From mboxrd@z Thu Jan 1 00:00:00 1970 From: George Dunlap Subject: Re: [PATCH 1/2] xl: neuter vcpu-set --ignore-host. Date: Thu, 26 Sep 2013 17:25:17 +0100 Message-ID: <52445FED.1020604@eu.citrix.com> References: <1380141617-8981-1-git-send-email-konrad.wilk@oracle.com> <1380141617-8981-2-git-send-email-konrad.wilk@oracle.com> <1380186391.29483.18.camel@kazak.uk.xensource.com> <20130926124814.GE5792@konrad-lan.dumpdata.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130926124814.GE5792@konrad-lan.dumpdata.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Konrad Rzeszutek Wilk , Ian Campbell Cc: xen-devel@lists.xensource.com, Ian.Jackson@eu.citrix.com List-Id: xen-devel@lists.xenproject.org On 26/09/13 13:48, Konrad Rzeszutek Wilk wrote: > On Thu, Sep 26, 2013 at 10:06:31AM +0100, Ian Campbell wrote: >> On Wed, 2013-09-25 at 16:40 -0400, Konrad Rzeszutek Wilk wrote: >>> When Xen 4.3 was released we had a discussion whether we should >>> allow the vcpu-set command to allow the user to set more than >>> physical CPUs for a guest (it didn't). The author brought up: >>> - Xend used to do it, >> IMHO xend is buggy here. If it were being maintained I encourage a patch >> to file this particular sharp edge off. >> >>> - If a user wants to do it, let them do it, >> We do, we have an option for those who know what they are doing to use >> in the tiny minority of cases where they need to do this. >> >>> - The original author of the change did not realize the >>> side-effect his patch caused this and had no intention of changing it. >> a happy accident then. >> >>> - The user can already boot a massively overcommitted guest by >>> having a large 'vcpus=' value in the guest config and we allow >>> that. >> IMHO this is an xl bug, I'd be happy to see a patch to fix this and >> require and override here too. > I think I posted one some time ago, but I don't recall anybody > commenting on it. Will repost it. >>> Since we were close to the release we added --ignore-host parameter >>> as a mechanism for a user to still set more vCPUs that the physical >>> machine as a stop-gate. >>> >>> This patch keeps said option but neuters the check so that we >>> can overcommit. In other words - by default the user is >>> allowed to set as many vCPUs as they would like. >> and why would a naive user want to do this? non-naive users can use the >> option if this is what they really want, and are probably grateful for >> the catch if they didn't intend to overcommit, which is almost always >> even for expert users. >> >> This change need far better rationalisation than "because xend did it" >> and "because we can". IMHO. > I am going to defer to George here. His viewpoint (I am going to > probably mangle it up) was that - if the user wants to do, let him/her > do it without us putting obstacles. > > And I think Ian Jackson was ambivalent here and was deferring to George. So I've gone back and read the original thread, and what I actually said was: "So I think the right thing to do long-term is to make it possible to do in xl. Having a "seatbelt" restriction by default that can be overridden would be OK with me, but I think a warning message when vcpus > pcpus would suffice." And my summary of mine and IanC's positions at the time (which IanC did not dispute) was: "We both agree that "vcpus > pcpus" is a bad configuration. I think ideally we should support it (because administrators should be allowed to shoot themselves in the foot) and Ian[C] seems to be making the case that we shouldn't support it." IanJ, as I understood him, agreed with me that it should be *possible*. As IanC points out, it is possible -- you just have to add "--ignore-host". So given what all of us think, keeping the "seatbelt" is probably the best compromise. IanC is happy that a hapless user will not accidentally shoot his own foot, and IanJ and I are happy that a skilled user can shoot her own foot if she really wants to. -George