higher speedup Comnet 2010
Input Queue Scheduling • First Goal : maximize throughput • Second Goal : control packet delay • Methods : • maximum matching • maximal matching • maximum/maximal weight matching • stable matchingusing Virtual Output Queuing and moderate speedup Comnet 2010
A 1 A 1 A 1 2 B 2 B 2 B 3 C 3 C 3 C 4 D 4 4 D D 5 E 5 5 E E 6 F 6 6 F F Example of Maximal Size Matching Maximal Size Matching Maximum Size Matching Comnet 2010
Maximum vs. Maximal • Maximum matching • Maximizes instantaneous throughput • Starvation • Time complexity is very high in Hardware (o(n3)) • Maximal matching • Can’t add any connection on the current match without alert existing connections • More practical Comnet 2010
1 1 1 1 2 2 2 2 #1 1 1 3 3 3 3 2 2 4 4 4 4 Grant Accept/Match 3 3 1 1 1 1 1 1 4 4 2 2 2 2 2 2 #2 3 3 3 3 3 3 4 4 4 4 4 4 Parallel Iterative Matching (PIM) Random Selection Random Selection Requests Comnet 2010
Improving PIM • RRM - Round-Robin Matching • Choose the link to Grant/Accept in RR manner • Simpler • More fair • Starvation is rare but exists due to synchronization Comnet 2010
1 1 1 1 1 10 2 2 2 2 1 w e i g h t M m m a x i u 3 3 3 3 1 10 4 4 4 4 1 Input QueueingLongest Queue First orOldest Cell First { = } Queue Length Weight 100% Waiting Time Comnet 2010
Non-uniform traffic Uniform traffic Avg Occupancy Avg Occupancy VOQ # VOQ # Input QueueingWhy is serving long/old queues better than serving maximum number of queues? • When traffic is uniformly distributed, servicing themaximum number of queues leads to 100% throughput. • When traffic is non-uniform, some queues become longer than others. • A good algorithm keeps the queue lengths matched, and services a large number of queues. Comnet 2010
Speedup: Context Memory Memory A generic switch The placement of memory gives • Output-queued switches • Input-queued switches • Combined input and output queued switches Comnet 2010
1 2 1 2 1 Using Speedup Comnet 2010
The Speedup Problem Find a compromise: 1 < Speedup << N • to get the performance of an OQ switch • close to the cost of an IQ switch Essential for high speed QoS switching Comnet 2010
What is exact mimicking? Apply same inputs to an OQ and a CIOQ switch • packet by packet Obtain same outputs • packet by packet Key concept: urgency value • urgency = departure time in OQ - present time Comnet 2010
Most Urgent Cell First (MUCF) The algorithm • Outputs try to get their most urgent packets • Inputs grant to output whose packet is most urgent, ties broken by port number • Loser outputs for next most urgent packet • Algorithm terminates when no more matches are possible Speedup of 4 is sufficient for exact emulation of FIFO OQ switches, with MUCF (Prabhakar & McKeown, 1997) Joined Preferred Matching (JPM) – Speedup 2 (Stoica & Zhang, 1998) Comnet 2010
