Messages in this thread | | | Date | Mon, 12 Dec 2011 15:23:18 +0400 | From | Cyrill Gorcunov <> | Subject | Re: [patch 0/3] kvm tool: Serial emulation overhaul |
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On Mon, Dec 12, 2011 at 12:12:22PM +0100, Ingo Molnar wrote: > > * Sasha Levin <levinsasha928@gmail.com> wrote: > > > 50k cycles for every single byte is pretty much as good as it > > will get with serial console. See slide #5 in Marcelo's KVM > > Forum 2010 presentation[1] where he timed a heavyweight exit > > to about 40k cycles. > > > [1] > > http://www.linux-kvm.org/wiki/images/e/ea/2010-forum-mtosatti_walkthrough_entry_exit.pdf > > But what we do here is a PIO exit. That, according to Marcelo's > measurements, is about 10K cycles, back to back. [*] > > So where does the extra overhead come from? > > We shouldn't care that there's virtio-console - the goal of > tools/kvm it speed everything up as much as possible, so we > should not jump to the next IO abstraction unless we know where > every cycle was spent with simpler IO models ... > > Thanks, > > Ingo > > [*] Also, those 10K cycles include some significant Qemu > overhead - a couple of thousand cycles - that should be much > lower in the tools/kvm case. >
FWIW, last time I was mcount'ing calltrace -- we spend a lot of time due to timer signals, ie because of
#define TIMER_INTERVAL_NS 1000000 /* 1 msec */
so, do we really need it being that hight? This poll includes enquiry if there some symbol a user typed in console. Maybe we should reduce this rate?
Cyrill
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