Merge pull request #2516 from franzpoeschel/fix-bp4-serializer-buffer…

…-shrinking

Do not shrink serialization buffer upon EndStep – Fix Performance Bug

source/adios2/toolkit/format/bp/bp4/BP4Serializer.cpp

@@ -372,13 +372,24 @@ void BP4Serializer::SerializeDataBuffer(core::IO &io) noexcept
     if (attributesSizeInData)
     {
         attributesSizeInData += 12; // count + length + end ID
-        m_Data.Resize(position + attributesSizeInData + 4,
-                      "when writing Attributes in rank=0\n");
+        // Take care not to shrink the buffer size in this.
+        // Otherwise, growing the buffer again later on is expensive
+        // - not because of reallocation, but because of (re)initialization
+        // with zeros by BufferSTL::Resize.
+        const size_t minSize = position + attributesSizeInData + 4;
+        if (m_Data.m_Buffer.size() < minSize)
+        {
+            m_Data.Resize(minSize, "when writing Attributes in rank=0\n");
+        }
         PutAttributes(io);
     }
     else
     {
-        m_Data.Resize(position + 12 + 4, "for empty Attributes\n");
+        const size_t minSize = position + 12 + 4;
+        if (m_Data.m_Buffer.size() < minSize)
+        {
+            m_Data.Resize(minSize, "for empty Attributes\n");
+        }
         // Attribute index header for zero attributes: 0, 0LL
         // Resize() already takes care of this
         position += 12;

testing/adios2/engine/bp/TestBPBufferSize.cpp

@@ -231,7 +231,8 @@ TEST_F(BPBufferSizeTest, SyncDeferredIdenticalUsage)
          * size
          * */
         const size_t TotalDataSize = Nx * sizeof(double) * 3;
-        const size_t MaxExtra = 16777216; /* 16MB extra allowed in buffer */
+        const size_t MaxExtra =
+            18 * 1024 * 1024; /* 18MB extra allowed in buffer */
         for (size_t step = 0; step < NSteps; ++step)
         {
             EXPECT_LT(bufsize_sync_beginstep[step], TotalDataSize + MaxExtra);