CODEWITHSUNDEEP
c++memory-management
user1451819
user1451819

Reputation: 69

Contiguous Memory Allocation

In an application i have to allocate two buffers of 480 MB each. Memory allocation is done using HeapAlloc method. The application works fine in the systems where not many applications are running. But in system where other applications are also running memory is not allocated because of non availability of contiguous memory. Even though the memory space(non contiguous) is available but it is not allocated.

Need help to allocate two buffers of 480 MB even if non contiguous memory is available.

Upvotes: 3

Views: 4514

Answers (3)

Steve
Steve

Reputation: 7271

You can use VirtualAlloc with fAllocation specified as MEM_LARGE_PAGES. This enables large page support, note that you must check GetLargePageMinimum to ensure that the system supports lage pages.

Also note that this is likely to be slow as this page details.

Large-page memory regions may be difficult to obtain after the system has been running for a long time because the physical space for each large page must be contiguous, but the memory may have become fragmented. Allocating large pages under these conditions can significantly affect system performance. Therefore, applications should avoid making repeated large-page allocations and instead allocate all large pages one time, at startup.

Upvotes: 2

Ben Voigt
Ben Voigt

Reputation: 283614

The situation you describe is not possible in a full featured OS which gives each process its own address space. It doesn't matter how many other applications are running, they won't affect contiguity of the free address space in your process. And virtual memory can map discontiguous physical memory addresses to a contiguous range in virtual address space.

Only in an embedded system without a memory management unit could the existence of other tasks cause your program to suffer memory fragmentation.

HeapAlloc() suggests Windows, which does give a separate address space to each process. The most likely explanation there is that your private address space is fragmented by libraries (DLLs) loading in scattered locations. You can rebase the libraries you use to avoid this and provide larger contiguous blocks of address space.

Upvotes: 3

Eloff
Eloff

Reputation: 21656

Use VirtualAlloc. The underlying memory that backs the virtual pages need not be contiguous and you will always have your full virtual address space (2GB on a 32 bit system, I think 8 or 16 TB on Windows x64, I can't remember.) HeapAlloc can become fragmented (through your process's use, not others.) Your address space can also become fragmented, so try allocating it early in your application. I actually don't recommend HeapAlloc for anything, you can just use new and delete (which call malloc and free) For large blocks like yours malloc will call VirtualAlloc on Windows.

Upvotes: 0

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