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    SubjectRe: Swap vs No Swap.
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    On Friday 23 November 2001 6:30 am, Charles Marslett wrote:
    > James A Sutherland wrote:
    > > On Thursday 22 November 2001 4:00 pm, war wrote:
    > > > Incorrect, my point is I have enough ram where I am not going to run
    > > > out for the things I do.
    > >
    > > There's more to it than "not run out". You have some fixed amount of RAM;
    > > if the VM is working properly, adding swap will IMPROVE performance,
    > > because that fixed amount of RAM is used more efficiently.
    > >
    > > Obviously, there are cases where removing swap breaks the system
    > > entirely, but even in other cases, adding swap should *never* degrade
    > > performance. (In theory, anyway; in practice, it still needs tuning...)
    > >
    > > > Using swap simply slows the system down!
    > >
    > > In which case, the VM isn't working properly; it SHOULD page out
    > > infrequently used data to make more room for caching frequently used
    > > files.
    > >
    > > James.
    >
    > I disagree. It is true that a VM could be designed sufficiently complex
    > that it would properly analyze every possible sequence of execution and
    > have perfect prescience. It would probably take a few hundred gigabytes of
    > table structure to do that and that in itself will slow down the VM just
    > scanning those tables, I dare say.

    That wasn't quite what I had in mind :)

    > In short, no VM is going to work perfectly -- it is extrapolating a model
    > of behavior to a real world sequence of events and as such there will
    > always be some real world set of programs and events that will make it
    > worse than some other model of behavior (VM), including the one that never
    > pages at all. We just want that to happen rarely (whatever that means).

    Yes, sometimes you'll get better behaviour in a specific case by "disabling"
    swap (i.e. forcing the kernel to page code instead), which in other cases
    causes nasty disk thrashing. In this case, though, I think the VM could do a
    much better job than it does presently; I've a feeling Rik's would perform
    better in this case, for example...

    > A VM that is working properly is one that satisfies the beholder (sort of
    > like beauty). And in fact, if you look at the various similar discussions
    > on Microsoft newsgroups (sorry ;-), you may notice they don't seem to be
    > able to come up with a mechanism that handles large uniform access working
    > sets and still works well with "normal" (highly peaked) working sets. So I
    > doubt it is an easy problem.

    Nobody said VM coding was easy - or that Microsoft had got it right :)


    James.
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