Windows Stalls When My Program Uses Swapfile

Question

I am running a user mode program on normal priority. My program is searching an NP problem, and as a result, uses up a lot of memory which eventually ends up in the swap file.

Then my mouse freezes up, and it takes forever for task manager to open up and let me end the process.

What I want to know is how I can stop my Windows operating system from completely locking up from this even though only 1 out of my 2 cores are being used.

Edit: Thanks for the replies.

I know that making it use less memory will help, but it just doesn't make sense to me that the whole OS should lock up.

Answer 1

The obvious answer is "use less memory". When your app uses up all the available memory, the OS has to page the task manager (etc.) out to make room for your app. When you switch programs, the OS has to page the other programs back in (as they are needed). Disk reads are slower than memory reads, so everything appears to be going slower.

If you want to avoid this, have your app manage its own memory, or use a better algorithm than brute force. (There are genetic algorithms, simulated annealing, etc.)

Answer 2

I don't think there's a programming answer to this question, aside from "restructure your app to use less memory." The swapfile problem is most likely due to the bottleneck in accessing the disk, especially if you're using an IDE HDD or a highly fragmented swapfile.

Answer 3

The problem is that when another program (e.g. explorer.exe) is going to execute, all of its code and memory has been swapped out. To make room for the other program Windows has to first write data that your program is using to disk, then load up the other program's memory. Every new page of code that is executed in the other program requires disk access, causing it to run slowly.

I don't know the access pattern of your program, but I'm guessing it touches all of its memory pages a lot in a random fashion, which makes the problem worse because as soon as Windows evicts a memory page from your program, suddenly you need it again and Windows has to find some other page to give the same treatment.

To give other processes more RAM to live in, you can use SetProcessWorkingSetSize to reduce the maximum amount of RAM that your program may use. Of course this will make your program run more slowly because it has to do more swapping.

Another alternative you could try is to add more drives to the system, and distribute the swap files over those. You may have a dual-core CPU, but you have only a single drive. Distributing the swap file over multiple drives allows Windows to balance work across them (although I don't have first-hand experience of how well it does this).

Answer 4

It's a bit extreme, but you could always minimise your swap file so you don't have all the disk thashing, and your program isn't allowed to allocate much virtual memory. Under Control panel / Advanced / Advanced tab / Perfromance / Virtual memory, set the page file to custom size and enter a value of 2mb (smallest allowed on XP). When an allocation fails, you should get an exception and be able exit gracefully. It doesn't quite fix your problem, just speeds it up ;)

Another thing worth considering would be if you are ona 32bit platform, port to a 64bit system and get a box with much more addressable RAM.