From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Magenheimer Subject: RE: Xen CPU limit? Date: Tue, 30 Mar 2010 09:14:54 -0700 (PDT) Message-ID: <43f035b5-977e-4910-a46e-608d76a9346f@default> References: <4B991586.30602@cancer.org.uk> <20100329154901.GE1878@reaktio.net> <4BB1C16A0200007800037BB9@vpn.id2.novell.com 20100330075306.GY1878@reaktio.net> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <20100330075306.GY1878@reaktio.net> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: =?UTF-8?B?IlBhc2kgS8Okcmtrw6RpbmVuIg==?= , Jan Beulich Cc: xen-devel@lists.xensource.com, Martin Lukasik List-Id: xen-devel@lists.xenproject.org > -----Original Message----- > From: Pasi K=E4rkk=E4inen [mailto:pasik@iki.fi] > Sent: Tuesday, March 30, 2010 1:53 AM > To: Jan Beulich > Cc: xen-devel@lists.xensource.com; Martin Lukasik > Subject: Re: [Xen-devel] Xen CPU limit? >=20 > On Tue, Mar 30, 2010 at 08:16:26AM +0100, Jan Beulich wrote: > > >>> Pasi K=E4rkk=E4inen 29.03.10 17:49 >>> > > >I think the number of virtual cpus (vcpus) per guest is (was) 32, > > >and now in Xen 4.0.0 you can have up to 64 vcpus per guest. > > > > I don't think the tools are up to anything beyond 32 yet. >=20 > Hmm.. what's actually missing from the tools to handle >32 vcpus? >=20 > > The (64-bit) hypervisor allows up to 8192 vCPU-s iirc (but that's a > truly > > theoretical limit, as Dom0 or a guest likely won't be able to bring > up > > that many due to there only being 4096 event channels; current > > Linux requires 5-6 of them per vCPU for IPIs and timer vIRQ). > > >=20 > Wow.. I wasn't aware the (theoretical) limit is that big :) But, if I am reading this correctly, 4096 event channels is the next scalability barrier. If, on a "big" machine, one tries to run 64 (nearly always idle) guests each configured with 16 vcpus (because they run a busy database load when they are not idle), it won't work due to the event channel limit? This scenario seems quite possible in a cloud/hosting environment.