Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754754Ab0FMUpn (ORCPT ); Sun, 13 Jun 2010 16:45:43 -0400 Received: from lennier.cc.vt.edu ([198.82.162.213]:35172 "EHLO lennier.cc.vt.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753430Ab0FMUpm (ORCPT ); Sun, 13 Jun 2010 16:45:42 -0400 X-Mailer: exmh version 2.7.2 01/07/2005 with nmh-1.2 To: Avi Kivity Cc: Ingo Molnar , "H. Peter Anvin" , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/4] Really lazy fpu In-Reply-To: Your message of "Sun, 13 Jun 2010 18:03:43 +0300." <1276441427-31514-1-git-send-email-avi@redhat.com> From: Valdis.Kletnieks@vt.edu References: <1276441427-31514-1-git-send-email-avi@redhat.com> Mime-Version: 1.0 Content-Type: multipart/signed; boundary="==_Exmh_1276461922_18203P"; micalg=pgp-sha1; protocol="application/pgp-signature" Content-Transfer-Encoding: 7bit Date: Sun, 13 Jun 2010 16:45:22 -0400 Message-ID: <39727.1276461922@localhost> X-Mirapoint-Received-SPF: 128.173.34.98 localhost Valdis.Kletnieks@vt.edu 2 pass X-Mirapoint-IP-Reputation: reputation=neutral-1, source=Fixed, refid=n/a, actions=MAILHURDLE SPF TAG X-Junkmail-Status: score=10/50, host=vivi.cc.vt.edu X-Junkmail-SD-Raw: score=unknown, refid=str=0001.0A020207.4C154364.0086,ss=1,fgs=0, ip=0.0.0.0, so=2009-09-22 00:05:22, dmn=2009-09-10 00:05:08, mode=single engine X-Junkmail-IWF: false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1705 Lines: 41 --==_Exmh_1276461922_18203P Content-Type: text/plain; charset=us-ascii On Sun, 13 Jun 2010 18:03:43 +0300, Avi Kivity said: > Currently fpu management is only lazy in one direction. When we switch into > a task, we may avoid loading the fpu state in the hope that the task will > never use it. If we guess right we save an fpu load/save cycle; if not, > a Device not Available exception will remind us to load the fpu. > > However, in the other direction, fpu management is eager. When we switch out > of an fpu-using task, we always save its fpu state. Does anybody have numbers on how many clocks it takes a modern CPU design to do a FPU state save or restore? I know it must have been painful in the days before cache memory, having to make added trips out to RAM for 128-bit registers. But what's the impact today? (Yes, I see there's the potential for a painful IPI call - anything else?) Do we have any numbers on how many saves/restores this will save us when running the hypothetical "standard Gnome desktop" environment? How common is the "we went all the way around to the original single FPU-using task" case? --==_Exmh_1276461922_18203P Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Exmh version 2.5 07/13/2001 iD8DBQFMFUNicC3lWbTT17ARApeGAJ4jL3ssObs3FStfyIfhawfKSKy4igCdGgwS QOPrCIyRHS7q5vKi8X0jzBA= =Z1It -----END PGP SIGNATURE----- --==_Exmh_1276461922_18203P-- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/