diff --git a/source/adios2/engine/bp5/BP5Engine.h b/source/adios2/engine/bp5/BP5Engine.h
index 7841caf5cf..6bc187675d 100644
--- a/source/adios2/engine/bp5/BP5Engine.h
+++ b/source/adios2/engine/bp5/BP5Engine.h
@@ -110,7 +110,7 @@ class BP5Engine
     MACRO(NodeLocal, Bool, bool, false)                                        \
     MACRO(verbose, Int, int, 0)                                                \
     MACRO(CollectiveMetadata, Bool, bool, true)                                \
-    MACRO(NumAggregators, UInt, unsigned int, 999999)                          \
+    MACRO(NumAggregators, UInt, unsigned int, 0)                               \
     MACRO(NumSubFiles, UInt, unsigned int, 999999)                             \
     MACRO(FileSystemPageSize, UInt, unsigned int, 4096)                        \
     MACRO(AggregationType, AggregationType, int,                               \
diff --git a/source/adios2/toolkit/aggregator/mpi/MPIShmChain.cpp b/source/adios2/toolkit/aggregator/mpi/MPIShmChain.cpp
index 6f61395ad5..7fe175e1f4 100644
--- a/source/adios2/toolkit/aggregator/mpi/MPIShmChain.cpp
+++ b/source/adios2/toolkit/aggregator/mpi/MPIShmChain.cpp
@@ -308,6 +308,10 @@ MPIShmChain::ShmDataBuffer *MPIShmChain::LockProducerBuffer()
 
 void MPIShmChain::UnlockProducerBuffer()
 {
+    m_Shm->lockSegment.lock();
+    ++m_Shm->NumBuffersFull;
+    m_Shm->lockSegment.unlock();
+
     if (m_Shm->producerBuffer == LastBufferUsed::A)
     {
         m_Shm->lockA.unlock();
@@ -316,7 +320,6 @@ void MPIShmChain::UnlockProducerBuffer()
     {
         m_Shm->lockB.unlock();
     }
-    ++m_Shm->NumBuffersFull;
 }
 
 MPIShmChain::ShmDataBuffer *MPIShmChain::LockConsumerBuffer()
@@ -365,6 +368,10 @@ MPIShmChain::ShmDataBuffer *MPIShmChain::LockConsumerBuffer()
 
 void MPIShmChain::UnlockConsumerBuffer()
 {
+    m_Shm->lockSegment.lock();
+    --m_Shm->NumBuffersFull;
+    m_Shm->lockSegment.unlock();
+
     if (m_Shm->consumerBuffer == LastBufferUsed::A)
     {
         m_Shm->lockA.unlock();
@@ -373,7 +380,6 @@ void MPIShmChain::UnlockConsumerBuffer()
     {
         m_Shm->lockB.unlock();
     }
-    --m_Shm->NumBuffersFull;
 }
 
 } // end namespace aggregator