Dynamic Verification of Sequential Consistency Albert Meixner Daniel J. Sorin Dept. of Computer Dept. of Electrical and Science Computer Engineering Duke University

Introduction Multithreaded systems becoming ubiquitous Commercial workloads rely heavily on parallel machines Reliability and availability are crucial Backward Error Recovery can provide high availability Recover to known good state upon error But can only recover from errors detected in time Memory system is of special interest Complex – Many components, large transistor count Numerous error hazards

Memory System Error Detection Must cover all memory system components DRAMs, caches, controllers, interconnect, and write buffers Mechanisms for individual components exist Storage structures: ECC Interconnect: checksums, sequence numbering Cache and memory controllers: replication Adding detection to all components is hard Complicates design of every component Requires good intuition of interactions and possible errors  Want comprehensive, end-to-end error detection

Dynamic Verification Dynamic verification Correct system operation constantly monitored at runtime End-to-end scheme Detects transient errors, design bugs, and manufacturing errors Differs from statically verifying that design is bug-free High level invariants are checked, instead of individual components Simplified design of system components Can detect any low-level error that violates invariant

Memory Consistency Memory consistency model Formal specification of memory system behavior in a multithreaded system Defines order in which memory accesses from different CPUs can become globally visible Many consistency models exist, we focus on one Verifying memory consistency = Verifying correctness of the memory system Ideal invariant for dynamic verification

Sequential Consistency (SC) Requires appearance of total global order of all loads and stores in system Each load must receive value of most recent store in total order to the same address Program order of all processors is preserved in total order SC is most intuitive consistency model Good for programmers Speculation can make SC almost as fast as more relaxed models Our contribution: Dynamic Verification of Sequential Consistency (DVSC)

Outline Introduction DVSC-Direct DVSC-Indirect Results Conclusion

Global Order Program Order ST A←2LD C→1 DVSC-Direct CPU 1 CPU 2 t=1.1 LD A→1 t=2.1 ST B←2 t=3.1 LD A→2 t=1.2 LD C→1 t=2.2 ST A←2 t=3.2 LD C→1 Verifier LD A→1ST B←2LD A→2 LD C→1ST A←2LD C→1 LD A→1ST B←2LD A→2LD C→1

DVSC-Indirect: Idea Verify conditions sufficient for Sequential Consistency In-order performance of memory operations Cache coherence Conditions formally defined and proven by Plakal et al. [SPAA 1998] Two mechanisms On-chip checker for in-order performance Distributed checker for cache coherence

In-Order Performance Verification A load of block B receives the value of… …the most recent local store to B or most recent global store to B performed after all local stores Trivially observed on in-order processor with coherent caches Modern processors execute out-of-order Results of ooo-execution are considered speculative until in- order re-execution and verification DVSC-Indirect uses DIVA checker core by Austin [Micro 1999] Could substitute other mechanisms

Cache Coherence All processors observe the same order of stores to a given memory location Difficult because the same memory location can exist in different caches Maintained by a coherence protocol Different protocols: MOSI, MSI, MOESI, Token Coherence, … Different maintenance mechanisms: directory, snooping Verification uses “divide and conquer” Verify conditions provably sufficient for cache coherence Initially defined for proof of sequential consistency by Plakal et al. [SPAA1998]

Cache Coherence Verification Coherence Conditions 1.Cache accesses are contained in an epoch Stores in read-write epochs Loads in read-write or read-only epochs 2.Read-write epochs do not overlap other epochs 3.Block data at beginning of epoch equals block data at end of last read-write epoch Verification Check if accesses are in appropriate epoch during DIVA-replay Collect epoch information at every node and send to verifier Verifier checks epoch history for overlaps and data propagation Epoch The time interval between obtaining and losing permissions on a block.

Implementation Overview CPU Core Cache Memory Verify Epochs Record Epochs Epoch History Collect Epochs DIVA CPU Core Cache Memory Verify Epochs Record Epochs Epoch History Collect Epoch DIVA CPU Core Cache Memory Verify Epochs Record Epochs Epoch History Collect Epochs DIVA Interconnect

At the Cache Controller All caches keep track of active epochs in the Cache Epoch Table (CET) Epoch Inform sent to the memory controller when epoch ends Begin and end data are hashed Every DIVA cache access checks CET for active epoch Ensure access is contained in epoch Verification off the critical path Second order performance effect from bandwidth usage Epoch Inform CET Type: read-write or read-only Begin time Begin data End time End data

At the Memory Controller Check for epoch overlaps and correct value propagation Generally requires entire block history → O(N) space If epoch informs are processed in order… Need end value of last read-write epoch for propagation check Need end time of last read-write and last read-only epoch for overlap check O(1) space Epochs arrive almost in order Fix remaining re-orderings in priority queue before verifications Epoch state in Memory Epoch Table (MET) Last end time of read-only epoch and read-write epoch, last value

Experimental Evaluation Empirically determine error detection capability Error injection into caches, controller, interconnect, switches, etc. Quantify error-free overhead Increase in interconnect bandwidth consumption Potential decrease in application performance

Simulation Methodology Full-system simulation of 8-CPU UltraSPARC SMP Simics functional simulation GEMS-based timing simulation 2 GB RAM, 4-way 32KB I+D L1, 4-way 1MB L2 SafetyNet for backward error recovery MOSI-Directory and MOSI- Snooping Benchmarks Apache 2Static web-server SpecJBB3-Tier Java system OLTPOnline transaction system with DB2 SlashcodeDynamic website with perl and mysql BarnesBarnes-Hut from SPLASH2

Bottleneck Link Bandwidth - Directory

Error-Free Runtime - Directory slower

Conclusions DVSC-Direct and DVSC-Indirect enable end-to-end verification of the memory system DVSC-Indirect imposes acceptable hardware and performance overhead An extension of DVSC-Indirect to relaxed consistency is currently under development

Questions?