CoNDA : Efficient Cache Coherence Support for Near-Data Accelerators

1. CoNDA: Efficient Cache Coherence Supportfor Near-Data Accelerators Amirali Boroumand SaugataGhose, MineshPatel, Hasan Hassan, Brandon Lucia, RachataAusavarungnirun, KevinHsieh,Nastaran Hajinazar, Krishna Malladi, Hongzhong Zheng, Onur Mutlu

2. Specialized Accelerators • Specialized accelerators are now everywhere! CPU DRAM ASIC NDA Recent advancement in 3D-stacked technology enabled Near-Data Accelerators (NDA) GPU FPGA ASIC NDA ASIC

3. Coherence For NDAs • Challenge: Coherence between NDAs and CPUs CPU CPU CPU (1) Large cost of off-chip communication DRAM L2 L1 CPU Compute Unit (2) NDA applications generate a large amount of off-chip data movement NDA ASIC It is impractical to use traditional coherence protocols

4. Existing Coherence Mechanisms • We extensively study existing NDA coherence mechanisms and make three key observations: These mechanisms eliminatea significant portion of NDA’s benefits 1 The majority of off-chip coherence traffic generated by these mechanisms is unnecessary 2 Much oftheoff-chip trafficcan be eliminatedif the coherence mechanism has insight into the memory accesses 3 ASIC

5. An Optimistic Approach • We find that an optimistic approach to coherence can address the challenges related to NDA coherence 2 1 Perform only the necessary coherence requests Gain insightsbefore anycoherence checks happens ASIC

6. CoNDA • We propose CoNDA, a mechanism that uses optimistic NDA execution to avoid unnecessary coherence traffic CPU NDA Time CPU Thread Execution Offload NDA kernel Concurrent CPU + NDA Execution Optimisticexecution No Coherence Request Signature Signature Send signatures • Coherence Resolution Commit or Re-execute

7. CoNDA • We propose CoNDA, a mechanism that uses optimistic NDA execution to avoid unnecessary coherence traffic CPU NDA Time CPU Thread Execution Offload NDA kernel Concurrent CPU + NDA Execution Optimisticexecution No Coherence Request Signature Signature Send signatures • Coherence Resolution • CoNDA comes within 10.4% and 4.4% of performance and energy of an ideal NDA coherence mechanism Commit or Re-execute

8. CoNDA: Efficient Cache Coherence Supportfor Near-Data Accelerators Amirali Boroumand SaugataGhose, MineshPatel, Hasan Hassan, Brandon Lucia, RachataAusavarungnirun, KevinHsieh,Nastaran Hajinazar, Krishna Malladi, Hongzhong Zheng, Onur Mutlu