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    SubjectRe: [patch 0/3] kvm tool: Serial emulation overhaul

    * Thomas Gleixner <> wrote:

    > On Sun, 11 Dec 2011, Ingo Molnar wrote:

    > > In theory a kvm driven serial console should be extremely
    > > fast, much faster than a real serial console, basically as
    > > fast as a local console. Yet this still does not seem to be
    > > the case.
    > >
    > > In fact even ssh-ing in to a box over Wifi and running the
    > > above top session is undistinguishable from top running in a
    > > local console. So IMO it cannot be virtualization overhead -
    > > there must still be some delay or serious lack of buffering
    > > somewhere.
    > Well, the difference between ssh and serial is, that ssh can
    > pack 1.5k worth of data into one frame, while serial has to
    > send it piecewise. And the emulation has to trap into kvm tool
    > for each tx byte, which doesnt help either. We cannot do much
    > with buffering on the kvm tool side as we have no clue how
    > much consecutive data will come in. That's why there is a
    > virtual console, which has the disadvantage that you cant see
    > the early boot messages.

    Okay, but look at it from another angle: the top output i
    generate is about 300k characters. 5000 msecs to execute it
    means 16 usecs overhead per character - or about 50k cycles - on
    a top of the class x86 CPU.

    50k cycles for every single byte. And as a user i notice that
    first hand.



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