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

* Thomas Gleixner <tglx@linutronix.de> 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.

Thanks,

Ingo


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