[e2e] end of interest
touch at ISI.EDU
Fri May 9 21:06:33 PDT 2008
jonathan at dsg.stanford.edu wrote:
> In message <48247C79.70103 at isi.edu>Joe Touch writes
>> VM, e.g., was originally to handle memory capacity limits, but has other
>> benefits that persist even when RAM is plentiful:
>> - providing a linear, contiguous memory view to processes
>> - sandboxing processes from each other
>> Overlays have very similar benefits to networking. [...]
> [[Caveat: on historical matters, I defer, before the fact, to those
> who were there before me, nevermind those there at the time! ]]
> Hi Joe,
> I don't think that's a good analogy. The contemporary literature -- as
> collected in Structured Computer Organization: Bell & Newell; or Bell,
> Newell and Siewioriek -- shows that sandboxing of process memory and
> "linearization" of process memory predates the Atlas and its
> "one-level store". Base/bounds registers go back much earlier, and
> provide both sandboxing and relocation. Hm. Didn't sharing of address
> spaces, via segments also predate demand-paged virtual memory?
Demand-paged VM is one version; some of the other mechanisms weren't
called VM at the time, but might be in retrospect. Current
virtualization, as you note, is cleaner than previous attempts, e.g., by
not only linearizing and sandboxing, but by preventing checkerboarding
(vs. base/bounds registers and segments).
> Last, As Seymour Cray put it, "Virtual memory is you don't have".
> Does this this VM ==> Overlay networks analogy stretch as far as:
> "Overlay networks you don't really have"?
> Joe, I'm guessing you'd disagree.
Correct. IMO, Cray is focusing on the paging part, not the
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