Detecting memory running low BEFORE allocations start failing on Windows

Question

We have an application that could potentially allocate a large number of small objects (depending on user input). Sometimes the application runs out of memory and effectively crashes.

However, if we knew that memory allocations were becoming tight there are some lower-priority objects which could be destroyed and thereby allow us to gracefully degrade the user results.

What's the best way to detect that memory for a process is running low before calls to 'new' actually fail? We could call API functions like GetProcessWorkingSetSize() or GetProcessMemoryInfo() but how do you know when the limits on a given machine are being reached (e.g. with 80% of maximum allocations)?

Answer 1

• At start up, allocate a memory reserve.
• Then use set_new_handler() to install a hook that will detect allocations failures.
• When one happens:
  • Free the reserve (so you have enough free memory to work with).
  • Run your code that finds and frees low priority objects.
  • When that has done its job, try to reallocate the reserve again (for next time).
  • Finally return to let the original allocation attempt retry.

Answer 2

If it's a 32-bit process then you would want to ensure that you don't use more than 1.6GB, which is 80% of 2.0GB, the max allowed for your process. Calling GlobalMemoryStatusEx will fill in the struct MEMORYSTATUSEX.ullAvailVirtual, when this is only 400MB available (or less) then you are at your threshold.