Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753103AbcDVNHN (ORCPT ); Fri, 22 Apr 2016 09:07:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38034 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751779AbcDVNHL (ORCPT ); Fri, 22 Apr 2016 09:07:11 -0400 Date: Fri, 22 Apr 2016 15:07:06 +0200 From: Radim =?utf-8?B?S3LEjW3DocWZ?= To: Wanpeng Li Cc: Greg Kurz , Paolo Bonzini , james.hogan@imgtec.com, Ingo Molnar , linux-mips@linux-mips.org, kvm , "linux-kernel@vger.kernel.org" , qemu-ppc@nongnu.org, Cornelia Huck , Paul Mackerras , David Gibson Subject: Re: [PATCH v3] KVM: remove buggy vcpu id check on vcpu creation Message-ID: <20160422130705.GD7202@potion> References: <146116689259.20666.15860134511726195550.stgit@bahia.huguette.org> <20160420182909.GB4044@potion> <20160421132958.0e9292d5@bahia.huguette.org> <20160421152916.GA30356@potion> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 959 Lines: 20 2016-04-22 09:40+0800, Wanpeng Li: > 2016-04-21 23:29 GMT+08:00 Radim Krčmář : >> x86 vcpu_id encodes APIC ID and APIC ID encodes CPU topology by >> reserving blocks of bits for socket/core/thread, so if core or thread >> count isn't a power of two, then the set of valid APIC IDs is sparse, > > ^^^^^^^^^^^^^^^^^^^ > ^^^^^^^ > Is this the root reason why recommand max vCPUs per vm is 160 and the > KVM_MAX_VCPUS is 255 instead of due to perforamnce concern? No, the recommended amout of VCPUs is 160 because I didn't bump it after PLE stopped killing big guests. :/ You can get full 255 VCPU guest with a proper configuration, e.g. "-smp 255" or "-smp 255,cores=8" and the only problem is scalability, but I don't know of anything that doesn't scale to that point. (Scaling up to 2^32 is harder, because you don't want O(N) search, nor full allocation on smaller guests. Neither is a big problem now.)