From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 18BEEC433DF for ; Tue, 2 Jun 2020 11:13:18 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id E64F3206C3 for ; Tue, 2 Jun 2020 11:13:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E64F3206C3 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jg4qu-00083M-NX; Tue, 02 Jun 2020 11:13:04 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jg4qt-00083H-RO for xen-devel@lists.xenproject.org; Tue, 02 Jun 2020 11:13:03 +0000 X-Inumbo-ID: 0615fc58-a4c2-11ea-abd4-12813bfff9fa Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 0615fc58-a4c2-11ea-abd4-12813bfff9fa; Tue, 02 Jun 2020 11:13:02 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id C97A3AD4D; Tue, 2 Jun 2020 11:13:03 +0000 (UTC) Subject: Re: [PATCH v2] tools/libxl: make default of max event channels dependant on vcpus [and 1 more messages] To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= References: <20200406082704.13994-1-jgross@suse.com> <24203.2251.628483.557280@mariner.uk.xensource.com> <26161282-7bad-5888-16c9-634647e6fde8@xen.org> <8a6f6e41-9395-6c68-eae9-4c1aeb7d96e2@suse.com> <24203.2546.728186.463143@mariner.uk.xensource.com> <24203.2996.819908.965198@mariner.uk.xensource.com> <799396b3-0304-e149-cc3f-45c5a46c7c0c@suse.com> <715f6143-38b3-3f70-b9e3-1ac4a240282f@suse.com> From: Jan Beulich Message-ID: <08eff8dd-59b2-9f3e-9664-ff126eecd123@suse.com> Date: Tue, 2 Jun 2020 13:12:59 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.8.1 MIME-Version: 1.0 In-Reply-To: <715f6143-38b3-3f70-b9e3-1ac4a240282f@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Anthony Perard , Ian Jackson , Julien Grall , Wei Liu , "xen-devel@lists.xenproject.org" Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 02.06.2020 13:06, Jürgen Groß wrote: > On 06.04.20 14:09, Jan Beulich wrote: >> On 06.04.2020 13:54, Jürgen Groß wrote: >>> On 06.04.20 13:11, Jan Beulich wrote: >>>> On 06.04.2020 13:00, Ian Jackson wrote: >>>>> Julien Grall writes ("Re: [PATCH v2] tools/libxl: make default of max event channels dependant on vcpus"): >>>>>> There are no correlation between event channels and vCPUs. The number of >>>>>> event channels only depends on the number of frontend you have in your >>>>>> guest. So... >>>>>> >>>>>> Hi Ian, >>>>>> >>>>>> On 06/04/2020 11:47, Ian Jackson wrote: >>>>>>> If ARM folks want to have a different formula for the default then >>>>>>> that is of course fine but I wonder whether this might do ARMk more >>>>>>> harm than good in this case. >>>>>> >>>>>> ... 1023 event channels is going to be plenty enough for most of the use >>>>>> cases. >>>>> >>>>> OK, thanks for the quick reply. >>>>> >>>>> So, Jürgen, I think everyone will be happy with this: >>>> >>>> I don't think I will be - my prior comment still holds on there not >>>> being any grounds to use a specific OS kernel's (and to be precise >>>> a specific OS kernel version's) requirements for determining >>>> defaults. If there was to be such a dependency, then OS kernel >>>> [variant] should be part of the inputs to such a (set of) formula(s). >>> >>> IMO this kind of trying to be perfect will completely block a sane >>> heuristic for being able to boot large guests at all. >> >> This isn't about being perfect - I'm suggesting to leave the >> default alone, not to improve the calculation, not the least >> because I've been implying ... >> >>> The patch isn't about to find an as stringent as possible upper >>> boundary for huge guests, but a sane value being able to boot most of >>> those. >>> >>> And how should Xen know the OS kernel needs exactly after all? >> >> ... the answer of "It can#t" to this question. >> >>> And it is not that we talking about megabytes of additional memory. A >>> guest with 256 vcpus will just be able to use additional 36 memory >>> pages. The maximum non-PV domain (the probably only relevant case >>> of another OS than Linux being used) with 128 vcpus would "waste" >>> 32 kB. In case the guest misbehaves. >> >> Any extra page counts, or else - where do you draw the line? Any >> single page may decide between Xen (not) being out of memory, >> and hence also not being able to fulfill certain other requests. >> >>> The alternative would be to do nothing and having to let the user >>> experience a somewhat cryptic guest crash. He could google for a >>> possible solution which would probably end in a rather high static >>> limit resulting in wasting even more memory. >> >> I realize this. Otoh more people running into this will improve >> the chances of later ones finding useful suggestions. Of course >> there's also nothing wrong with trying to make the error less >> cryptic. > > Reviving this discussion. > > I strongly disagree with your reasoning. > > Rejecting to modify tools defaults for large guests to make them boot > is a bad move IMO. We are driving more people away from Xen this way. > > The fear of a misbehaving guest of that size to use a few additional > pages on a machine with at least 100 cpus is fine from the academical > point of view, but should not be weighed higher than the usability > aspect in this case IMO. Very simple question then: Where do you draw the boundary if you don't want this to be a pure "is permitted" or "is not permitted" underlying rule? If we had a model where _all_ resources consumed by a guest were accounted against its tool stack requested allocation, things would be easier. Jan