modular sram-based binary content-addressable memories

Modular SRAM-based Binary Content-Addressable Memories

Ameer M.S. Abdelhadi and Guy G.F. LemieuxDepartment of Electrical and Computer Engineering

University of British Columbia

Vancouver, Canadaa place of mindTHE UNIVERSITY OFBRITISH COLUMBIA

Binary Content-Addressable Memory (BCAM)

1/17

Hardware-based Single-Cycle Parallel Search EnginesWrite

Stores new data at specific address

MatchSearch all addresses for a

given data (pattern)

Found in ‘2’

BCDA

0123B C A M

Search for ‘D’

BCDA

0123B C A M

Store ‘C’ in ‘1’

BCAM Applications

Memory management

Associative caches

Translation lookaside

buffers (TLBs)

Databases

Eliminates memory

bottleneck

Networking

IP lookup in routing/

forwarding tables

Intrusion detection

• detect predefined suspicious packages

Packet Classification

Pattern matching

e.g. DNA sequence

lookup

Data compression

find and shorten

redundant patterns

Data encryption

find and encrypt specific patterns

2/17

Motivation - FPGAs

3/17

1000’sMemory

Blocks

100,000’sLogic

Elements

N o d e d i c a t e d B C A M r e s o u r c e s i n F P G A s

Objectives

BCAMs

Massively parallel memory search

Require high memory bandwidth

FPGAs

Block RAMs are main storage

Limited memory bandwidth

4/17

Use BRAMs to construct BCAMs

• Modular and flexible• Storage efficient• Single-cycle• Performance oriented

Associative Arrays

Algorithmic Heuristics

5/17

HashesSearch Trees:Tries, BSTs, …

Multi-unpredictable-cycle

Data dependent performance

Variable search depth Misses due to conflicts

Register-Based BCAM

Concurrent register read and compare

Single-cycle Limited resources Complex routing Fits small BCAMs

6/17

⌊log2CD⌋

WPatt

MIn

dc

Ad

dre

ssD

eco

der

WAddr

PW

=

MPatt

CD

=PW

PW

=

⌊log2CD⌋

MAddr

Match

PWPWQ

Q

QEnD Reg0

EnD Reg1

EnD RegCD-1

Pri

ori

ty

Enco

der

Brute-Force Transposed-Indicators-RAM (1)A Traditional BRAM-based BCAM

7/17

Key idea: Transposed RAM - data becomes addresses

WriteWrite ‘0’ to location ‘B’

MatchRead location ‘D’ for match

‘2’

3012

ABCD‘D’

3012

ABCD

‘0’ to ‘B’

* Xilinx App Notes

Brute-Force Transposed-Indicators-RAM (2)Storing Data to Multiple Addresses

• How can we store data to multiple addresses?• Specify addresses using one-hot coding

• Each bit indicates a match or “store at location”

PROBLEM: Depth of CAM is limited by data width of RAM• e.g. to build 1M deep CAM, we need 1M bits wide

• In FPGAs: 1000 BRAMs x 32bit wide = 32K deep CAM

8/17

BRAM-based Single-cycle Depth of CAM is limited by RAM width

1

3210

DCBA

BCAM Cascading

• PROBLEM:• Patterns are encoded as RAM addresses

RAM depth is exponential to pattern width

• Solution: Cascading1. Divide pattern into smaller slices

2. Search for each slice separately

3. If all slices are found pattern match!

RAM depth is linear to pattern width

9/17

RAM Depth = 2Pattern Width

RAM Depth = 2Slice Widthx (Pattern Width / Slice Width)

CAM CAM CAM

SliceSlice SliceM a t c h e d P a t t e r n

Slic

e M

atch

……

…

Slic

e M

atch

Slic

e M

atch

Pattern Match

Hierarchical Search 2D BCAM (1)Narrow and Deep BCAM

10/17

Key idea: Hierarchical search1D BCAM 2D BCAM

4Mtoo

deep

2k

2k

Find a set (row) with match using a 1D BCAM

Search this set (row) in parallel for a specific match

Hierarchical Search 2D BCAM (2)Example

• Divide address space into segments• RAM: each segment in a line

• Transposed-RAM: indicates “pattern in segment?”

• Hierarchical Search:1. Find a row (segment) with

match using a 1D BCAM

2. Search this row (segment) in parallel for a specific match

11/17

0 0 0 00 0 1 11 0 0 00 1 0 0

0 1 2 30123

addresses

pa

tte

rns2

311

0123ad

dre

sses

patterns

Transposed-RAMRAM


• Divide address space into sets• RAM: each segment in a line





11/176

0 0 0 00 0 1 11 0 0 00 1 0 0

0 1 2 30123

addresses

pa

tte

rns2

311

0123ad

dre

sses

patterns

Transposed-RAMRAM


• Divide address space into sets• RAM: each set in a line





11/17

0 0 0 00 0 1 11 0 0 00 1 0 0

0 1 2 30123

addresses

pa

tte

rns2 3

1 101se

ts

patterns

Transposed-RAMRAM



• Transposed-RAM: indicates “pattern in set?”




11/17

0 00 111 01 0

0 10123

sets

pa

tte

rns2 3

1 101

patterns

Transposed-RAMRAMse

ts




• Hierarchical Search:1. Find a set (row) with match

using a 1D BCAM


11/17

0 00 111 01 0

0 10123p

att

ern

s2 31 1

01

patterns

Transposed-RAMRAMse

ts

sets

Match pattern ‘3’




• Hierarchical Search:1. Find a set (row) with match

using a 1D BCAM

2. Search this set (row) in parallel for a specific match

11/17

0 00 111 01 0

0 10123p

att

ern

s2 31 1

01

patterns

Transposed-RAMRAMse

ts

sets

Hierarchical Search 2D BCAM (3)Pros and Cons

Single match only

Cannot be

cascaded

RAM depth is exponential to pattern

width

Inefficient for wide patterns

12/17

BRAM-Based Single-cycleEfficient for deep CAMs

Indirectly-Indexed 2D (II2D) BCAM (1)Cascadable Wide and Deep BCAM

13/17

PROBLEM: is it possible to regenerate matches for all addresses?

patte

rns addresses

11 1

1 11 1 1


13/17


Key observation

Transposed RAMis a sparse matrix

ncolumns (set of addresses)accommodates nmatches (1’s) at most!

patte

rns addresses

11 1

1 11 1 1


13/17

Key idea: use indirect indices to point to intra-set matches

Cascadable

Scalable (linear growth)

Supports wider patterns


Key observation



patte

rns S e t s

11

11

11

11

Intra-set Match Indicators


13/17

Key idea: use indirect indices to point to intra-set matches

Cascadable

Scalable (linear growth)

Supports wider patterns


Key observation



patte

rns S e t s

11

11

11

11

BRAMLUTRAM


01

10

Indirectly-Indexed 2D (II2D) BCAM (2)Example

• Divide address space into sets

• Store sets with a match in Indicators-RAM

• Transposed-RAM stores indices to all matches in set

• Hierarchical Search:• Find indices of all matching sets in

Transposed-RAM

• Read Indicators-RAM using indices from Transposed-RAM

14/17

0 0 0 00 0 0 1

0 01 0

0 1 2 30123

addresses

patt

erns

Transposed-RAM

2331

0123

patterns

RAM (reference)

addr

esse

s






Transposed-RAM


14/17

0 0 0 00 0

1 0

0 0

0 1

0 1 2 30123

addresses

patt

erns

Transposed-RAM

Indicators-RAMs

0 11 0

2331

0123

patterns

RAM (reference)

addr

esse

s






Transposed-RAM


14/17

- --

1 0

-

0 1

0 10123

s e t s

patt

erns

Transposed-RAM

Indicators-RAMs

0 11 0

2331

0123

patterns

RAM (reference)

addr

esse

s






Transposed-RAM


14/16

- --

1 0

-

0 1

0 10123

s e t s

patt

erns

Transposed-RAM

Indicators-RAMs

0 11 0

2331

0123

patterns

RAM (reference)

addr

esse

s







Transposed-RAM


14/17

- --

1 0

-

0 1

0 10123

s e t s

patt

erns

Transposed-RAM

Indicators-RAMs

0 11 0

2331

0123

patterns

RAM (reference)

addr

esse

s







Transposed-RAM


14/17

- --

1 0

-

0 1

0 10123

s e t s

patt

erns

Transposed-RAM

Indicators-RAMs

0 11 0

2331

0123

patterns

RAM (reference)

addr

esse

s


Found in ‘1’ and ‘2’0 1 1 00 1 2 3

Indirectly-Indexed 2D (II2D) BCAM (3)Area and Performance

Except for very a narrow HS,II2D exhibits higher Fmax

register-based BCAMregister consumption

II2D linear ALM consumption;similar to other methods

HS exponential BRAM consumption

II2D linear BRAM consumption

II2D supports wider patterns

15/17

0

0.5

1

1.5

2

2.5

3

91

82

73

64

55

46

37

28

19

09

91

08

11

71

26

13

51

44

15

3 91

82

73

64

55

46

37

2 91

82

73

6 9

16K 32k 64k 128k

M2

0K

s (1

00

0's

)

050

100150200250300350

ALM

s (10

00's)

0

100

200

300

400

500

Fmax

(M

Hz)

Reg-basedBF-BCAMHS-BCAMII2D-BCAMDevice Limit

PW

CD

Open Source

16/17

http://ece.ubc.ca/~lemieux/downloads/

Modular and parametric Verilog files

Run-in-batch simulation and

synthesis manager

Conclusions

17/17

Brute-Force Transposed-RAM BRAM-based

Single-cycle

Deep

Wide

Cascadable

pa

tte

rns

a d d r e s s e s

Scalable

Conclusions

17/17

Hierarchical Search 2D BCAM BRAM-based

Single-cycle

Deep

Wide

Cascadable

pa

tte

rns

s e t s

Scalable

Conclusions

17/17

Indirectly-Indexed 2D (II2D) BCAMBRAM-based

Single-cycle

Deep

Wide

Cascadable

pa

tte

rns

s e t s

Scalable


Multi-unpredictable-cycle

Thank You!

Backup Slides

16

II2DHierarchicalBrute-Force

patte

rns addresses

patte

rns s e t s

patte

rns s e t s

Match IndicatorsBRAM-based

Single-cycle

Deep

Wide

Conclusions

modular sram-based binary content-addressable memories

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