WoperTM: Got nacks? Use them!

Abstract

The simplicity of requester-wins has made it the preferred choice for conflict resolution in commercial implementations of Hardware Transactional Memory (HTM), which typically have relied on conventional locking to escape from conflict-induced livelocks. Prior work advocates for combining requester-wins and requester-loses to ensure progress for higher-priority transactions, yet it fails to take full advantage of the available features, namely, protocol support for nacks. This paper introduces WoperTM, a dual-policy, best-effort HTM design that resolves conflicts using requester-loses policy in the common case. Our key insight is that, since nacks are required to support priorities in HTM, performance can be improved at nearly no extra cost by allowing regular transactions to benefit from requester-loses, instead of only those involving a high-priority transaction. Experimental results using gem5 and STAMP show that WoperTM can significantly reduce squashed work and improve execution times by 12% with respect to power transactions, with negligible hardware overhead.