1

Buffering Strategies Buffering Strategies in ATM Switchesin ATM Switches

Carey WilliamsonDepartment of Computer Science

University of Calgary

2

IntroductionIntroduction

Up to now, we have assumed bufferless switches and bufferless switch fabrics

When contention occurs, cells are dropped

Not practical to do this!

3

AlternativesAlternatives

Buffering– a cell that cannot be transmitted on

its desired path or port right now can wait in a buffer to try again later

– several possibilities: input buffering, output buffering, crosspoint (internal) buffering, combination thereof

4

Alternatives (Cont’d)Alternatives (Cont’d)

Recirculation– a cell that cannot be transmitted on

its desired path or port right now is sent back to the input ports using a recirculation line to try again in the next time slot (with higher priority)

– hopefully will get through next time

5

Alternatives (Cont’d)Alternatives (Cont’d)

Deflection routing– a cell that cannot be transmitted

on its desired path or port right now is sent out “another” (available) port instead, in the hope that it will find an alternate path to its destination

– example: tandem banyan

6

Alternatives (Cont’d)Alternatives (Cont’d)

Redundant paths– design a switch fabric with multiple

possible paths from each input port to each output port (e.g., Benes)

– greater freedom for path selection– flexible, adaptive, less contention– works well with deflection routing

7

Buffering IssuesBuffering Issues

There are three main factors that affect the performance of switch buffering strategies

Buffer location Buffer size Buffer management strategy

8

Buffer LocationBuffer Location

Several choices Input buffering Output buffering Internal buffering Combination of the above

9

Input BufferingInput Buffering In the event of output port

contention (which can be detected ahead of time at the input ports), let one of the contending cells (chosen at random) go ahead, and hold the other(s) at the input ports

Others try to go through the switch fabric the next chance they get

10

Input Buffering (Cont’d)Input Buffering (Cont’d) Can be a poor choice! Input buffering suffers from the

Head of the Line (HOL) blocking problem

Can significantly degrade the performance of the switch

11

HOL Blocking ProblemHOL Blocking Problem The cell at the head of the input

queue cannot go because of output port contention

Because of the FCFS nature of the queue, all cells behind the head cell are also blocked from going

Even if the output port that they want is idle!!!!

12

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

OUTPUT 0

OUTPUT 1

INPUT 1

INPUT 0

13

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

0

1Two arrivals

14

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

0

1Two departures

15

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

1

0Two arrivals

16

HOL Blocking ExampleHOL Blocking Example

0

1

Two departures

17

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

0

0Two arrivals

18

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

0

0

One departure

19

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

1

01Two arrivals

20

HOL Blocking ExampleHOL Blocking Example

0

11

Two departures

21

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

1

11Two arrivals

22

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

11

1

One departure

23

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

1

10

One arrival

24

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

1

10

One departure

HOL Blocking

25

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

10

0One arrival

26

HOL Blocking ExampleHOL Blocking Example

0

1

0

Two departures

27

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

0

No arrivals

28

HOL Blocking ExampleHOL Blocking Example

2 x 2Switch

0

One departure

29

HOL Blocking: SummaryHOL Blocking: Summary

Cells can end up waiting at input ports even if their desired output port is idle

How often can this happen? For a 100% loaded 2x2 switch,

HOL blocking happens 25% of time Effective throughput: 0.75

30

HOL Blocking (Cont’d)HOL Blocking (Cont’d)

The HOL blocking problem does NOT go away on larger mesh sizes

In fact, it even gets worse!!!

N Maximum Throughput

1 12 0.753 0.68254 0.65535 0.63996 0.63027 0.62348 0.6184 0.5858

Maximum Throughput for Input Buffering

33

Solutions for HOL Solutions for HOL BlockingBlocking Non-FIFO service discipline Lookahead “windowing” schemes

– e.g., if front cell is blocked, then try the next cell, and so on

– maximum lookahead W (e.g., W=8)– called “HOL bypass”

34

Solutions forSolutions forHOL Blocking (Cont’d)HOL Blocking (Cont’d)

Don’t use input buffering! Use output buffering instead

35

Output BufferingOutput Buffering In the event of output port

contention, send all the cells through the switch fabric, letting one of the contending cells (chosen at random) use the output port, but holding the other(s) in the buffers at the output ports

36

Output Buffering Output Buffering (Cont’d)(Cont’d) Main difference: cells have already

gone through the switch fabric As soon as the port is idle, the

cells go out (i.e., work conserving) Nothing else can get in their way Achieves maximum possible

throughput

37

Buffer SizingBuffer Sizing

Need buffer size large enough to keep cell loss below an acceptable threshold (e.g., CLR = 0.000001)

Purpose of buffers is to handle short term statistical fluctuations in queue length

38

Buffer Sizing (Cont’d)Buffer Sizing (Cont’d) Obvious fact #1: the larger the

buffer size, the lower the cell loss Obvious fact #2: the larger the

buffer size, the larger the maximum possible queuing delay (and the cost of the switch!)

Tradeoff: cell loss versus cell delay (and cell delay jitter) (and cost)

39

Buffer Sizing (Cont’d)Buffer Sizing (Cont’d)

Reality: finite buffers– e.g., 100’s or 1000’s of cells per port

Buffers need to be large enough to handle the bursty characteristics of integrated ATM traffic

General rule of thumb: buffer size = 10 x max burst size

40

Buffer ManagementBuffer Management

In a shared memory switch, for example, there is a choice of using dedicated buffers for each port (called partitioned buffering) or using a common pool of buffers shared by all ports (called shared buffering)

Partitioned BuffersPartitioned BuffersSHARED MEMORY

Shared BuffersShared BuffersSHARED MEMORY

43

Buffer Mgmt (Cont’d)Buffer Mgmt (Cont’d)

Shared buffering offers MUCH better cell loss performance

Partititioned is perhaps easier to design and build

Shared is more complicated to design, build, and control

Shared is superior (uniform traffic)

44

SummarySummary There are a wide range of

choices to make for buffering in ATM switches

Main issues:– buffer location– buffer size– buffer management strategy

Major impact on performance