From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751443Ab3EBEiF (ORCPT ); Thu, 2 May 2013 00:38:05 -0400 Received: from mail-qa0-f42.google.com ([209.85.216.42]:51627 "EHLO mail-qa0-f42.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751330Ab3EBEhs (ORCPT ); Thu, 2 May 2013 00:37:48 -0400 MIME-Version: 1.0 In-Reply-To: <517EEF72.5090100@valvesoftware.com> References: <517B1153.8000401@valvesoftware.com> <517B2FB4.30605@redhat.com> <20130427024248.GA1229@cmpxchg.org> <517EEBD1.503@valvesoftware.com> <517EEF72.5090100@valvesoftware.com> From: Sonny Rao Date: Wed, 1 May 2013 21:37:27 -0700 X-Google-Sender-Auth: 9mt0XD1kFuAIqnh_mh0B1Pgfd-A Message-ID: Subject: Re: IO regression after ab8fabd46f on x86 kernels with high memory To: "Pierre-Loup A. Griffais" Cc: Linus Torvalds , Johannes Weiner , Rik van Riel , Linux Kernel Mailing List , KAMEZAWA Hiroyuki , Andrew Morton Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 29, 2013 at 3:08 PM, Pierre-Loup A. Griffais wrote: > 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. > Sorry just saw this (my stupid gmail filters for lkml) The slow-down we ran into wasn't even on PAE -- it was *just* with highmem on a 2GB system. The non-zero amount (90MB? or so) of highmem was enough to cause major problems due to that particular underflow. I would say regardless of how much memory you have, if the system can use a 64-bit kernel, then it almost certainly should. I've seen some very minor performance impacts on 64-bit capable Atom systems with tiny L2 caches, but it's almost in the noise and not worth the pain. > Thanks, > - Pierre-Loup > >> >> Linus >> >