Re: [RFC] VM: I have a dream...

["Barry K. Nathan" <barryn@pobox.com>]

On 1/31/06, Al Boldi <a1426z@gawab.com> wrote:
> Faulty, because we are currently running a legacy solution to workaround an
> 8,16,(32) arch bits address space limitation, which does not exist in
> 64bits+ archs for most purposes.

In the early 1990's (and maybe even the mid 90's), the typical hard
disk's storage could theoretically be byte-addressed using 32-bit
addresses -- just as (if I understand you correctly) you are arguing
that today's hard disks can be byte-addressed using 64-bit addresses.

If this was going to be practical ever (on commodity hardware anyway),
I would have expected someone to try it on a 32-bit PC or Mac when
hard drives were in the 100MB-3GB range... That suggests to me that
there's a more fundamental reason (i.e. other than lack of address
space) that caused people to stick with the current scheme.

[snip]
> There is a lot to gain, for one there is no more swapping w/ all its related
> side-effects.  You're dealing with memory only.  You can also run your fs
> inside memory, like tmpfs, which is definitely faster.  And there may be
> lots of other advantages, due to the simplified architecture applied.

tmpfs isn't "definitely faster". Remember those benchmarks where Linux
ext2 beat Solaris tmpfs?
http://www.tux.org/lkml/#s9-12

Also, the only way I see where "there is no more swapping" and
"[y]ou're dealing with memory only" is if the disk *becomes* main
memory, and main memory becomes an L3 (or L4) cache for the CPU [and
as a consequence, main memory also becomes the main form of long-term
storage]. Is that what you're proposing?

If so, then it actually makes *less* sense to me than before -- with
your scheme, you've reduced the speed of main memory by 100x or more,
then you try to compensate with a huge cache. IOW, you've reduced the
speed of *main* memory to (more or less) the speed of today's swap!
Suddenly it doesn't sound so good anymore...

--
-Barry K. Nathan <barryn@pobox.com>