Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760144Ab3D2WQn (ORCPT ); Mon, 29 Apr 2013 18:16:43 -0400 Received: from mx3.valvesoftware.com ([208.64.203.145]:35194 "EHLO mx3.valvesoftware.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760119Ab3D2WQl (ORCPT ); Mon, 29 Apr 2013 18:16:41 -0400 Message-ID: <517EEF72.5090100@valvesoftware.com> Date: Mon, 29 Apr 2013 15:08:50 -0700 From: "Pierre-Loup A. Griffais" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130221 Thunderbird/17.0.3 MIME-Version: 1.0 To: Linus Torvalds CC: Johannes Weiner , Rik van Riel , Linux Kernel Mailing List , , KAMEZAWA Hiroyuki , Andrew Morton Subject: Re: IO regression after ab8fabd46f on x86 kernels with high memory References: <517B1153.8000401@valvesoftware.com> <517B2FB4.30605@redhat.com> <20130427024248.GA1229@cmpxchg.org> <517EEBD1.503@valvesoftware.com> In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-EXCLAIMER-MD-CONFIG: 86b76815-e903-4403-b95d-5abb05264373 X-Mlf-Version: 7.3.6.7163 X-Mlf-UniqueId: o201304292211060086769 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2338 Lines: 51 On 04/29/2013 03:03 PM, Linus Torvalds wrote: > On Mon, Apr 29, 2013 at 2:53 PM, Pierre-Loup A. Griffais > wrote: >> >> Other than this particular concern, what's the high-level take-away? Is PAE >> support in the Linux kernel a false promise than distros should not be >> shipping by default, if at all? Should it be removed from the kernel >> entirely if these configurations are knowingly broken by commits like this? > > PAE is "make it barely work". The whole concept is fundamentally > flawed, and anybody who runs a 32-bit kernel with 16GB or RAM doesn't > even understand *how* flawed and stupid that is. > > Don't do it. Upgrade to 64-bit, or live with the fact that IO > performance will suck. The fact that it happened to work better under > your particular load with one particular IO size is entirely just > "random noise". > > Yeah, the difference between "we can cache it" and "we have to do IO" > is huge. With a 32-bit kernel, we do IO much earlier now, just to > avoid some really nasty situations. That makes you go from the "can > sit in the cache" to the "do lots of IO" situation. Tough. > > Seriously, you can compile yourself a 64-bit kernel and continue to > use your 32-bit user-land. And you can complain to whatever distro you > used that it didn't do that in the first place. But we're not going to > bother with trying to tune PAE for some particular load. It's just not > worth it to anybody. All of this came from me trying to reproduce slowdowns reported by other people; I personally run a 64-bit kernel and understand how bad of an idea it is to attempt to run 32-bit kernels with PAE enabled on modern machines. However, my goal is to avoid ending up with a variety of end-users that don't necessarily understand this getting bitten by it and breaking their systems by upgrading their kernels. I will indeed bring this up with distributors and point out than shipping PAE kernels by default is not a good idea given these problems and your stance on the matter. Thanks, - Pierre-Loup > > Linus > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/