From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752102Ab1LKPzf (ORCPT ); Sun, 11 Dec 2011 10:55:35 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:54804 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752051Ab1LKPzd (ORCPT ); Sun, 11 Dec 2011 10:55:33 -0500 Date: Sun, 11 Dec 2011 16:53:47 +0100 From: Ingo Molnar To: Thomas Gleixner Cc: Pekka Enberg , LKML , Sasha Levin Subject: Re: [patch 0/3] kvm tool: Serial emulation overhaul Message-ID: <20111211155347.GA1297@elte.hu> References: <20111210132220.083204833@linutronix.de> <20111211103047.GA19299@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=AWL,BAYES_00 autolearn=no SpamAssassin version=3.3.1 -2.0 BAYES_00 BODY: Bayes spam probability is 0 to 1% [score: 0.0000] 0.0 AWL AWL: From: address is in the auto white-list Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Thomas Gleixner 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