Re: Returning memory to the OS
On 09/10/09 22:17, Jan Bielawski wrote:
I have a multi-platform STL question (multi-platform means Windows 32,
64, Linux 64-bit, Mac OS). I'd like for an STL vector to return the
memory to the OS - not just to the heap - upon deallocation, like
exiting the scope. I wrote an allocator with the usual ::operator
delete in its deallocate() function:
void deallocate(pointer p, size_type num) {
::operator delete((void *)p);
}
...but this does not necessarily return the memory to the OS. I found
the function _heapmin() on MSDN web site and it seems to work for
Windows XP 32-bit:
void deallocate(pointer p, size_type num) {
::operator delete((void *)p);
#ifdef WIN32
_heapmin(); // memory goes away stepping past this line
#endif
}
My first question is: is this the way to go about it in general?
You would need to create another allocator that would do the required
memory management and instantiate std::vector<> with it. Not very
practical for your purpose though because one of the design goals of
std::vector<> is to hide memory management details from the user while
you seem to want to manage memory explicitly.
If you just need a huge array you could allocate it directly from the OS
using POSIX mmap() call or VirtualAlloc() call on Windoze. The
interesting bit here is that operating systems like Linux and Windoze do
demand paging, i.e. mmap() call only reserves a portion of process
virtual address space without actually allocating any memory. Memory is
allocated on demand by pages (most often 4096 bytes on x86)
transparently when you first actually write or read that mmap()ed
memory, thus demand paging. http://en.wikipedia.org/wiki/Demand_paging
On 64-bit platforms this demand paging feature lets you do very
interesting things. Like reserving ridiculously large amounts of memory
and using that as object memory pools. For example, if you application
can theoretically handle maximum of say 1.000.000.000 objects of size 64
bytes, one can reserve 64 * 1.000.000.000 bytes of virtual address
memory space using mmap(). And then allocate objects from the beginning
of that array and deallocate objects back into a free-list. This way you
only use as little memory as possible at the start of the huge array,
eliminating any memory waste as memory is allocated only when you
actually use it.
> And
if yes, what are the corresponding system calls for Linux and Mac?
See http://www.opengroup.org/onlinepubs/000095399/functions/mmap.html
--
Max