iSLIP Switch Scheduler Ali Mohammad Zareh Bidoki April 2002

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iSLIP Switch Scheduler Ali Mohammad Zareh
BidokiApril 2002
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Table of Contents

• The place Buffer in Crossbar Switches
• Example of Fabrics
• PIM
• iSLIP (in CISCO 12000 ,5Gb/s router and Tiny Tera
  0.5 Tb/s)
• RRM
• WFA
• PP_VOQ
• Multicasting
• A 2.5Tb/s Router

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The place of Buffer in Crossbar

• Output Buffer
• Shared Buffer
• Input buffer

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InterconnectsTwo basic techniques
Input Queueing
Output Queueing
Usually a non-blocking switch fabric (e.g.
crossbar)
Usually a fast bus
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InterconnectsInput Queueing with Crossbar
Arbiter
Data In
Data Out
configuration
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Input QueueingHead of Line Blocking
Delay
Load
100
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Head of Line Blocking
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Virtual output Queuing
Queue scheduler
To port 1
To port 2
To port n
To port 1
To port 2
To port n
To port 1
To port 2
To port n
Input queues
To port 1
To port n
Port 2 queue
Port n queue
Port 1 queue
Input port 1
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Input QueueingVirtual output queues
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Input QueueingVirtual Output Queues
Delay
Load
100
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Which is better?

• Virtual output Queue (input queue).
• Ideal Output queue.

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Input QueueingVirtual output queues
Arbiter
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VOQ

• Arbiter
• Input memory management

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Problem Definition (bipartite)
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Maximum or Maximal matching
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Maximum or Maximal matching

• Maximum matching
• Maximizes instantaneous throughput
• Starvation
• Time complexity is very high in Hardware (o(n3))
• Maximal matching
• Cant add any connection on the current match
  without alert existing connections
• More practical (e.g. WFA, PIM, iSLIP, DRR,RRM)

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Matching Algorithms
3. iSLIP Iterative Serial-Line IP(base on
PIM and RRM)
2. RRM Round-Robin Matching
1. PIM - Parallel Iterative Matching
We will discuss three different matching algo.
Each algo. is evaluated by four
parameters 1. Latency(Throughput). 2. Starvation
free. 3. Fast. 4. Implementation.
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PIM - Parallel Iterative Matching
When no new matching can be found, the
algorithm stops.
3. Accept - If an input receives a grant,
it accepts one by selecting an output randomly
among those that granted to this output..
2. Grant - If an unmatched output receives
any requests, it grants to one by randomly
selecting a request uniformly over all requests.
1. Request - Each unmatched input sends a
request to every output for which it has a queued
cell.
The basic matching algorithm. Each iteration of
the algorithm follows these three steps
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PIM

• Each iteration will eliminate at least ¾ of the
  remaining connections
• Converge in O(logN) iterations
• No input queue is starved if service
• No memory or state is used
• At the beginning of each cell time, the match
  begins over, independently of the matches that
  were made in previous cell times
• PIM does not perform well for a single iteration
  it limits the throughput to approximately 63,
  only slightly higher than for a FIFO switch.
• This is because the probability that an input
  will remain ungranted is (N-1/N)N , hence as N
  increases, the throughput tends to .63 (1-(1/e))
• Implementation is hard in Hardware

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RRM Round-Robin Matching
The pointer gi to the highest priority
element of the round-robin schedule is
incremented (modulo N) to one location beyond the
granted input.
2. Grant - If an output receives any requests,
it chooses the one that appears next in a fixed,
round-robin schedule starting from the highest
priority element. The output notifies each input
whether or not its request was granted.
1. Request - Each unmatched input sends a request
to every output for which it has a queued cell.
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RRM Round-Robin Matching
The pointer ai to the highest
priority element of the round-robin schedule is
incremented (modulo N) to one location beyond the
accepted output.
3. Accept - If an input receives a grant,
it accepts the one that appears next in a fixed,
round-robin schedule starting from the highest
priority element.
The pointer gi to the highest priority
element of the round-robin schedule is
incremented (modulo N) to one location beyond the
granted input.
2. Grant - If an output receives any requests,
it chooses the one that appears next in a fixed,
round-robin schedule starting from the highest
priority element. The output notifies each input
whether or not its request was granted.
1. Request - Each unmatched input sends a request
to every output for which it has a queued cell.
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RRM Round-Robin Matching
The pointer ai to the highest
priority element of the round-robin schedule is
incremented (modulo N) to one location beyond the
accepted output.
3. Accept - If an input receives a grant,
it accepts the one that appears next in a fixed,
round-robin schedule starting from the highest
priority element.
The pointer gi to the highest priority
element of the round-robin schedule is
incremented (modulo N) to one location beyond the
granted input.
2. Grant - If an output receives any requests,
it chooses the one that appears next in a fixed,
round-robin schedule starting from the highest
priority element. The output notifies each input
whether or not its request was granted.
1. Request - Each unmatched input sends a request
to every output for which it has a queued cell.
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RRM Round-Robin Matching
The RRM is not starvation free In the following
example, we assume there are always cells waiting
to be transferred. The destination is always the
same.
g1
a1
First cycle
a2
g2
g3
a3
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RRM Round-Robin Matching
The RRM is not starvation free In the following
example, we assume there are always cells waiting
to be transferred. The destination is always the
same.
a1
First cycle
a2
g1
a3
g2
g3
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RRM Round-Robin Matching
The RRM is not starvation free In the following
example, we assume there are always cells waiting
to be transferred. The destination is always the
same.
First cycle
a1
g1
a2
a3
g2
g3
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RRM Round-Robin Matching
The RRM is not starvation free In the following
example, we assume there are always cells waiting
to be transferred. The destination is always the
same.
First cycle
a1
g2
g1
a2
a3
g3
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RRM Round-Robin Matching
The RRM is not starvation free In the following
example, we assume there are always cells waiting
to be transferred. The destination is always the
same.
First cycle
a1
g2
g1
a2
a3
g3
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RRM Round-Robin Matching
The RRM is not starvation free In the following
example, we assume there are always cells waiting
to be transferred. The destination is always the
same.
Second cycle
a1
g2
g1
a2
a3
g3
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RRM Round-Robin Matching
The RRM is not starvation free In the following
example, we assume there are always cells waiting
to be transferred. The destination is always the
same.
Second cycle
a1
g1
a2
g2
a3
g3
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RRM Round-Robin Matching
The RRM is not starvation free In the following
example, we assume there are always cells waiting
to be transferred. The destination is always the
same.
a1
Second cycle
g1
a2
g2
a3
g3
At this point the sequence of the events will
repeat itself Outputs 1 and 3 will always grant
input 1, while output 2 will always grant input 1
at the first iteration of the first cycle, but
input 1 will select output 1 indefinitely,
leaving output 2 to grant either input 2 or input
3. Thus the cell from input 1 to output 2 will
never be granted.
In order to solve this starvation the iSlip
algorithm was developed.
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RRM

• RRM overcomes two problem
• Complexity
• Unfairness
• the round-robin arbiters are much simpler and can
  perform faster than random arbiters.
• The rotating priority aids the algorithm in
  assigning bandwidth equally and more fairly among
  requesting connections.
• Its throughput is about 63

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2x2 switch with RRM algorithm under heavy load.

• synchronization of output arbiters leads to a
  throughput of just 50.

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Performance
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Synchronization
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iSLIP Iterative Serial-Line IP
2. Grant - If an output receives any requests, it
chooses the one that appears next in a fixed,
round-robin schedule starting from the highest
priority element. The output notifies each input
whether or not its request was granted.
The pointer gi to the highest priority
element of the round-robin schedule is
incremented (modulo N) to one location beyond the
granted input if and only if the grant is
accepted in Step 3 of the first iteration.
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iSLIP Iterative Serial-Line IP
The pointer gi to the highest priority
element of the round-robin schedule is
incremented (modulo N) to one location beyond the
granted input if and only if the grant is
accepted in Step 3 of the first iteration.
2. Grant - If an output receives any requests, it
chooses the one that appears next in a fixed,
round-robin schedule starting from the highest
priority element. The output notifies each input
whether or not its request was granted.
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iSLIP properties

• Property 1. Lowest priority is given to the most
  recently made connection.
• If input i successfully connects to output j,
  both a i and g j are updated and the connection
  from input i to output j becomes the lowest
  priority connection in the next cell time.
• Property 2. No connection is starved. This is
  because an input will continue to request an
  output until it is successful. The output will
  serve at most other inputs first, waiting at most
  N cell times to be accepted by each input.
  Therefore, a requesting input is always served in
  less than N 2 cell times.
• Property 3. Under heavy load, all queues with a
  common output have the same throughput. This is a
  consequence of Property 2 the output pointer
  moves to each requesting input in a fixed order,
  thus pr-viding each with the same throughput.

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iSLIP properties

• Simple to implement in hardware
• Starvation free
• Its throughput is about 100
• It is fair
• As the load increases, the number of synchronized
  arbiters decreases (see Figure), leading to a
  large sized match.
• Under uniform 100 offered load the iSLIP
  arbiters adapt to a time-division multiplexing
  scheme.
• It converge in O(1)

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Bursty Arrivals
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Burstiness Reduction

• Results indicate that iSLIP reduces the average
  burst length, and will tend to be more
  burst-reducing as the offered load increases.
• This is because the probability of switching
  between multiple connections increases as the
  utilization increases.
• As the load increases, the contention increases
  and bursts are interleaved at the output. In
  fact, if the offered load exceeds approximately
  70, the average burst length drops to exactly
  one cell.

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Burstiness Reduction
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Multiple Iteration

• The pointer gi to the highest priority element of
  the round-robin schedule is incremented (modulo
  N) to one location beyond the granted input if
  and only if the grant is accepted in Step 3 of
  the first iteration.
• Note that pointers g i and a i are only updated
  for matches found in the first iteration.
• It converge in O(logN)

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Multiple Iteration
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All with 4 iterations
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Implementation
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Implementation(2N arbiters)
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Implementation(N arbiters)Each arbiter is used
for both inputand output arbitration. In this
case, each arbiter contains two registers to hold
pointers gi and ai .
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Implementation
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Priority in iSLIP
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Why iSLIP is good for high speed?

• input buffers are separated
• Separated scheduler for each input and output
• Each work independently

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Multicasting

• Fanout splitting higher throughput, but not as
  simple
• Non-fanout splittingEasy, but low throughput

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Multicasting (ESLIP Combining Unicast and
Multicast-use in CISCO 12000)
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IP packet in iSLIP switch (2N2 Queue)
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LCS Ingress Flow control(2.5Tb/s)
Linecard
Switch Port
Switch Fabric
LCS
LCS
Switch Scheduler
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LCS Over Optical Fiber 10Gb/s Linecards
10Gb/s Linecard
10Gb/s Switch Port
2.5Gb/s LVDS
12 multimode fibers
Switch Fabric
LCS
12 multimode fibers
Switch Scheduler
GENET Quad Serdes
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2.56Tb/s IP router
1000ft/300m
Port 1
LCS
Port 256
Linecards
2.56Tb/s switch core
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Switch core architecture
Port 1
Scheduler
Port 256