Messages in this thread | | | Subject | Re: [RFC] VM: I have a dream... | From | Bryan Henderson <> | Date | Thu, 2 Feb 2006 14:33:18 -0800 |
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>So w/ 1GB RAM, no swap, and 1TB disk mmap'd, could this mmap'd space be added >to the total memory available to the OS, as is done w/ swap?
Yes.
>And if that's possible, why not replace swap w/ mmap'd disk-space?
Because mmapped disk space has a permanent mapping of address to disk location. That's how the earliest virtual memory systems worked, but we moved beyond that to what we have now (what we've been calling swapping), where the mapping gets established at the last possible moment, which means we can go a lot faster. E.g. when the OS needs to steal 10 page frames used for malloc pages which are scattered across the virtual address space, it could write all those pages out in a single cluster wherever a disk head happens to be at the moment.
Also, given that we use multiple address spaces (my shell and your shell both have an Address 0, but they're different pages), there'd be a giant allocation problem in assigning a contiguous area of disk to each address space.
-- Bryan Henderson IBM Almaden Research Center San Jose CA Filesystems
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