Āpta: Fault-tolerant object-granular CXL disaggregated memory for accelerating FaaS.

As cloud workloads increasingly adopt the fault-tolerant Function-as-a-Service (FaaS) model, demand for improved performance has increased. Alas, the performance of FaaS applications is heavily bottlenecked by the remote object store in which FaaS objects are maintained. We identify that the upcoming CXL-based cache-coherent disaggregated memory is a promising technology for maintaining FaaS objects. Our analysis indicates that CXL's low-latency, high-bandwidth access characteristics coupled with compute-side caching of objects, provides significant performance potential over an in-memory RDMA-based object store. We observe however that CXL lacks the requisite level of fault-tolerance necessary to operate at an inter-server scale within the datacenter. Furthermore, its cache-line granular accesses impose inefficiencies for object-granular data store accesses. We propose Apta, a CXL-based object-granular memory interface for maintaining FaaS objects. Apta's key innovation is a novel fault-tolerant coherence protocol for keeping the cached objects consistent without compromising availability in the face of compute server failures. Our evaluation of Apta using 6 full FaaS application workflows (totaling 26 functions) indicates that it outperforms a state-of-the-art fault-tolerant object caching protocol on an RDMA-based system by 21-90% and an uncached CXL-based system by 15-42%.