E81 CSE 532S: Advanced Multi-Paradigm Software Development

1. E81 CSE 532S: Advanced Multi-Paradigm Software Development C++11 Concurrency Design Chris Gill Department of Computer Science and Engineering Washington University in St. Louis cdgill@cs.wustl.edu

2. Dividing Work Between Threads • Static partitioning of data can be helpful • Makes threads (mostly) independent, ahead of time • Threads can read from and write to their own locations • Some partitioning of data is necessarily dynamic • E.g., Quicksort uses a pivot at run-time to split up data • May need to launch (or pass data to) a thread at run-time • Can also partition work by task-type • E.g., hand off specific kinds of work to specialized threads • E.g., a thread-per-stage pipeline that is efficient once primed • Number of threads to use is a key design challenge • E.g., std::thread::hardware_concurrency() is only a starting point (blocking, scheduling, etc. also matter)

3. Factors Affecting Performance • Need at least as many threads as hardware cores • Too few threads makes insufficient use of the resource • Oversubscription increases overhead due to task switching • Need to gauge for how long (and when) threads are active • Data contention and cache ping-pong • Performance degrades rapidly as cache misses increas • Need to design for low contention for cache lines • Need to avoid false sharing of elements (in same cache line) • Packing or spreading out data may be needed • E.g., localize each thread’s accesses • E.g., separate a shared mutex from the data that it guards

4. Additional Considerations • Exception safety • Affects both lock based and lock-free synchronization • Use std::packaged_taskand std::future to allow for an exception being thrown in a thread (see listing 8.3) • Scalability • How much of the code is actually parallizable? • Various theoretical formulas (including Amdahl’s) apply • Hiding latency • If nothing ever blocks you may not need concurrency • If something does, concurrency makes parallel progress • Improving responsiveness • Giving each thread its own task may simplify, speed up tasks