Full Page Malloc

Question

I am trying to optimize the memory allocation of my program by using entire pages at a time.

I am grabbing the page size like this: sysconf(_SC_PAGESIZE); then calculating the total number of elements that will fit in a page like this: elements=pageSize/sizeof(Node);

I was thinking that when I actually go to malloc my memory I would use malloc(elements*sizeof(Node)); It seems like the multiplication and division of sifeof(Node) would cancel out, but with integer division, I do not believe that that is the case.

Is this the best way to malloc an entire page at a time?

Thanks

Answer 1

The malloc function doesn't have any concept of pagesize. Unless you are allocating pages that are ALSO aligned to a page-boundary, you will not get ANY benefit from calling malloc in this way. Just malloc as many elements as you need, and stop worrying about micro-optimising something that almost certainly won't give you any benefit at all.

Yes, the Linux kernel does things like this all the time. There are two reasons for that:

1. You don't want to allocate blocks LARGER than a page, since that significantly increases the risk of allocation failure.
2. The kernel allocation is made on a per-page basis, rather than like the C library, which allocates a large amount of memory in one go, and then splits it into small components.

If you really want to allocate page-size amount of memory, then use the result from sysconf(_SC_PAGESIZE) as your size argument. But it is almost certain that your allocation straddles two pages.

Answer 2

Your computation elements=pageSize/sizeof(Node); doesn't take account of the malloc() metadata that are added to any block/chunk of memory returned by malloc(). In many cases, malloc() will return a memory block likely aligned at least on min(sizeof(double),2 * sizeof(void *)) boundary (32 bytes is becoming quite common btw ...). If malloc() gets a memory block aligned on a page, adds its chunk (with padding), and you write a full page size of data, the last bytes are off the first page: so you're ending up using 2 pages.

Want a whole page, just for you, without concerns about wasting memory, without using mmap() / VirtualAlloc() as suggested in the comments ? Here you are:

int ret;
void *ptr = NULL;
size_t page_size = sysconf(_SC_PAGESIZE);

ret = posix_memalign(&ptr, page_size, page_size);
if (ret != 0 || ptr == NULL) {
    fprintf(stderr, "posix_memalign failed: %s\n", strerror(ret));
}

By the way, this is probably about micro-optimization. You probably still haven't checked your Node have a size multiple of a cache-line, nor how to improve cache-locality, nor found a way to reduce memory fragmentation. So you're probably going in the wrong way: make it works first, profil, optimize your algorithms, profil, micro-optimize at the last option.