From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH kvm-unit-tests] pmu: fixes for Sandy Bridge hosts Date: Mon, 03 Jun 2013 09:08:46 +0200 Message-ID: <51AC40FE.5020005@redhat.com> References: <1369935788-19069-1-git-send-email-pbonzini@redhat.com> <20130602153209.GJ24773@redhat.com> <51AC38A9.5010703@redhat.com> <20130603063854.GU4725@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org To: Gleb Natapov Return-path: Received: from mail-wg0-f45.google.com ([74.125.82.45]:40539 "EHLO mail-wg0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750861Ab3FCHI6 (ORCPT ); Mon, 3 Jun 2013 03:08:58 -0400 Received: by mail-wg0-f45.google.com with SMTP id n12so2844939wgh.0 for ; Mon, 03 Jun 2013 00:08:57 -0700 (PDT) In-Reply-To: <20130603063854.GU4725@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: Il 03/06/2013 08:38, Gleb Natapov ha scritto: > On Mon, Jun 03, 2013 at 08:33:13AM +0200, Paolo Bonzini wrote: >> Il 02/06/2013 17:32, Gleb Natapov ha scritto: >>> On Thu, May 30, 2013 at 07:43:07PM +0200, Paolo Bonzini wrote: >>>> This patch includes two fixes for SB: >>>> >>>> * the 3rd fixed counter ("ref cpu cycles") can sometimes report >>>> less than the number of iterations >>>> >>> Is it documented? It is strange for "architectural" counter to behave >>> differently on different architectures. >> >> It just counts the CPU cycles. If the CPU can optimize the loop better, >> it will take less CPU cycles to execute it. >> > We should try and change the loop so that it will not be so easily optimized. > Making the test succeed if only 10% percent of cycles were spend on a loop > may result in the test missing the case when counter counts something > different. Any hard-to-optimize loop risks becoming wrong on the other side (e.g. if something stalls the pipeline, a newer chip with longer pipeline will use more CPU cycles). Turbo boost could also contribute to lowering the number of cycles; a boosted processor has ref cpu cycles that are _longer_ than the regular cycles (thus they count in smaller numbers). Maybe that's why "core cycles" didn't go below N. The real result was something like 0.8*N (780-830000). I used 0.1*N because it is used for the "ref cpu cycles" gp counter, which is not the same but similar. Should I change it to 0.5*N or so? Paolo