High-Throughput Low-Energy Content-Addressable Memory Based on Self-Timed Overlapped Search Mechanism

Transcription

1 18 th IEEE International Symposium on Asynchronous Circuits and Systems (ASYNC), May 7-9, 2012, Copenhagen! High-Throughput Low-Energy Content-Addressable Memory Based on Self-Timed Overlapped Search Mechanism Dr. Naoya Onizawa Postdoctoral Fellow, McGill Univerisity Collaborators: S. Matsunaga, V. Gaudet, and T. Hanyu Acknowledgements This research was supported by Japan Science Technology Agency (JST) Development of Dependable Network-on-Chip Platform in Core Research for Evolutional Science and Technology (CREST)

2 Content-Addressable Memory (CAM)! Associative memory! Parallel searching! Applications! Cache, Virus checking! Packet forwarding (40G, 100Gbps) Search word Match match Location (Address) # entries word length 2

3 Hardware-Implementation Issue Speed restriction! Packet length - 32bit (IPv4), 128,144bit (IPv6) Search word 144-b data " Large matching delay Pipelined approach K. Pagiamtzis, et al (JSSC 04 vol.39-9) Search word " Large area and power dissipation Goal: High-throughput low-overhead CAM 3

4 Concept! Operate as comparable to small CAM Search If Match Slow access High-speed access to small CAM Large CAM Word 1 Word 2 Word 3 Large Large Large Small matching delay time Hide large matching delay to improve throughput 4 In most cases, search words match

5 Approach! Assign search words to unused blocks After searching first few bits, most blocks are mismatched " If unused blocks are found, it doesn t need to wait to search new words until the current search is complete. Pre-computation block (Find unused blocks before sending) 5.57x higher throughput at 8% cost of area 5 Unused In use Unused Unused Partitioning Search new words after searching few bits

6 Table of Contents " Introduction to content-addressable memory " Overlapped search mechanism " Word overlapped search " Phase overlapped processing " Hardware implementation " Evaluation " Conclusion and future prospect 6

7 CAM word blocks bits (IPv6) stored word 0 stored word 1 stored word w-1 match lines encoder match location log 2 w bits Operation 1. Search all words in parallel 2. Find a matched word block Input controller n bits Search word search lines Word-parallel search in single cycle 7 3. Output a matched location (address)

8 CAM Word Block Search word (n bits) Sense amplifier Conditions! Match! Stored word (n bits) Series of CAM cells (NAND-type CAM) Throughput determined by word length in conventional CAM Long word length degrades throughput 8

9 CAM Characteristics Matching probability of word blocks after k-bit search is! p matched = 1 $ # & " 2 % Most word blocks are not used after k-bit search (We set k=8 in the hardware implementation) " Use unused blocks to improve throughput k Most word blocks are unused (mismatched). 9

10 Word Overlapped Search (WOS) CAM architecture based on segmentation method k-bit 1st-stage sub-word block ML1 0 ML1 1 CAM block (self-timed circuit) MLS1 0 MLS1 1 (n-k)-bit 2nd-stage sub-word block ML2 0 ML Partition word block to: a) small k-bit block and b) large (n-k)-bit block by segmentation block ML1 w-1 MLS1 w-1 Segmentation circuit ML1 w-1 2. Segmentation block stores Its k-bit matched result Input controller word block SL1 SL2 (synchronous circuit) 3. Latter block operates when the first block matches Word block partitioned by segmentation block 10

11 WOS operation Match Input connects all word blocks Search sub-word 1 1. First k-bit sub-word search 2. Some sub-word block matched 11

12 WOS operation Latter bits matching Match Match Assign to unused block Search sub-word 2 Search sub-word 1 After k-bit search, new search starts 12

13 WOS operation Still operates Match Match Latter bits matching (word 2) Match Search sub-word 3 Assign to unused block Search sub-word 2 Should not affect the old matched block How to assign search words to unused blocks? 13

14 Categorize Word Blocks Same stored k-bit data Group A Group B Categorize based on the first k-bit stored word 14

15 Pre-Computation ctrl SL1 Search line registers k bits Search data SL2 Search line registers Search line registers (n-k) bits Comparator enable Mode controller Input controller (m=1) mode Compare m consecutive k-bit search words If they are different, they are in different groups (Category 1: fast mode) Otherwise, they are in the same group (Category 2: slow mode) Categorize search words using comparator 15

16 Timing Diagram (fast mode) Ctrl SL1 Data1 Data2 Data3 Data4 SL2 ML1 0 MLS1 0 ML2 0 ML1 1 MLS1 1 ML2 1 Data0 Data1 Data2 Data3 Match Hold Match Match Hold Send search words based on short delay T tst Consecutive words are assigned to unused blocks (a) High-speed searching based on T 1st 16

17 Timing Diagram (slow mode) Ctrl SL1 Data1 Data2 Data3 SL2 Data0 Data1 Data2 mode Slow fast ML1 0 Match Match MLS1 0 Hold Hold ML2 0 Match Two consecutive words use the same word block Wait until the current search is complete 17

18 Average Search Delay! Category 1 fast mode Send search words based on the first k-bit delay (T 1st )! Category 2 slow mode Send a new word after the current n-bit search is complete (T slow ) " " T sa = T 1st 1! m 1 % $ $ ' # # 2 & k % & ' +T " m " 1 % slow $ $ # 2 ' # & k % ' & 18

19 Table of Contents " Introduction to content-addressable memory " Overlapped search mechanism " Word overlapped search " Phase overlapped processing " Hardware implementation " Evaluation " Conclusion and future prospect 19

20 Word Circuit (precharge) NAND-type word circuit NAND-type CAM cell ML C C C Low Charge Low WL SL BL BL SL CAM cell! Dynamic logic! Series of pass transistors Charge capacitance on match line 20 ML! Match ON,! OFF

21 Word Circuit (evaluate) Match operation operation Search word High High High Low Discharge Discharging capacitance on match line " Output goes high Not discharging " Output remains low Match line remains high in mismatched case 21

22 Synchronous Control (conventional) Evaluate Search data Clk High Precharge Low ML 0 Low Low ML 1 High Low ML 2 Low Low All word circuits are controlled by a global clock signal 2 phases are required every search 22

23 Phase Overlapped Processing (POP) Lclk 0 High Lclk 1 High Lclk 2 High Low High Each circuit is independently controlled using local control signals! Matched word circuit Move on to precharge phase! ed word circuit Stay in evaluate phase Low #Lowering switching activity of pre-charging signals ed blocks always process new word 23

24 WOS based POP Input can be changed at every phase Input are assigned to unused block (WOS scheme) Evaluate Lctrl 0 High High Lctrl 1 High Low High Low Precharge (Local) High Low Lctrl 2 High High Low Unused block can process without waiting precharge phase Searching words requires just 1 phase Low 24

25 Throughput Ratio Conventional T CS = 2T SS = 2(T reg +T 1st +T 2nd ) Proposed " " T CA = T SA = T 1st 1! m 1 % $ $ ' # # 2 &! T 1st Throughput ratio = T CS T CA = 2(T reg +T 1st +T 2nd ) T 1st k % & ' +T " m " 1 % slow $ $ # 2 ' # & k % ' & T SS T SA Synchronous search delay (evaluate phase) Asynchronous search delay 25

26 Table of Contents " Introduction to content-addressable memory " Overlapped search mechanism " Word overlapped search " Phase overlapped processing " Hardware implementation " Evaluation " Conclusion and future prospect 26

27 Circuit Implementation! 144-bit CAM word block with self-precharge circuit! Self-precharge circuit pre-charges after 2 nd stage is complete.! Hierarchical 2 nd stage block (17 local and 1 global match circuit) 8 bits 1st-stage sub-word circuits prec Local match circuit C C C C C C Segment (a) circuit NAND-type cell (b) x8 weak feedback transistor x8 ML1 0 prec ML1 0 LML14 0 Store local matched result ML1 0 Self-precharge circuit (c) prec LML0 0 Global match circuit prec LML15 0 LML16 0 (d) ML2 0 It isn t affected by input changing Self-precharge circuit controls its word circuit 27

28 Simulated Waveforms Ctrl ML1 0 LML1 0 ML2 0 prec 0 ML1 1 LML1 1 ML2 1 prec 1 Voltage [V] CAM operates based on T 1st (259ps) T CA =261ps Time [ns] HSPICE simulation under a 90nm CMOS technology 28 Global match circuit uses only local matched result. After search is complete, self pre-charging is locally done.

29 Performance Comparison 256-word 144-bit binary CMOS 1.5 Precharge Evaluate Conventional Proposed Cycle delay [ps] Average cycle time [ns] % -64.1% -82.0% -100% -64.1% Energy [fj/bit/ search] Match Search Control Total Area [Trs.] 372K 408K 0 Synchronous WOS WOS + POP Independent control reduces switching activity of pre-charging 5.57x throughput and 38% energy saving 29

30 Performance Comparison Cycle time [ns] V DD =0.6V 0.7V 0.8V 0.9V 65nm@JSSC vol V Conventional Energy dissipation [fj/bit/search] Better energy-delay product 30

31 Conclusion High-throughput low-energy CAM! Word overlapped search! Use unused word block! Assign based on pre-computation! Phase overlapped search! Independent control of each word block! Search without waiting for precharge " 5.57x throughput, 38% energy saving, 8% cost of area 31

32 Future Prospects! Circuit design considerations! Number of partitions! Timing robustness! Extend to Ternary CAM (TCAM)! Redesign input controller! Application specific design! Cache (TLB), virus checker 32