From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753237AbbAXBCo (ORCPT ); Fri, 23 Jan 2015 20:02:44 -0500 Received: from aserp1040.oracle.com ([141.146.126.69]:23405 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750960AbbAXBCm (ORCPT ); Fri, 23 Jan 2015 20:02:42 -0500 Message-ID: <54C2EF09.3090407@oracle.com> Date: Fri, 23 Jan 2015 20:02:01 -0500 From: Sasha Levin User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0 MIME-Version: 1.0 To: WANG Chao , Davidlohr Bueso CC: Rik van Riel , Vlastimil Babka , Andrew Morton , Ingo Molnar , Peter Zijlstra , Michel Lespinasse , Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm, vmacache: Add kconfig VMACACHE_SHIFT References: <1421908189-18938-1-git-send-email-chaowang@redhat.com> <1421912761.4903.22.camel@stgolabs.net> <20150122075742.GA11335@dhcp-129-179.nay.redhat.com> <1421943573.4903.24.camel@stgolabs.net> <54C123CF.2070107@redhat.com> <20150123051422.GC8670@dhcp-129-179.nay.redhat.com> In-Reply-To: <20150123051422.GC8670@dhcp-129-179.nay.redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Source-IP: ucsinet22.oracle.com [156.151.31.94] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/23/2015 12:14 AM, WANG Chao wrote: > On 01/22/15 at 11:22am, Rik van Riel wrote: >> > On 01/22/2015 11:19 AM, Davidlohr Bueso wrote: >>> > > On Thu, 2015-01-22 at 15:57 +0800, WANG Chao wrote: >>>> > >> Hi, Davidlohr >>>> > >> >>>> > >> On 01/21/15 at 11:46pm, Davidlohr Bueso wrote: >>>>> > >>> On Thu, 2015-01-22 at 14:29 +0800, WANG Chao wrote: >>>>>> > >>>> Add a new kconfig option VMACACHE_SHIFT (as a power of 2) to specify the >>>>>> > >>>> number of slots vma cache has for each thread. Range is chosen 0-4 (1-16 >>>>>> > >>>> slots) to consider both overhead and performance penalty. Default is 2 >>>>>> > >>>> (4 slots) as it originally is, which provides good enough balance. >>>>>> > >>>> >>>>> > >>> >>>>> > >>> Nack. I don't feel comfortable making scalability features of core code >>>>> > >>> configurable. >>>> > >> >>>> > >> Out of respect, is this a general rule not making scalability features >>>> > >> of core code configurable? >>> > > >>> > > I doubt its a rule, just common sense. Users have no business >>> > > configuring such low level details. The optimizations need to >>> > > transparently work for everyone. >> > >> > There may sometimes be a good reason for making this kind of >> > thing configurable, but since there were no performance >> > numbers in the changelog, I have not seen any such reason for >> > this particular change :) > True. I didn't run any kind of benchmark, thus no numbers here. This is > purely hypothetical. > > I'm glad to run some tests. For the sake of consistency, could you > please show me a hint how do you measure at the first place? I can do > hit-rate, but I don't know how you measure cpu cycles. Could you > elaborate? I don't think there's a need to look for problems where there are none. Have you observed a performance issue that might be improved by changing the shift here? Thanks, Sasha