Fix bug with MLMG solver, always pass ghost cells to `SumBoundary` (#4078)

* Must pass number of ghost cells to call `SumBoundary`

* Apply suggestions from #4093

* Add inline comments on possible performance optimization

1 files changed, 5 insertions, 1 deletions

diff --git a/Source/Particles/MultiParticleContainer.cpp b/Source/Particles/MultiParticleContainer.cpp
index 7bbb973bb..529a78c8e 100644
--- a/Source/Particles/MultiParticleContainer.cpp
+++ b/Source/Particles/MultiParticleContainer.cpp
@@ -613,7 +613,11 @@ MultiParticleContainer::GetChargeDensity (int lev, bool local)
     }
     if (!local) {
         const Geometry& gm = allcontainers[0]->Geom(lev);
-        ablastr::utils::communication::SumBoundary(*rho, WarpX::do_single_precision_comms, gm.periodicity());
+        // Possible performance optimization:
+        // pass less than `rho->nGrowVect()` in the fifth input variable `dst_ng`
+        ablastr::utils::communication::SumBoundary(
+            *rho, 0, rho->nComp(), rho->nGrowVect(), rho->nGrowVect(),
+            WarpX::do_single_precision_comms, gm.periodicity());
     }
 
     return rho;