Switch off loop splitting if child clique is small

Author: filikat

highs/ipm/hipo/factorhighs/FactorHiGHSSettings.h

@@ -50,6 +50,9 @@ const double kDynamicDiagCoeff = 1e-24;
 // metis
 const Int kMetisSeed = 42;
 
+// parallel assembly
+const Int kParallelAssemblyThresh = 100;
+
 struct Regul {
   double primal{};
   double dual{};

highs/ipm/hipo/factorhighs/HybridPackedFormatHandler.cpp

@@ -141,7 +141,7 @@ void HybridPackedFormatHandler::assembleChild(Int child_sn,
 
   bool parallel = S_->parTree() || S_->parNode();
 
-  if (parallel) {
+  if (parallel && child_clique_size > kParallelAssemblyThresh) {
     // The loop over the columns of the child clique has completely independent
     // iterations. The loop is split evenly among the available threads.