My understanding of mmap is very limited, let me know which of the following are correct. For the following scenario in a piece of program:
1. Process starts, call mmap() // this is not actually loading anything from disk,
// just allocates memory?
2. access data in the file // this actually triggers the load from disk so
// it takes longer?
3. at this point, the process is killed and restarted
4. Process starts, call mmap() // this is not loading but the memory pointer
// allocated is likely to be different?
5. access data in the file // it takes roughly the same amount of time
// as the first time
Is my understanding correct? I am especially confused about the part after the process is killed and restarted. Thanks!
mmap
"creates a new mapping in the virtual address space of the calling process". Unless you use MAP_POPULATE
nothing is read from the file backing the mapping. ( man page )
Accessing the file-backed mapping needs to bring in the data obviously. Whether physical I/O happens at this point depends on whether the OS has the page you're trying to access in its cache.
So I'd say your statements 1, 2, and 4 are true, while 5 might be not.
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