Will Windows be still able to allocate memory when free space in physical memory is very low?

Question

On Windows 32-bit system the application is being developed using Visual Studio:

Lets say lots of other application running on my machine and they have occupied almost all of physical memory and only 1 MB memory is left free. If my application (which has not yet allocated any memory) tries to allocate, say 2 MB, will the call be successful?

My guess: In theory, each Windows application has 2GB of virtual memory available. So I believe this call should be successful (regardless how much physical memory is available). But I am not sure on this. That's why asking here.

Answer 1

Windows gives a rock-hard guarantee that this will always work. A process can only allocate virtual memory when Windows can commit space in the paging file for the allocation. If necessary, it will grow the paging file to make the space available. If that fails, for example when the paging file grows beyond the preset limit, then the allocation fails as well. Windows doesn't have the equivalent of the Linux "OOM killer", it does not support over-committing that may require an operating system to start randomly killing processes to find RAM.

Do note that the "always works" clause does have a sting. There is no guarantee on how long this will take. In very extreme circumstances the machine can start thrashing where just about every memory access in the running processes causes a page fault. Code execution slows down to a crawl, you can lose control with the mouse pointer frozen when Explorer or the mouse or video driver start thrashing as well. You are well past the point of shopping for RAM when that happens. Windows applies quotas to processes to prevent them from hogging the machine, but if you have enough processes running then that doesn't necessarily avoid the problem.

Answer 2

Of course. It would be lousy design if memory had to be wasted now in order to be used later. Operating systems constantly re-purpose memory to its most advantageous use at any moment. They don't have to waste memory by keeping it free just so that it can be used later.

Answer 3

This is one of the benefits of virtual memory with a page file. Because the memory is virtual, the system can allocate more virtual memory than physical memory. Virtual memory that cannot fit in physical memory, is pushed out to the page file.

So the fact that your system may be using all of the physical memory does not mean that your program will not be able to allocate memory. In the scenario that you describe, your 2MB memory allocation will succeed. If you then access that memory, the virtual memory will be paged in to physical memory and very likely some other pages (maybe in your process, maybe in another process) will be pushed out to the page file.

Answer 4

Well, it will succeed as long as there's some memory for it - apart from physical memory, there's also the page file.

However, once you reach the limit of both RAM and the page file, you're done for and that's when the out of memory situation really starts being fun.

Now, systems like Windows Vista will try to use all of your available RAM, pretty much for caching. That's a good thing, and when there's a request for memory from an application, the cache will be thrown away as needed.

As for virtual memory, you can request much more than you have available, regardless of your RAM or page file size. Only when you commit the memory does it actually need some backing - either RAM or the page file. On 64-bit, you can easily request terabytes of virtual memory - that doesn't mean you'll get it when you try to commit it, though :P

Answer 5

If your application is unable to allocate a physical memory (RAM) block to store information, the operating system takes over and 'pages' or stores sections that are in RAM on disk to free up physical memory so that your program is able to perform the allocation. This is done automatically and is completely invisible to your applications.

So, in your example, on a system that has 1MB RAM free, if your application tries to allocate memory, the operating system will page certain contents of physical memory to disk and free up RAM for your application. Your application will not crash in this case.

This, obviously is much more complicated than that.

There are several ways to configure a page file on Windows (fixed size, variable size and on which disk). If you run out of physical memory, and out of hard drive space (because your page file has grown very large due to excessive 'paging') or reach the limit of your paging file (if it is a static limit) then your applications will fail due out an out-of-memory exception. With today's systems with large local storage however, this is a rare event.

Be sure to read about paging for the full picture. Check out:

http://en.wikipedia.org/wiki/Paging

In certain cases, you will notice that you have sufficient free physical memory. Say 100MB and your program tries to allocate a 10MB block to store a large object but fails. This is caused by physical memory fragmentation. Although the total free memory is 100MB, there is no single contiguous block of 10MB that can be used to store your object. This will result in an exception that needs to be handled in your code. If you allocate large objects in your code you may want to separate the allocation into smaller blocks to facilitate allocation, and then aggregate them back in your code logic. For example, instead of having a single 10m vector, you can declare 10 x 1m vectors in an array and allocate memory for each individual one.