SoftMC A Flexible and Practical Open-Source Infrastructure for Enabling Experimental DRAM Studies

Transcription

1 SoftMC A Flexible and Practical Open-Source Infrastructure for Enabling Experimental DRAM Studies Hasan Hassan, Nandita Vijaykumar, Samira Khan, Saugata Ghose, Kevin Chang, Gennady Pekhimenko, Donghyuk Lee, Oguz Ergin, Onur Mutlu 1

2 Executive Summary Two critical problems of DRAM: Reliability and Performance Recently-discovered bug: RowHammer Characterize, analyze, and understand DRAM cell behavior We design and implement SoftMC, an FPGA-based DRAM testing infrastructure Flexible and Easy to Use (C++ API) Open-source (github.com/cmu-safari/softmc) We implement two use cases A retention time distribution test An experiment to validate two latency reduction mechanisms SoftMC enables a wide range of studies 2

3 Outline 1. DRAM Basics & Motivation 2. SoftMC 3. Use Cases Retention Time Distribution Study Evaluating Recently-Proposed Ideas 4. Future Research Directions 5. Conclusion 3

4 DRAM Operations DRAM Cell Memory Bus DRAM Row Memory Controller Activate Read Precharge CPU Sense Amplifier 4

5 DRAM Latency DRAM Cell Sense Amplifier 0 (refresh) 64 ms time Activate Read Precharge Activate Ready-to-access Retention Time: Latency The interval during which Precharge the data Latency is retained correctly Activation in the Latency DRAM cell without accessing it 5

6 Latency vs. Reliability DRAM Cell Sense Amplifier time Activate Read Precharge Activate Ready-to-access Latency Precharge Latency affects Activation DRAM Latency reliability Violating latencies negatively 6

7 Other Factors Affecting Reliability and Latency Temperature Voltage Inter-cell Interference Manufacturing Process Retention Time To develop new mechanisms improving reliability and latency, we need to better understand the effects of these factors 7

8 Characterizing DRAM Many of the factors affecting DRAM reliability and latency cannot be properly modeled We need to perform experimental studies of real DRAM chips 8

9 Outline 1. DRAM Basics & Motivation 2. SoftMC 3. Use Cases Retention Time Distribution Study Evaluating Recently-Proposed Ideas 4. Future Research Directions 5. Conclusion 9

10 Goals of a DRAM Testing Infrastructure Flexibility Ability to test any DRAM operation Ability to test any combination of DRAM operations and custom timing parameters Ease of use Simple programming interface (C++) Minimal programming effort and time Accessible to a wide range of users who may lack experience in hardware design 10

11 SoftMC: High-level View FPGA-based memory characterization infrastructure Prototype using Xilinx ML605 Easily programmable using the C++ API 11

12 SoftMC: Key Components 1. SoftMC API 2. PCIe Driver 3. SoftMC Hardware 12

13 SoftMC API Writing data to DRAM: InstructionSequence iseq; iseq.insert(genact(bank, row)); iseq.insert(genwait(trcd)); iseq.insert(genwr(bank, col, data)); iseq.insert(genwait(tcl + tbl + twr)); iseq.insert(genpre(bank)); iseq.insert(genwait(trp)); iseq.insert(genend()); iseq.execute(fpga); Instruction generator functions 13

14 SoftMC: Key Components 1. SoftMC API 2. PCIe Driver* Communicates raw data with the FPGA 3. SoftMC Hardware * Jacobsen, Matthew, et al. "RIFFA 2.1: A reusable integration framework for FPGA accelerators." TRETS,

15 PCIe Controller Instruction Dispatcher SoftMC Hardware Wait (Ready-to-access Activate Latency) Instructions Instruction Receiver Instruction Queue DRAM Host Machine Autorefresh Controller Calibration Controller Read Capture SoftMC Hardware (FPGA) DDR PHY Data 15

16 Outline 1. DRAM Basics & Motivation 2. SoftMC 3. Use Cases Retention Time Distribution Study Evaluating Recently-Proposed Ideas 4. Future Research Directions 5. Conclusion 16

17 Retention Time Distribution Study Write Reference Data to a Row Wait (Refresh Interval) Read Back Observe Errors Increase the refresh interval Can be implemented with just ~100 lines of code 17

18 Number of Erroneous Bytes Retention Time Test: Results ~20 ⁰ C (room temperature) Module A Module B Module C Validates the correctness of the 0 SoftMC Infrastructure Refresh Interval (s) 18

19 Outline 1. DRAM Basics & Motivation 2. SoftMC 3. Use Cases Retention Time Distribution Study Evaluating Recently-Proposed Ideas 4. Future Research Directions 5. Conclusion 19

20 Accessing Highly-charged Cells Faster NUAT (Shin+, HPCA 2014) ChargeCache (Hassan+, HPCA 2016) A highly-charged cell can be accessed with low latency 20

21 How a Highly-Charged Cell Is Accessed Faster? DRAM Cell Sense Amplifier 0 (refresh) 64 ms time Activate Read Precharge Activate Ready-to-access Latency Activation Latency Precharge Latency 21

22 Ready-to-access Latency Test Longer wait Shorter wait Lower cell charge Higher cell charge Write Reference Data Wait for the Wait Interval Read Back Observe Errors Change the Wait Interval With custom ready-to-access latency parameter Can be implemented with just ~150 lines of code 22

23 Number of Erroneous Bytes Number of Erroneous Bytes Ready-to-access Latency: Results Latency (cycles) Latency (cycles) ⁰ C temperature Expected Curves Real Curves We do not observe the expected latency reduction effect Refresh Interval in existing DRAM chips Wait Interval (ms) 23

24 charge Why Don t We See the Latency Reduction Effect? The memory controller cannot externally control when a sense amplifier gets enabled in existing DRAM chips Cell Ready to Access Ready to Access Charge Level Sense Amp Data 1 Potential Reduction Enabling the Sense Amplifier Fixed Latency! Data 0 ACT R/W time 24

25 Outline 1. DRAM Basics & Motivation 2. SoftMC 3. Use Cases Retention Time Distribution Study Evaluating Recently-Proposed Ideas 4. Future Research Directions 5. Conclusion 25

26 Future Research Directions More Characterization of DRAM How are the cell characteristics changing with different generations of technology nodes? What types of usage accelerate aging? Characterization of Non-volatile Memory Extensions Memory Scheduling Workload Analysis Testbed for in-memory Computation 26

27 Outline 1. DRAM Basics & Motivation 2. SoftMC 3. Use Cases Retention Time Distribution Study Evaluating Recently-Proposed Ideas 4. Future Research Directions 5. Conclusion 27

28 Conclusion SoftMC: First publicly-available FPGA-based DRAM testing infrastructure Flexible and Easy to Use Implemented two use cases Retention Time Distribution Study Evaluation of two recently-proposed latency reduction mechanisms SoftMC can enable many other studies, ideas, and methodologies in the design of future memory systems Download our first prototype github.com/cmu-safari/softmc 28

29 SoftMC A Flexible and Practical Open-Source Infrastructure for Enabling Experimental DRAM Studies Hasan Hassan, Nandita Vijaykumar, Samira Khan, Saugata Ghose, Kevin Chang, Gennady Pekhimenko, Donghyuk Lee, Oguz Ergin, Onur Mutlu 29

30 Backup Slides 30

31 Key SoftMC Instructions 31

32 Github 32

33 Number of Erroneous Bytes Number of Erroneous Bytes Number of Erroneous Bytes Ready-to-Access Latency Test Results Latency (cycles) Module A Latency (cycles) Module B Wait Interval (ms) Latency (cycles) Wait Interval (ms) Module C Wait Interval (ms) 33

34 Activation Latency Test With low activation latency parameter Write Reference Data Wait for the Wait Interval ACT- PRE Change the wait interval Observe Errors Read Back Wait for the Wait Interval 34

35 Number of Erroneous Bytes Number of Erroneous Bytes Number of Erroneous Bytes Activation Latency Test Results Latency (cycles) Module A Latency (cycles) Module B Wait Interval (ms) Wait Interval (ms) 2400 Latency (cycles) Module C Wait Interval (ms) 35