From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752010Ab1LKOEm (ORCPT ); Sun, 11 Dec 2011 09:04:42 -0500 Received: from www.linutronix.de ([62.245.132.108]:51027 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751698Ab1LKOEl (ORCPT ); Sun, 11 Dec 2011 09:04:41 -0500 Date: Sun, 11 Dec 2011 15:04:37 +0100 (CET) From: Thomas Gleixner To: Ingo Molnar cc: Pekka Enberg , LKML , Sasha Levin Subject: Re: [patch 0/3] kvm tool: Serial emulation overhaul In-Reply-To: <20111211103047.GA19299@elte.hu> Message-ID: References: <20111210132220.083204833@linutronix.de> <20111211103047.GA19299@elte.hu> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanks, tglx