I have a program that loads a file (anywhere from 10MB to 5GB) a chunk at a time (ReadFile), and for each chunk performs a set of mathematical operations (basically calculates the hash).

After calculating the hash, it stores info about the chunk in an STL map (basically <chunkID, hash>) and then writes the chunk itself to another file (WriteFile).

That's all it does. This program will cause certain PCs to choke and die. The mouse begins to stutter, the task manager takes > 2 min to show, ctrl+alt+del is unresponsive, running programs are slow.... the works.

I've done literally everything I can think of to optimize the program, and have triple-checked all objects.

What I've done:

• Tried different (less intensive) hashing algorithms.
• Switched all allocations to nedmalloc instead of the default new operator
• Switched from stl::map to unordered_set, found the performance to still be abysmal, so I switched again to Google's dense_hash_map.
• Converted all objects to store pointers to objects instead of the objects themselves.
• Caching all Read and Write operations. Instead of reading a 16k chunk of the file and performing the math on it, I read 4MB into a buffer and read 16k chunks from there instead. Same for all write operations - they are coalesced into 4MB blocks before being written to disk.
• Run extensive profiling with Visual Studio 2010, AMD Code Analyst, and perfmon.
• Set the thread priority to THREAD_MODE_BACKGROUND_BEGIN
• Set the thread priority to THREAD_PRIORITY_IDLE
• Added a Sleep(100) call after every loop.

Even after all this, the application still results in a system-wide hang on certain machines under certain circumstances.

Perfmon and Process Explorer show minimal CPU usage (with the sleep), no constant reads/writes from disk, few hard pagefaults (and only ~30k pagefaults in the lifetime of the application on a 5GB input file), little virtual memory (never more than 150MB), no leaked handles, no memory leaks.

The machines I've tested it on run Windows XP - Windows 7, x86 and x64 versions included. None have less than 2GB RAM, though the problem is always exacerbated under lower memory conditions.

I'm at a loss as to what to do next. I don't know what's causing it - I'm torn between CPU or Memory as the culprit. CPU because without the sleep and under different thread priorities the system performances changes noticeably. Memory because there's a huge difference in how often the issue occurs when using unordered_set vs Google's dense_hash_map.

What's really weird? Obviously, the NT kernel design is supposed to prevent this sort of behavior from ever occurring (a user-mode application driving the system to this sort of extreme poor performance!?)..... but when I compile the code and run it on OS X or Linux (it's fairly standard C++ throughout) it performs excellently even on poor machines with little RAM and weaker CPUs.

What am I supposed to do next? How do I know what the hell it is that Windows is doing behind the scenes that's killing system performance, when all the indicators are that the application itself isn't doing anything extreme?

Any advice would be most welcome.