Christopher Smith wrote:
David Brown wrote:
With a 64-bit architecture, just memory mapping the harddrive can be an
excellent way to implement a filesystem.
That seems true but desktop processors tend not to actually be able to
manage the full 2^64 virtual address space, and at the same time they
tend to have large drives attached to them. While right now all is well,
I could see where we could encounter problems at some point that are
similar to the ones we have encountered at the end of the 2^32 era.
Another problem, of course, is the issue of programs that need the
virtual address space for other things, although if I remember the Mach
driver model the driver gets its own address space, so that may be a
non-issue. Finally, one should not ignore the overhead of managing all
those page tables....
I dunnoh. I don't think it is as clear cut a bad idea as others have
implied, but it is not without its shortcomings.
Well, you're not likely to run out of space, if that's what you're
implying. 2^64 is large even compared with Moore's law growth from
where we are now. 10^12 is approx 2^36. You need 28 more iterations of
Moore's law, so we would need 28 more years (at least) to hit that kind
of limit, and I believe there are some energy considerations before we
get there.
The bigger issue is simply that disk doesn't act like memory and
attempting to make it do so breaks the abstraction. The memory
subsystem is tuned for accessing memory, and you have to put in nasty
hacks to start working around those tunings to make going to disk via
memory map work.
Want proof? Go take a look at the XFS on ARM fiasco. They're *still*
trying to work out all of the kinks in mmap() to make it work. Linux
has a lot of x86 assumptions baked into both memory and disk subsystems.
Stripping those out slows x86 performance, so people have had a very
hard time convincing Linus to take the kernel changes required (a couple
new invalidation options need to be added to mmap(), but supporting them
on x86 slows things down).
Deciding when an abstraction is useful is a tough problem. There was a
time when the idea that disk and memory are "random access" was good
enough and you could treat disk like memory. I'm pretty sure that
doesn't work anymore. If anything, we need to start treating memory
more like a disk (random access with slow, error-prone burstable response).
-a
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