Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760309Ab3HNUft (ORCPT ); Wed, 14 Aug 2013 16:35:49 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:54862 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760173Ab3HNUfr (ORCPT ); Wed, 14 Aug 2013 16:35:47 -0400 Date: Wed, 14 Aug 2013 13:35:46 -0700 From: Greg Kroah-Hartman To: Dave Hansen Cc: Seth Jennings , Nathan Fontenot , Cody P Schafer , Andrew Morton , Lai Jiangshan , "Rafael J. Wysocki" , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC][PATCH] drivers: base: dynamic memory block creation Message-ID: <20130814203546.GA6200@kroah.com> References: <1376508705-3188-1-git-send-email-sjenning@linux.vnet.ibm.com> <20130814194348.GB10469@kroah.com> <520BE30D.3070401@sr71.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <520BE30D.3070401@sr71.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2176 Lines: 46 On Wed, Aug 14, 2013 at 01:05:33PM -0700, Dave Hansen wrote: > On 08/14/2013 12:43 PM, Greg Kroah-Hartman wrote: > > On Wed, Aug 14, 2013 at 02:31:45PM -0500, Seth Jennings wrote: > >> ppc64 has a normal memory block size of 256M (however sometimes as low > >> as 16M depending on the system LMB size), and (I think) x86 is 128M. With > >> 1TB of RAM and a 256M block size, that's 4k memory blocks with 20 sysfs > >> entries per block that's around 80k items that need be created at boot > >> time in sysfs. Some systems go up to 16TB where the issue is even more > >> severe. > > > > The x86 developers are working with larger memory sizes and they haven't > > seen the problem in this area, for them it's in other places, as I > > referred to in my other email. > > The SGI guys don't run normal distro kernels and don't turn on memory > hotplug, so they don't see this. I do the same in my testing of > large-memory x86 systems to speed up my boots. I'll go stick it back in > there and see if I can generate some numbers for a 1TB machine. > > But, the problem on x86 is at _worst_ 1/8 of the problem on ppc64 since > the SECTION_SIZE is so 8x bigger by default. > > Also, the cost of creating sections on ppc is *MUCH* higher than x86 > when amortized across the number of pages that you're initializing. A > section on ppc64 has to be created for each (2^24/2^16)=256 pages while > one on x86 is created for each (2^27/2^12)=32768 pages. > > Thus, x86 folks with our small pages and large sections tend to be > focused on per-page costs. The ppc folks with their small sections and > larger pages tend to be focused on the per-section costs. Ah, thanks for the explaination, now it makes more sense why they are both optimizing in different places. But a "cleanup" patch first, and then the "change the logic to go faster" would be better here, so that we can review what is really happening. thanks, greg k-h -- 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/