diff --git a/include/runtime/arena.h b/include/runtime/arena.h
index afbfca21b..eea6f9aff 100644
--- a/include/runtime/arena.h
+++ b/include/runtime/arena.h
@@ -12,7 +12,7 @@
 
 extern "C" {
 
-size_t const HYPERBLOCK_SIZE = (size_t)BLOCK_SIZE * 1024 * 1024;
+size_t const hyperblock_size = (size_t)BLOCK_SIZE * 1024 * 1024;
 
 // An arena can be used to allocate objects that can then be deallocated all at
 // once.
@@ -24,9 +24,9 @@ class arena {
   }
 
   ~arena() {
-    munmap(current_addr_ptr, HYPERBLOCK_SIZE);
+    munmap(current_addr_ptr, hyperblock_size);
     if (collection_addr_ptr)
-      munmap(collection_addr_ptr, HYPERBLOCK_SIZE);
+      munmap(collection_addr_ptr, hyperblock_size);
   }
 
   char *evacuate(char *scan_ptr);
@@ -124,6 +124,10 @@ class arena {
       = nullptr; // pointer to start of collection address space
   size_t num_collection_blocks
       = 0; // notional number of BLOCK_SIZE blocks in collection semispace
+  //
+  //	hyperblock_size is set to non-zero value just once, at runtime, by any thread.
+  //
+  static size_t hyperblock_size;
 };
 
 inline char arena::get_arena_semispace_id_of_object(void *ptr) {
@@ -135,7 +139,7 @@ inline char arena::get_arena_semispace_id_of_object(void *ptr) {
   //	Set the low bits to 1 to get the address of the last byte in the hyperblock.
   //
   uintptr_t end_address
-      = reinterpret_cast<uintptr_t>(ptr) | (HYPERBLOCK_SIZE - 1);
+      = reinterpret_cast<uintptr_t>(ptr) | (hyperblock_size - 1);
   return *reinterpret_cast<char *>(end_address);
 }
 
@@ -166,7 +170,7 @@ inline void *arena::kore_arena_alloc(size_t requested) {
     //	We move the tripwire to 1 past the end of our hyperblock so that we have
     //	a well defined comparison that will always be false until the next arena swap.
     //
-    tripwire = current_addr_ptr + HYPERBLOCK_SIZE;
+    tripwire = current_addr_ptr + hyperblock_size;
   }
   void *result = allocation_ptr;
   allocation_ptr += requested;
diff --git a/runtime/alloc/arena.cpp b/runtime/alloc/arena.cpp
index fbfd7e527..1dd10bb2a 100644
--- a/runtime/alloc/arena.cpp
+++ b/runtime/alloc/arena.cpp
@@ -23,11 +23,38 @@ thread_local bool time_for_collection = false;
 thread_local bool gc_enabled = true;
 #endif
 
+size_t arena::hyperblock_size = 0;
+
 void arena::initialize_semispace() {
+  if (hyperblock_size == 0) {
+    //
+    //	No thread has initialized hyperblock_size at this moment so we will.
+    //	We get the size of the memory from the OS as number of physical pages * page size.
+    //
+    size_t pages = sysconf(_SC_PHYS_PAGES);
+    size_t page_size = sysconf(_SC_PAGE_SIZE);
+    size_t v = pages * page_size;
+    //
+    //	We require hyperblock_size to be a power of 2 for bit twiddling, so we
+    //	round up to the next power of 2.
+    //
+    --v;
+    v |= v >> 1;
+    v |= v >> 2;
+    v |= v >> 4;
+    v |= v >> 8;
+    v |= v >> 16;
+    v |= v >> 32;
+    v++;
+    //
+    //	Writing to a class static variable might be a race with another thread.
+    //
+    hyperblock_size = v;
+  }
   //
   //	Current semispace is uninitialized so mmap() a big chuck of address space.
   //
-  size_t request = 2 * HYPERBLOCK_SIZE;
+  size_t request = 2 * hyperblock_size;
   void *addr = mmap(
       nullptr, // let OS choose the address
       request, // Linux and MacOS both allow up to 64TB
@@ -46,12 +73,12 @@ void arena::initialize_semispace() {
   auto *start_block = reinterpret_cast<char *>(addr);
   auto *end_block = start_block + request;
   //
-  //	We allocated 2 * HYPERBLOCK_SIZE worth of address space but we're only going to use 1, aligned on a
-  //	HYPERBLOCK_SIZE boundry. This is so we can get end of the hyperblock by setting the low bits of any
+  //	We allocated 2 * hyperblock_size worth of address space but we're only going to use 1, aligned on a
+  //	hyperblock_size boundry. This is so we can get end of the hyperblock by setting the low bits of any
   //	address within the space to 1.
   //
   current_addr_ptr = reinterpret_cast<char *>(
-      std::align(HYPERBLOCK_SIZE, HYPERBLOCK_SIZE, addr, request));
+      std::align(hyperblock_size, hyperblock_size, addr, request));
   //
   //	Release any unused address space at the start of the mmap()ed block.
   //
@@ -61,7 +88,7 @@ void arena::initialize_semispace() {
   //
   //	Release any unused address space at the end of the mmap()ed block.
   //
-  auto *end_aligned = current_addr_ptr + HYPERBLOCK_SIZE;
+  auto *end_aligned = current_addr_ptr + hyperblock_size;
   if (size_t back_slop = end_block - end_aligned) {
     munmap(end_aligned, back_slop);
   }
@@ -69,7 +96,7 @@ void arena::initialize_semispace() {
   //	We put a semispace id in the last byte of the hyperblock so we can identify which semispace an address
   //	belongs to by setting the low bits to 1 to access this id.
   //
-  current_addr_ptr[HYPERBLOCK_SIZE - 1] = allocation_semispace_id;
+  current_addr_ptr[hyperblock_size - 1] = allocation_semispace_id;
   allocation_ptr = current_addr_ptr;
   //
   //	We set the tripwire for this space so we get trigger a garbage collection when we pass BLOCK_SIZE of memory