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How does the iOS platform handle memory-mapped files during low-memory scenarios? By low-memory scenarios, I mean when the OS sends the UIApplicationDidReceiveMemoryWarningNotification notification to all observers in the application.
Our files are mapped into memory using +[NSData dataWithContentsOfMappedFile:], the documentation for which states:
A mapped file uses virtual memory techniques to avoid copying pages of the file into memory until they are actually needed.
Does this mean that the OS will also unmap the pages when they're no longer in use? Is it possible to mark pages as being no longer in use? This data is read-only, if that changes the scenario. How about if we were to use mmap() directly? Would this be preferable?
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Memory-mapping is a mechanism that maps a portion of a file, or an entire file, on disk to a range of addresses within an application's address space. The application can then access files on disk in the same way it accesses dynamic memory.
Memory-mapped files are casual special files in Java that help to access content directly from memory. Java Programming supports memory-mapped files with java. nio package. Memory-mapped I/O uses the filesystem to establish a virtual memory mapping from the user directly to the filesystem pages.
File mapping is the process of mapping the disk sectors of a file into the virtual memory space of a process. Once mapped, your app accesses the file as if it were entirely resident in memory.
Memory-mapped files copy data from disk into memory a page at a time. Unused pages are free to be swapped out, the same as any other virtual memory, unless they have been wired into physical memory using mlock(2). Memory mapping leaves the determination of what to copy from disk to memory and when to the OS.
Dropping from the Foundation level to the BSD level to use mmap is unlikely to make much difference, beyond making code that has to interface with other Foundation code somewhat more awkward.
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(This is not an answer, but it would be useful information.)
From @ID_AA_Carmack tweet,
@ID_AA_Carmack are iOS memory mapped files automatically unmapped in low memory conditions? (using +[NSData dataWithContentsOfMappedFile]?)
ID_AA_Carmack replied for this,
@KhrobEdmonds yes, that is one of the great benefits of using mapped files on iOS. I use mmap(), though.
I'm not sure that is true or not...
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