From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: Why do additional cores reduce performance? Date: Tue, 16 Dec 2014 10:26:05 +0100 Message-ID: <548FFAAD.5040601@redhat.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit To: Oleg Ovechko , qemu-discuss@nongnu.org, kvm@vger.kernel.org Return-path: Received: from mail-wi0-f170.google.com ([209.85.212.170]:40107 "EHLO mail-wi0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750794AbaLPJ0K (ORCPT ); Tue, 16 Dec 2014 04:26:10 -0500 Received: by mail-wi0-f170.google.com with SMTP id bs8so13009985wib.5 for ; Tue, 16 Dec 2014 01:26:09 -0800 (PST) In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: On 16/12/2014 00:40, Oleg Ovechko wrote: > A. Host Windows, 6 cores (no HT, turbo boost off): 6:23 (+- 10 secs) > B. Host Windows, 1 CPU core (other are turned off in BIOS): 7:13 (+-10 secs) > C. Host 1 core, Guest Windows 1 core: 7:15 - same as B, no degradation > D. Host 6 cores, Guest Windows 1 core: 7:57 > E. Host 6 cores, Guest Windows 4 cores: 8:17 What is your benchmark? Windows sometimes has scalability problems due to the way it does timing. Try replacing "-cpu host" with "-no-hpet -cpu host,hv_time,hv_vapic". > 3. Also I am unsure about HT. When I specify "cores=2", I suppose you mean "threads=2". > is there any > guaranty that whole core with both HT parts is passed to VM? Or it can be > mix of two real cores with separate caches? It will be a mix. Do not specify HT in the guest, unless you have HT in the host _and_ you are pinning the two threads of each guest core to the two threads of a host core. Paolo