diff --git a/roxie/roxiemem/roxiemem.cpp b/roxie/roxiemem/roxiemem.cpp
index 6e4926a1593..ce9477b1c35 100644
--- a/roxie/roxiemem/roxiemem.cpp
+++ b/roxie/roxiemem/roxiemem.cpp
@@ -5155,18 +5155,23 @@ class CChunkingRowManager : public CRowManager
             unsigned pageLimit = getPageLimit();
             if (totalPages <= pageLimit)
             {
+                unsigned newHeapPages;
                 if (pageLimit != UNLIMITED_PAGES)
                 {
                     //Use compare_exchange so that only one thread can increase the number of pages at a time.
                     //(Don't use atomic_add because we need to check the limit hasn't been exceeded.)
-                    if (!totalHeapPages.compare_exchange_weak(numHeapPages, numHeapPages + numRequested, std::memory_order_relaxed))
+                    newHeapPages = numHeapPages + numRequested;
+                    if (!totalHeapPages.compare_exchange_weak(numHeapPages, newHeapPages, std::memory_order_relaxed))
                         continue;
                 }
                 else
                 {
                     //Unlimited pages => just increment the total
-                    totalHeapPages.fetch_add(numRequested, std::memory_order_relaxed);
+                    newHeapPages = totalHeapPages.fetch_add(numRequested, std::memory_order_relaxed) + numRequested;
                 }
+
+                //Ensure the total is correct if two threads allocate big blocks of memory at the same time
+                totalPages = dataBuffPages + newHeapPages;
                 break;
             }
 
@@ -5203,9 +5208,15 @@ class CChunkingRowManager : public CRowManager
 
         if (totalPages > peakPages)
         {
-            if (trackMemoryByActivity)
-                getPeakActivityUsage();
-            peakPages = totalPages;
+            unsigned value = totalPages;
+            //Atomically update the peak - this could temporarily reduce the value, but it will eventually be correct.
+            while (value > peakPages.load())
+                value = peakPages.exchange(value, std::memory_order_acq_rel);
+
+            //Check if this thread actually updated the peak value - or did another thread beat us to it.
+            if (totalPages == peakPages)
+                if (trackMemoryByActivity)
+                    getPeakActivityUsage();
         }
         return totalPages;
     }