Architectural support for message queue task parallelism

Wu, Qinzhe
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The scaling of threads is an attractive way to exploit task-level parallelism and boost performance. From the perspective of software programming, many applications (e.g., network package processing, SQL queries) could be composite of a set of small tasks. Those tasks are arranged in a data flow graph and each task is undertaken by some threads. Message queues are often used to coordinate the tasks among the threads. On the other side, thread scaling is in favor of the hardware advancing trend that there are more Processing Elements (PE) in modern Chip Multiprocessors (CMP) than ever before. This is because single PE cannot simply run faster due to power and thermal limitations; instead architects have to use more transistors for increasing number of PEs, in order to improve the overall computing power of a processor. Unfortunately, this paradigm using message queues to drive parallel tasks sometime leads to diminishing performance returns due to issues lying in the architecture and system design. Particularly, the conventional coherent shared-memory architectures let task-parallel workloads suffer from unnecessary synchronization overhead and load-to-use latency. For instance, when passing messages through queues, multiple threads could contend for the exclusivity of the cacheline where the shared queue data structure stays. The more threads, the more severe the contention is, because every transition upgrading a cacheline from shared to exclusive state needs to invalidate more copies in the private caches of other cores, and waits for the acknowledgements from more cores. Such a overhead hurts the scalability of threads synchronizing via message queues. Adding to the coherence overhead, the load-to-use latency (from a consumer requesting data until the data being moved to the consumer to use) is often on the critical path, slowing down the computation. This is because the cache hierarchy in modern processors creates some layers of local storage to buffer data separately for different cores. Therefore, serving message queue data in an ondemand manner incurs longer load-to-use latency. It is also challenging to schedule message-driven tasks to use cores efficiently when arrival rate and service rate mismatch. It wastes CPU cycles if a runtime system leaves tasks blocked on full/empty message queues, while switching tasks has additional scheduling overheads. Diverse system topologies further complicate the problem, as the scheduling also needs to take data locality into consideration. This dissertation explores architectural supports for enhancing the scalability of message queue task parallelism, reducing the load-to-use latency, as well as avoiding blocking. Specifically, this dissertation designs and evaluates a message queue architecture that lowers the overhead of synchronization on shared queue states, a speculation technique to hide the load-to-use latency, as well as a locality-aware message queue runtime system with low overhead on scheduling and buffer resizing. The first contribution of the dissertation is Virtual-Link scalable message queue architecture (VL). Instead of having threads access the shared queue state variables (i.e., head, tail, or lock) atomically, VL provides configurable hardware support, providing both data transfer and synchronization. Unlike other hardware queue architectures with dedicated network, VL reuses the existing cache coherence network and delivers a virtualized channel as if there were a direct link (or route) between two arbitrary PEs. VL facilitates efficient synchronized data movement between M:N producers and consumers with several benefits: (i) the number of sharers on synchronization primitives is reduced to zero, eliminating a primary bottleneck of traditional lock-free queues, (ii) memory spills, snoops, and invalidations are reduced, (iii) data stays on the fast path (inside the interconnect) a majority of the time. Another contribution of the dissertation is SPAMeR speculation mechanism. SPAMeR has the capability to speculatively push messages in anticipation of consumer message requests. With the speculation, the latency of moving data from the source to the consumer that needs the data could be partially or fully overlapped with the message processing time. Unlike pre-fetch approaches which predict what addresses to fetch next, with a queue we know exactly what data is needed next but not when it is needed; SPAMeR proposes algorithms to learn from queue operation history in order to predict this. Finally the dissertation contributes ARMQ locality-aware runtime. ARMQ collects a set of approaches that avoids message queue blocking, ranging from the most general yielding, to dynamically resizing the buffer, and to spawning helper tasks. On one hand, ARMQ minimizes the overheads (e.g., wasteful polling, context switch, memory allocation and copying etc.) with a few techniques (e.g., userspace threading, chunk-based ringbuffer etc.) On the other hand, ARMQ schedules the message-driven tasks precisely and opportunely, in order to maximize the data locality preserved (in favor of cache) and balance the resource allocation.