CODEWITHSUNDEEP
c++memoryvector
user1558064
user1558064

Reputation: 887

when will virtual memory be used (windows)?

I am debugging a program which crashed because no contiguous memory can be used for my vector which needs to be reallocated. So I have a question how come the virtual memory isnot used? In which way can virtual memory be used? Thanks.

Upvotes: 0

Views: 121

Answers (3)

ScottMcP-MVP
ScottMcP-MVP

Reputation: 10415

The problem has nothing to do with memory, or even with virtual memory. An array needs a contiguous range of addresses. The address space (normally 2 GB in a Win32 program) is fragmented so that there is not a large enough space available.

If you could get the addresses Windows would automatically provide the virtual memory to go with them.

It is time to move your app up to 64 bits.

Upvotes: 0

Mats Petersson
Mats Petersson

Reputation: 129314

Virtual memory is the only memory you ever get as a program running on a modern OS (Linux, Unix, Windows, MacOS, Symbian, etc).

It sounds like your problem is that there isn't one contiguous virtual address range that is large enough for your vector [1]. I suspect what is happening is that you need, say, more than 1.5GB in a 32-bit process, which can only use 2GB at once, so there isn't much "room" on either end to stuff other bits into before the "middle" is smaller than 1.5GB - in particular, if you have a vector that is growing, you will need two copies of the vector, one at it's current size, and one at double the size to copy into.

A simple solution, assuming you know how big the vector needs to be is to set it's size, e.g. vector<int> vec(some_size);

If you don't know, there are some more solutions:

If you have a 64-bit OS, you could try setting the LARGEADDRESSAWARE flag for the executable (assuming it's Windows). That should give you a fair bit more memory, since the 64-bit OS doesn't have to reserve a large chunk of memory space for the OS itself (that lives well outside the 32-bit address range. In a 32-bit OS, you need to boot the OS with /3GB, and set the above flag.

Or compile the code as 64-bit (after upgrading to a 64-bit OS, if needed).

[1] Unless of course, you are writing a driver and trying to allocate many megabytes of physical memory as a buffer to use for DMA - but I think you would have said so.

Upvotes: 1

nullptr
nullptr

Reputation: 11058

Virtual memory is used automatically by OS. You don't need to care about this.

In your case, it's most likely that you run a 32-bit application. User address space for a 32-bit process in Windows is limited to 2 GB (well, 3 GB if Windows is booted with a specific key). If your vector requires more than several hundred megabytes of contiguous address space, this may become a problem (due to address space fragmentation).

Of course, any process can run out of memory (even while using virtual memory and swap file and whatever else). Take a look at memory usage of your program in Task Manager.

Upvotes: 2

Related Questions