From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joao Eduardo Luis Subject: Re: less cores more iops / speed Date: Thu, 08 Nov 2012 00:00:37 +0000 Message-ID: <509AF625.8040207@inktank.com> References: <509ADA6A.9000807@profihost.ag> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Return-path: Received: from mail-wi0-f172.google.com ([209.85.212.172]:56461 "EHLO mail-wi0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753738Ab2KHAB0 (ORCPT ); Wed, 7 Nov 2012 19:01:26 -0500 Received: by mail-wi0-f172.google.com with SMTP id hq12so5865957wib.1 for ; Wed, 07 Nov 2012 16:01:25 -0800 (PST) In-Reply-To: <509ADA6A.9000807@profihost.ag> Sender: ceph-devel-owner@vger.kernel.org List-ID: To: Stefan Priebe Cc: "ceph-devel@vger.kernel.org" On 11/07/2012 10:02 PM, Stefan Priebe wrote: > Hello again, > > I've noticed something really interesting. > > I get 5000 iops / VM for rand. 4k writes while assigning 4 cores on a > 2.5 Ghz Xeon. > > When i move this VM to another kvm host with 3.6Ghz i get 8000 iops > (still 8 cores) when i then LOWER the assigned cores from 8 to 4 i get > 14.500 iops. If i assign only 2 cores i get 16.000 iops... > > Why does less kvm cores mean more speed? Totally going on a limb here, but might be related to the cache maybe? When you have more cores your threads may bounce around the cores and invalidate cache entries as they go by; will less cores you might end up with some sort of twisted, forced cpu affinity that allows you to take advantage of caching. But I don't know, really. I would be amazed if what I just wrote had an ounce of truth, and would be completely astonished if that was the cause for such a sudden increase on iops. -Joao > > Greets, > Stefan > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html