Virtual Timers: Using Hardware Physical Timers for Profiling Kernel Code-Paths

When evaluating the performance of commercial work- loads it is important to be able to examine overheads induced by the operating system kernel. Currently the only method for understanding kernel overheads is by using sample-based profiling tools. Although profiling can be very useful, one of its main limitations is that it does not allow the precise pro- filing of specific code-paths. Using hardware timers with a simple API allows addressing this issue, however, does not deal with context switch operations that incur during code execution, due to synchronous or asynchronous events. In this work we propose a new interface for virtualizing physical timers. Our interface takes into account context switches and provides the system programmer with simple calls to profile specific code-paths. We implement this API in the Linux kernel and demonstrate how it can be used to profile the system overhead of MySQL running a subset of TPC-H over iSCSI. Using our virtual timers we identify the time is spent in I/O related code-paths in the kernel with little effort. We also examine the overhead of the instrumentation code and find that it is less than 3% of the execution time in all experiments we perform.