Page faults occurring within transactions jeopardize concurrency in Intel Restricted Transactional Memory (RTM). To make progress in spite of page-fault-induced aborts, the program must resort to the non-speculative fallback path and re-execute the affected transaction. Since the atomicity of a non-speculative transaction is guaranteed by impeding the execution of any other speculative transactions until the former completes, taking the fallback path is particularly harmful for performance. Therefore, such page-fault-induced aborts currently lead to thread serialization during the potentially long period of time taken to resolve them. In this work we propose PfTouch, a simple extension to RTM that allows page-fault handling to be moved out of non-speculative transactional execution in mutual exclusion. Our proposal sidesteps taking the fallback path in these cases and thus avoids its associated performance loss, by triggering page faults in the abort handler while other speculative transactions can run concurrently. PfTouch requires minimal modifications in the Intel RTM specification and keeps the OS unaltered. Through full-system simulation, we show that PfTouch achieves average reductions in execution time of 7.7% (up to 24.4%) for the STAMP benchmarks, closely matching the performance of the more complex suspended transactional mode in the IBM Power ISA.

事务中发生的页面错误会损害英特尔限制事务存储器（RTM）中的并发性。尽管页面错误导致中止，但仍要取得进展，程序必须诉诸非推测性的后备路径并重新执行受影响的事务。由于非投机交易的原子性是通过阻止其他投机交易的执行直到交易完成才得以保证的，因此采用后备路径对性能尤其有害。因此，这种由页面错误引起的中止当前会在解决它们所花费的潜在较长时间内导致线程序列化。在这项工作中，我们提出了PfTouch，它是RTM的简单扩展，它允许页面错误处理在相互排斥的情况下脱离非推测性事务执行。我们的建议在这些情况下回避了使用后备路径，从而通过在其他推测性事务可以同时运行时中止处理程序中触发页面错误来避免其相关的性能损失。PfTouch只需对Intel RTM规范进行最少的修改即可保持操作系统不变。通过全系统仿真，我们证明PfTouch可以将STAMP基准测试的执行时间平均减少7.7％（最多24.4％），与IBM Power ISA中更复杂的挂起事务模式的性能非常匹配。