Title: iSLIP Switch Scheduler Ali Mohammad Zareh Bidoki April 2002
1iSLIP Switch Scheduler Ali Mohammad Zareh
BidokiApril 2002
2Table 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
3The place of Buffer in Crossbar
- Output Buffer
- Shared Buffer
- Input buffer
4InterconnectsTwo basic techniques
Input Queueing
Output Queueing
Usually a non-blocking switch fabric (e.g.
crossbar)
Usually a fast bus
5InterconnectsInput Queueing with Crossbar
Arbiter
Data In
Data Out
configuration
6Input QueueingHead of Line Blocking
Delay
Load
100
7Head of Line Blocking
8Virtual 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
9Input QueueingVirtual output queues
10Input QueueingVirtual Output Queues
Delay
Load
100
11Which is better?
- Virtual output Queue (input queue).
- Ideal Output queue.
12Input QueueingVirtual output queues
Arbiter
13VOQ
- Arbiter
- Input memory management
14Problem Definition (bipartite)
15Maximum or Maximal matching
16Maximum 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)
17Matching 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.
18PIM - 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
19PIM
- 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
20RRM 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.
21RRM 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.
22RRM 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.
23RRM 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
24RRM 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
25RRM 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
26RRM 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
27RRM 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
28RRM 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
29RRM 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
30RRM 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.
31RRM
- 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
322x2 switch with RRM algorithm under heavy load.
- synchronization of output arbiters leads to a
throughput of just 50.
33Performance
34Synchronization
35iSLIP 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.
36iSLIP 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.
37iSLIP 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.
38iSLIP 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)
39Bursty Arrivals
40Burstiness 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.
41Burstiness Reduction
42Multiple 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)
43Multiple Iteration
44All with 4 iterations
45Implementation
46Implementation(2N arbiters)
47Implementation(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 .
48Implementation
49Priority in iSLIP
50Why iSLIP is good for high speed?
- input buffers are separated
- Separated scheduler for each input and output
- Each work independently
51Multicasting
- Fanout splitting higher throughput, but not as
simple - Non-fanout splittingEasy, but low throughput
52Multicasting (ESLIP Combining Unicast and
Multicast-use in CISCO 12000)
53IP packet in iSLIP switch (2N2 Queue)
54LCS Ingress Flow control(2.5Tb/s)
Linecard
Switch Port
Switch Fabric
LCS
LCS
Switch Scheduler
55LCS 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
562.56Tb/s IP router
1000ft/300m
Port 1
LCS
Port 256
Linecards
2.56Tb/s switch core
57Switch core architecture
Port 1
Scheduler
Port 256