Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754624Ab2BGPRk (ORCPT ); Tue, 7 Feb 2012 10:17:40 -0500 Received: from mail-pw0-f46.google.com ([209.85.160.46]:47866 "EHLO mail-pw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751217Ab2BGPRj (ORCPT ); Tue, 7 Feb 2012 10:17:39 -0500 Message-ID: <4F31408F.80901@codemonkey.ws> Date: Tue, 07 Feb 2012 09:17:35 -0600 From: Anthony Liguori User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.23) Gecko/20110922 Lightning/1.0b2 Thunderbird/3.1.15 MIME-Version: 1.0 To: Avi Kivity CC: Rob Earhart , linux-kernel , KVM list , qemu-devel Subject: Re: [Qemu-devel] [RFC] Next gen kvm api References: <4F2AB552.2070909@redhat.com> <4F2E80A7.5040908@redhat.com> <4F3025FB.1070802@codemonkey.ws> <4F31132F.3010100@redhat.com> In-Reply-To: <4F31132F.3010100@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1373 Lines: 37 On 02/07/2012 06:03 AM, Avi Kivity wrote: > On 02/06/2012 09:11 PM, Anthony Liguori wrote: >> >> I'm not so sure. ioeventfds and a future mmio-over-socketpair have to put the >> kthread to sleep while it waits for the other end to process it. This is >> effectively equivalent to a heavy weight exit. The difference in cost is >> dropping to userspace which is really neglible these days (< 100 cycles). > > On what machine did you measure these wonderful numbers? A syscall is what I mean by "dropping to userspace", not the cost of a heavy weight exit. I think a heavy weight exit is still around a few thousand cycles. Any nehalem class or better processor should have a syscall cost of around that unless I'm wildly mistaken. > > But I agree a heavyweight exit is probably faster than a double context switch > on a remote core. I meant, if you already need to take a heavyweight exit (and you do to schedule something else on the core), than the only additional cost is taking a syscall return to userspace *first* before scheduling another process. That overhead is pretty low. Regards, Anthony Liguori > > -- 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/