global: implemented rudimentary memory manager to use available system memory

without forcing the OS to page swap

app/render/memorymanager.cpp

@@ -0,0 +1,58 @@
+#include "memorymanager.h"
+
+#include <QDebug>
+
+#ifdef Q_OS_WINDOWS
+#include <windows.h>
+#endif
+
+MemoryManager* MemoryManager::instance_ = nullptr;
+
+const uint64_t MemoryManager::minimum_available_memory_ = 2147483648;
+
+MemoryManager::MemoryManager(QObject* parent) :
+  QObject(parent)
+{
+}
+
+void MemoryManager::CreateInstance()
+{
+  instance_ = new MemoryManager();
+}
+
+MemoryManager *MemoryManager::instance()
+{
+  return instance_;
+}
+
+void MemoryManager::DestroyInstance()
+{
+  delete instance_;
+  instance_ = nullptr;
+}
+
+bool MemoryManager::ShouldFreeMemory()
+{
+  return (GetAvailableMemory() < minimum_available_memory_);
+}
+
+void MemoryManager::ConsumedMemory()
+{
+  if (ShouldFreeMemory()) {
+    emit FreeMemory();
+  }
+}
+
+uint64_t MemoryManager::GetAvailableMemory()
+{
+#ifdef Q_OS_WINDOWS
+  MEMORYSTATUSEX status;
+  status.dwLength = sizeof(status);
+  GlobalMemoryStatusEx(&status);
+  return status.ullAvailPhys;
+#else
+  long pages = sysconf(_SC_PHYS_PAGES);
+  long page_size = sysconf(_SC_PAGE_SIZE);
+  return pages * page_size;
+#endif
+}