Branch prediction and simultaneous multithreading

In this paper, we examine the behavior of three of the best performing branch prediction strategies proposed in the literature while executing simultaneously several threads of instructions. Our simulations show that in a multiprogramming environment, if the sizes of the tables (PHT/BTB) are proportional to the number of active threads, there are very few interactions. With parallel workloads, we could have expected a beneficial sharing effect. In fact, it is very dependent an the branch predictors and in the best case, the gains stay very limited. We also show that, for the three predictors, whether in multiprogramming or in parallel processing, if the sizes of the tables are kept small, conflicts in the BTB induce a significant increase in mispredictions. However, for parallel processing with the gshare scheme, the resulting misprediction ratios for 2 or 4 threads stay below those exhibited by 1 thread. Finally, we study the impact of the addition of one Return Address Stack per context and show that a 12-deep stack per thread is sufficient to enhance greatly the accuracy of branch prediction.