Messages in this thread | | | Date | Sun, 11 Dec 2011 16:53:47 +0100 | From | Ingo Molnar <> | Subject | Re: [patch 0/3] kvm tool: Serial emulation overhaul |
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* 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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