Editing GHC/Data Parallel Haskell (section)

==== Parallel execution ====

By default, a Haskell program uses only one OS thread, and hence, also only one CPU core for execution.  To use multiple cores, we need to invoke the executable with an explicit RTS command line option — e.g., <code>./dotp +RTS -N2</code> uses two cores.  (Strictly speaking, it uses two OS threads, which will be scheduled on two separate cores if available.)  To determine the runtime of parallel code, measuring CPU time, as demonstrated in the [[GHC/Data Parallel Haskell/MainTimed|timed variant of the dot product example]], is not sufficient anymore.  We need to measure wall clock times instead.  For proper benchmarking, it is advisable to use a library, such as [http://hackage.haskell.org/package/criterion criterion].

A beautiful property of DPH is that the number of threads used to execute a program only affects its performance, but not the result.  So, it is fine to do all debugging concerning correctness with just one core and to move to multiple cores only for performance debugging.

Data Parallel Haskell –and more generally, GHC's multi-threading support– currently only aims at multicore processors or uniform memory access (UMA) multi-processors.  Performance on non-uniform memory access (NUMA) machines is often bad as GHC's runtime makes no effort at optimising placement.