diff --git a/src/shrpx_downstream.cc b/src/shrpx_downstream.cc
index afeee3e1..e4236e54 100644
--- a/src/shrpx_downstream.cc
+++ b/src/shrpx_downstream.cc
@@ -157,6 +157,8 @@ Downstream::Downstream(Upstream *upstream, MemchunkPool *mcpool,
   upstream_wtimer_.data = this;
   downstream_rtimer_.data = this;
   downstream_wtimer_.data = this;
+
+  rcbufs_.reserve(32);
 }
 
 Downstream::~Downstream() {