Issue with low-memory notification

The low-memory notification was getting into an infinite loop.  This
fixes the loop termination, and provides a test for platforms which
support low-memory notification.

src/mem/largealloc.h

@@ -263,7 +263,7 @@ namespace snmalloc
             {
               lazy_decommit();
             }
-          } while (old_epoch <= new_epoch);
+          } while (old_epoch < new_epoch);
         }
       }
 #endif

src/test/perf/low_memory/low-memory.cc

@@ -0,0 +1,127 @@
+#include <snmalloc.h>
+#include <test/measuretime.h>
+#include <test/setup.h>
+#include <unordered_set>
+
+#include <vector>
+#include <iostream>
+
+using namespace snmalloc;
+
+struct Node
+{
+  Node* next;
+};
+
+class Queue
+{
+  Node* head;
+  Node* tail;
+
+  Node* new_node(size_t size)
+  {
+    auto result = (Node*)ThreadAlloc::get()->alloc(size);
+    result->next = nullptr;
+    return result;
+  }
+
+public:
+  Queue()
+  {
+    head = new_node(1);
+    tail = head;
+  }
+
+  void add(size_t size)
+  {
+    tail->next = new_node(size);
+    tail = tail->next;
+  }
+
+  void try_remove()
+  {
+    if (head->next == nullptr)
+      return;
+
+    Node* next = head->next;
+    ThreadAlloc::get()->dealloc(head);
+    head = next;
+  }
+
+};
+
+bool has_pressure()
+{
+  static thread_local uint64_t epoch;
+  
+  if constexpr (!pal_supports<LowMemoryNotification, GlobalVirtual>)
+  {
+    return false;
+  }
+
+  uint64_t current_epoch = default_memory_provider.low_memory_epoch();
+  bool result = epoch != current_epoch;
+  epoch = current_epoch;
+  return result;
+}
+
+
+void reach_pressure(Queue& allocations)
+{
+  size_t size = 1;
+  size_t i = 0;
+  while(!has_pressure())
+  {
+    i++;
+    allocations.add(size);
+    allocations.try_remove();
+    allocations.add(size);
+
+    if (i % 8192 == 0)
+      size ++;
+  }
+}
+
+void reduce_pressure(Queue& allocations)
+{
+  size_t size = 1;
+  size_t i = 0;
+
+  for (size_t n = 0; n < 1000; n++)
+  {
+    allocations.try_remove();
+    allocations.try_remove();
+    allocations.add(size);
+
+    if (i % 8192 == 0)
+      size ++;
+  }
+}
+
+
+int main(int, char**)
+{
+
+  Queue allocations;
+
+
+  if constexpr (!pal_supports<LowMemoryNotification, GlobalVirtual>)
+  {
+    std::cout << "Pal does not support low-memory notification! Test not run" 
+      << std::endl;
+    return 0;
+  }
+
+  setup();
+
+  for(size_t i = 0; i < 10; i++)
+  {
+    reach_pressure(allocations);
+    std::cout << "Pressure " <<  i << std::endl;
+
+    reduce_pressure(allocations);
+
+  }
+
+  return 0;
+}