Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756193AbZFRRu3 (ORCPT ); Thu, 18 Jun 2009 13:50:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753208AbZFRRuN (ORCPT ); Thu, 18 Jun 2009 13:50:13 -0400 Received: from x35.xmailserver.org ([64.71.152.41]:45583 "EHLO x35.xmailserver.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752158AbZFRRuM (ORCPT ); Thu, 18 Jun 2009 13:50:12 -0400 X-AuthUser: davidel@xmailserver.org Date: Thu, 18 Jun 2009 10:44:02 -0700 (PDT) From: Davide Libenzi X-X-Sender: davide@makko.or.mcafeemobile.com To: Gregory Haskins cc: "Michael S. Tsirkin" , kvm@vger.kernel.org, Linux Kernel Mailing List , avi@redhat.com, paulmck@linux.vnet.ibm.com, Ingo Molnar Subject: Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface In-Reply-To: <4A3A48AB.2080206@novell.com> Message-ID: References: <20090616022041.23890.90120.stgit@dev.haskins.net> <20090616022956.23890.63776.stgit@dev.haskins.net> <20090616140240.GA9401@redhat.com> <4A37A7FC.4090403@novell.com> <20090616143816.GA18196@redhat.com> <4A37B0BB.3020005@novell.com> <20090616145502.GA1102@redhat.com> <4A37B832.6040206@novell.com> <20090616154150.GA17494@redhat.com> <4A37C592.2030407@novell.com> <4A37CFDA.4000602@novell.com> <4A3927C0.5060607@novell.com> <4A39415C.9060803@novell.com> <4A39649C.4020602@novell.com> <4A3A48AB.2080206@novell.com> User-Agent: Alpine 1.10 (DEB 962 2008-03-14) X-GPG-FINGRPRINT: CFAE 5BEE FD36 F65E E640 56FE 0974 BF23 270F 474E X-GPG-PUBLIC_KEY: http://www.xmailserver.org/davidel.asc MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1913 Lines: 47 On Thu, 18 Jun 2009, Gregory Haskins wrote: > Actually there is only one (the tx-thread) aside from the eventfd > imposed workqueue one. Incidentally, I would love to get rid of the > other thread too, so I am not just picking on eventfd ;). The other is > a lot harder since it has to update the virtio-ring and may need to page > in guest memory to do so. No, there is the interface rx softirq too, that makes two. Plus, the process of delivering (especially for KVM & Co.) does not involve only ctx switching, there's other stuff in the middle too. You also talk about latency. Really? Probably RT people aren't looking into KVM if RT is even a mild requirement. > To flip it around on you: try telling a group like the netdev guys that > they should put extra context switches into the stack because they don't > really matter. Be sure to wear extra thick asbestos. ;) They already do. So you've got to ask yourself why they can achieve Gbps throughput already, why can't KVM live with it and being compelled to litter an existing interface. > The fact is that eventfd is a really neat general signaling idiom. > However, its currently geared towards "signaling = wakeup". As we have > proven with this KVM *fd effort, this is not necessarily accurate to > describe all use cases, nor is it optimal. I'd like to address that. > An alternative, of course, is that we make a private anon-fd solution > within KVM. However, it will be so similar to eventfd so it just seems > wasteful if it can be avoided. Even though you take that route, you'll have to prove with replicable real life benchmarks, that the bloating make sense. - Davide -- 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/