Improving server software support for simultaneous multithreaded processors

Abstract

Simultaneous multithreading (SMT) represents a fundamental shift in processor capability. SMT's ability to execute multiple threads simultaneously within a single CPU offers tremendous potential performance benefits. However, the structure and behavior of software affects the extent to which this potential can be achieved. Consequently, just like the earlier arrival of multiprocessors, the advent of SMT processors prompts a needed re-evaluation of software that will run on them. This evaluation is complicated, since SMT adopts architectural features and operating costs of both its predecessors (uniprocessors and multiprocessors). The crucial task for researchers is to determine which software structures and policies - multi-processor, uniprocessor, or neither - are most appropriate for SMT.This paper evaluates how SMT's changes to the underlying hardware affects server software, and in particular, SMT's effects on memory allocation and synchronization. Using detailed simulation of an SMT server implemented in three different thread models, we find that the default policies often provided with multiprocessor operating systems produce unacceptably low performance. For each area that we examine, we identify better policies that combine techniques from both uniprocessors and multi-processors. We also uncover a vital aspect of multi-threaded synchronization (interaction with operating system thread scheduling) that previous research on SMT synchronization had overlooked. Overall, our results demonstrate how a few simple changes to applications' run-time support libraries can dramatically boost the performance of multi-threaded servers on SMT, without requiring modifications to the applications themselves.