diff --git a/paddle/gserver/layers/CudnnConvLayer.cpp b/paddle/gserver/layers/CudnnConvLayer.cpp
index a395a7223e394fec8f2365c6a20b3a2db040103e..0f932f960f6bacb5fc80273e5dfedf86bfb9d152 100644
--- a/paddle/gserver/layers/CudnnConvLayer.cpp
+++ b/paddle/gserver/layers/CudnnConvLayer.cpp
@@ -133,6 +133,8 @@ void CudnnConvLayer::reshape(int batchSize) {
   getOutput().setFrameHeight(outputH_);
   getOutput().setFrameWidth(outputW_);
 
+  // if the batchSize remains the same, set isSelectAlgo_ true.
+  // Otherwise, set isSelectAlgo_ false and select algo again.
   isSelectAlgo_ = (batchSize == batchNum_);
   batchNum_ = batchSize;