diff --git a/src/allocator.h b/src/allocator.h
index 2f48f38b..998653be 100644
--- a/src/allocator.h
+++ b/src/allocator.h
@@ -36,8 +36,12 @@ struct MemBlock {
   uint8_t *begin, *last, *end;
 };
 
-template <size_t BLOCK_SIZE> struct BlockAllocator {
-  BlockAllocator() : retain(nullptr), head(nullptr) {}
+struct BlockAllocator {
+  BlockAllocator(size_t block_size, size_t isolation_threshold)
+      : retain(nullptr),
+        head(nullptr),
+        block_size(block_size),
+        isolation_threshold(std::min(block_size, isolation_threshold)) {}
 
   ~BlockAllocator() {
     for (auto mb = retain; mb;) {
@@ -59,14 +63,14 @@ template <size_t BLOCK_SIZE> struct BlockAllocator {
   }
 
   void *alloc(size_t size) {
-    if (size >= BLOCK_SIZE) {
+    if (size >= isolation_threshold) {
       auto mb = alloc_mem_block(size);
       mb->last = mb->end;
       return mb->begin;
     }
 
     if (!head || head->end - head->last < static_cast<ssize_t>(size)) {
-      head = alloc_mem_block(BLOCK_SIZE);
+      head = alloc_mem_block(block_size);
     }
 
     auto res = head->last;
@@ -79,6 +83,11 @@ template <size_t BLOCK_SIZE> struct BlockAllocator {
 
   MemBlock *retain;
   MemBlock *head;
+  // size of single memory block
+  size_t block_size;
+  // if allocation greater or equal to isolation_threshold bytes is
+  // requested, allocate dedicated block.
+  size_t isolation_threshold;
 };
 
 template <typename BlockAllocator>
@@ -101,8 +110,6 @@ StringRef concat_string_ref(BlockAllocator &alloc, const StringRef &a,
   return StringRef{dst, a.size() + b.size()};
 }
 
-using DefaultBlockAllocator = BlockAllocator<1024>;
-
 } // namespace aria2
 
 #endif // ALLOCATOR_H
diff --git a/src/shrpx_downstream.cc b/src/shrpx_downstream.cc
index 11462d7c..ed1edf2e 100644
--- a/src/shrpx_downstream.cc
+++ b/src/shrpx_downstream.cc
@@ -116,6 +116,7 @@ Downstream::Downstream(Upstream *upstream, MemchunkPool *mcpool,
     : dlnext(nullptr),
       dlprev(nullptr),
       response_sent_body_length(0),
+      balloc_(1024, 1024),
       req_(balloc_),
       resp_(balloc_),
       request_start_time_(std::chrono::high_resolution_clock::now()),
@@ -357,9 +358,8 @@ void add_header(size_t &sum, HeaderRefs &headers, const StringRef &name,
 } // namespace
 
 namespace {
-void append_last_header_key(DefaultBlockAllocator &balloc, bool &key_prev,
-                            size_t &sum, HeaderRefs &headers, const char *data,
-                            size_t len) {
+void append_last_header_key(BlockAllocator &balloc, bool &key_prev, size_t &sum,
+                            HeaderRefs &headers, const char *data, size_t len) {
   assert(key_prev);
   sum += len;
   auto &item = headers.back();
@@ -371,7 +371,7 @@ void append_last_header_key(DefaultBlockAllocator &balloc, bool &key_prev,
 } // namespace
 
 namespace {
-void append_last_header_value(DefaultBlockAllocator &balloc, bool &key_prev,
+void append_last_header_value(BlockAllocator &balloc, bool &key_prev,
                               size_t &sum, HeaderRefs &headers,
                               const char *data, size_t len) {
   key_prev = false;
@@ -926,6 +926,6 @@ void Downstream::set_assoc_stream_id(int32_t stream_id) {
 
 int32_t Downstream::get_assoc_stream_id() const { return assoc_stream_id_; }
 
-DefaultBlockAllocator &Downstream::get_block_allocator() { return balloc_; }
+BlockAllocator &Downstream::get_block_allocator() { return balloc_; }
 
 } // namespace shrpx
diff --git a/src/shrpx_downstream.h b/src/shrpx_downstream.h
index 7b9dfa83..2ea0bf2e 100644
--- a/src/shrpx_downstream.h
+++ b/src/shrpx_downstream.h
@@ -52,7 +52,7 @@ struct BlockedLink;
 
 class FieldStore {
 public:
-  FieldStore(DefaultBlockAllocator &balloc, size_t headers_initial_capacity)
+  FieldStore(BlockAllocator &balloc, size_t headers_initial_capacity)
       : content_length(-1),
         balloc_(balloc),
         buffer_size_(0),
@@ -112,7 +112,7 @@ public:
   int64_t content_length;
 
 private:
-  DefaultBlockAllocator &balloc_;
+  BlockAllocator &balloc_;
   HeaderRefs headers_;
   // trailer fields.  For HTTP/1.1, trailer fields are only included
   // with chunked encoding.  For HTTP/2, there is no such limit.
@@ -126,7 +126,7 @@ private:
 };
 
 struct Request {
-  Request(DefaultBlockAllocator &balloc)
+  Request(BlockAllocator &balloc)
       : fs(balloc, 16),
         recv_body_length(0),
         unconsumed_body_length(0),
@@ -182,7 +182,7 @@ struct Request {
 };
 
 struct Response {
-  Response(DefaultBlockAllocator &balloc)
+  Response(BlockAllocator &balloc)
       : fs(balloc, 32),
         recv_body_length(0),
         unconsumed_body_length(0),
@@ -376,7 +376,7 @@ public:
 
   DefaultMemchunks pop_response_buf();
 
-  DefaultBlockAllocator &get_block_allocator();
+  BlockAllocator &get_block_allocator();
 
   enum {
     EVENT_ERROR = 0x1,
@@ -397,7 +397,7 @@ public:
   int64_t response_sent_body_length;
 
 private:
-  DefaultBlockAllocator balloc_;
+  BlockAllocator balloc_;
 
   Request req_;
   Response resp_;