Server applications must process requests as quickly as pos-sible. Because some requests depend on earlier requests, there is often a tension between increasing throughput and maintaining the proper semantics for dependent requests. Operating system transactions make it easier to write reli-able, high-throughput server applications because they allow the application to execute non-interfering requests in paral-lel, even if the requests operate on OS state, such as file data. By changing less than 200 lines of application code, we improve performance of a replicated Byzantine Fault Toler-ant (BFT) system by up to 88% using server-side specula-tion, and we improve concurrent performance up to 80% for an IMAP email server by changing only 40 lines. Achiev-ing these results requires substantial enhancements to system transactions, including the ability to pause and resume trans-actions, and an API to commit transactions in a pre-defined order.
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