> On Jul 4, 2016, at 8:12 PM, Steve Sisak <[email protected]> wrote: > > I think there may be more information in the guidelines, but don’t have time > to re-read the full document at the moment. >
Found it: https://developer.apple.com/library/prerelease/content/documentation/Performance/Conceptual/ManagingMemory/Articles/MemoryAlloc.html#//apple_ref/doc/uid/20001881-99765 Allocating Large Memory Blocks using Malloc For large memory allocations, where large is anything more than a few virtual memory pages, malloc automatically uses the vm_allocateroutine to obtain the requested memory. The vm_allocate routine assigns an address range to the new block in the logical address space of the current process, but it does not assign any physical memory to those pages right away. Instead, the kernel does the following: <>It maps a range of memory in the virtual address space of this process by creating a map entry; the map entry is a simple structure that defines the starting and ending addresses of the region. The range of memory is backed by the default pager. The kernel creates and initializes a VM object, associating it with the map entry. At this point there are no pages resident in physical memory and no pages in the backing store. Everything is mapped virtually within the system. When your code accesses part of the memory block, by reading or writing to a specific address in it, a fault occurs because that address has not been mapped to physical memory. In OS X, the kernel also recognizes that the VM object has no backing store for the page on which this address occurs. The kernel then performs the following steps for each page fault: <> It acquires a page from the free list and fills it with zeroes. It inserts a reference to this page in the VM object’s list of resident pages. <> <>It maps the virtual page to the physical page by filling in a data structure called the pmap. The pmap contains the page table used by the processor (or by a separate memory management unit) to map a given virtual address to the actual hardware address. The granularity <> of large memory blocks is equal to the size of a virtual memory page, or 4096 bytes. In other words, any large memory allocations that are not a multiple of 4096 are rounded up to this multiple automatically. Thus, if you are allocating large memory buffers, you should make your buffer a multiple of this size to avoid wasting memory. <>
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