Add comments for Workspace2D destructor & test tearDowns

Refs #8216

Code/Mantid/Framework/DataObjects/src/Workspace2D.cpp

@@ -28,6 +28,16 @@ namespace Mantid
     Workspace2D::~Workspace2D()
     {
       // Clear out the memory
+
+      // The omp loop is here primarily for MSVC. On MSVC 2012 
+      // when you allocate memory in a multithreaded loop, like our cow_ptrs will do,
+      // the deallocation time increases by a huge amount if the memory is just
+      // naively deallocated in a serial order. This is because when it was allocated
+      // in the omp loop then the actual memory ends up being interleaved and
+      // then trying to deallocate this serially leads to lots of swapping in and out of
+      // memory.
+      // See http://social.msdn.microsoft.com/Forums/en-US/2fe4cfc7-ca5c-4665-8026-42e0ba634214/visual-studio-2012-slow-deallocation-when-new-called-within-openmp-loop?forum=vcgeneral
+
       PARALLEL_FOR1(this)
       for (int64_t i=0; i < static_cast<int64_t>(data.size()); i++)
       {

Code/Mantid/Framework/TestHelpers/src/TearDownWorld.cpp

@@ -5,6 +5,11 @@
 #include "MantidAPI/AlgorithmManager.h"
 #include "MantidAPI/AnalysisDataService.h"
 
+// On MSVC all workspaces must be deleted by the time main() exits as the
+// Workspace destruction can call to an OpenMP loop which is not allowed
+// on MSVC after main() exits.
+// See Workspace2D::~Workspace2D()
+
 namespace
 {
   /// Define single ClearAlgorithmManager object