linux-kernel - Re: [patch 0/3] kvm tool: Serial emulation overhaul

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Date:	Mon, 12 Dec 2011 15:23:18 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Pekka Enberg <penberg@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch 0/3] kvm tool: Serial emulation overhaul

On Mon, Dec 12, 2011 at 12:12:22PM +0100, Ingo Molnar wrote:
> 
> * Sasha Levin <levinsasha928@...il.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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