Balancing register allocation across threads for a multithreaded network processor

• Authors: Xiaotong Zhuang;Santosh Pande
• Venue: Proceedings of the ACM SIGPLAN 2004 conference on Programming language design and implementation
• Year: 2004

Modern network processors employ multi-threading to allow concurrency amongst multiple packet processing tasks. We studied the properties of applications running on the network processors and observed that their imbalanced register requirements across different threads at different program points could lead to poor performance. Many times application needs demand some threads to be more performance critical than others and thus by controlling the register allocation across threads one could impact the performance of the threads and get the desired performance properties for concurrent threads. This prompts our work.Our register allocator aims to distribute available registers to different threads according to their needs. The compiler analyzes the register needs of each thread both at the point of a context switch as well as internally. Compiler then designates some registers as shared and some as private to each thread. Shared registers are allocated across all threads explicitly by the compiler. Values that are live across a context switch can not be kept in shared registers due to safety reasons; thus, only those live ranges that are internal to the context switch can be safely allocated to shared registers. Spill can cause a context switch. and thus, the problems of context switch and allocation are closely coupled and we propose a solution to this problem. The proposed interference graphs (GIG,BIG,IIG) distinguish variables that must use a thread's private registers from those that can use shared registers. We first estimate the register requirement bounds, then reduce from the upper bound gradually to achieve a good register balance among threads. To reduce the register needs, move insertions are inserted at program points that split the live ranges or the nodes on the interference graph. We show that the lower bound is reachable via live range splitting and is adequate for our benchmark programs for simultaneously assigning them on different threads. As our objective, the number of move instructions is minimized.Empirical results show that the compiler is able to effectively control the register allocation across threads by maximizing the number of shared registers. Speed-up for performance critical threads ranges from 18 to 24% whereas degradation for performance of non-critical threads ranges only from 1 to 4%.