From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with SMTP id D8B7B6B02A7 for ; Fri, 30 Jul 2010 05:43:59 -0400 (EDT) Received: by gwj16 with SMTP id 16so694890gwj.14 for ; Fri, 30 Jul 2010 02:43:58 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <1280450338.16922.11735.camel@nimitz> References: <20100728155617.GA5401@barrios-desktop> <20100728225756.GA6108@barrios-desktop> <20100729161856.GA16420@barrios-desktop> <20100729170313.GB16420@barrios-desktop> <20100729183320.GH18923@n2100.arm.linux.org.uk> <1280436919.16922.11246.camel@nimitz> <20100729221426.GA28699@n2100.arm.linux.org.uk> <1280450338.16922.11735.camel@nimitz> Date: Fri, 30 Jul 2010 18:43:58 +0900 Message-ID: Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v4 From: Minchan Kim Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org To: Dave Hansen Cc: Russell King - ARM Linux , Christoph Lameter , KAMEZAWA Hiroyuki , Milton Miller , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Mel Gorman , Johannes Weiner , Kukjin Kim List-ID: On Fri, Jul 30, 2010 at 9:38 AM, Dave Hansen wrot= e: > On Thu, 2010-07-29 at 23:14 +0100, Russell King - ARM Linux wrote: >> What we need is something which allows us to handle memory scattered >> in several regions of the physical memory map, each bank being a >> variable size. > > Russell, it does sound like you have a pretty pathological case here. :) > It's not one that we've really attempted to address on any other > architectures. > > Just to spell it out, if you have 4GB of physical address space, with > 512k sections, you need 8192 sections, which means 8192*8 bytes, so it'd > eat 64k of memory. =A0That's the normal SPARSEMEM case. > > SPARSEMEM_EXTREME would be a bit different. =A0It's a 2-level lookup. > You'd have 16 "section roots", each representing 256MB of address space. > Each time we put memory under one of those roots, we'd fill in a > 512-section second-level table, which is designed to always fit into one > page. =A0If you start at 256MB, you won't waste all those entries. > > The disadvantage of SPARSEMEM_EXTREME is that it costs you the extra > level in the lookup. =A0The space loss in arm's case would only be 16 > pointers, which would more than be made up for by the other gains. > > The other case where it really makes no sense is when you're populating > a single (or small number) of sections, evenly across the address space. > For instance, let's say you have 16 512k banks, evenly spaced at 256MB > intervals: > > =A0 =A0 =A0 =A0512k@0x00000000 > =A0 =A0 =A0 =A0512k@0x10000000 > =A0 =A0 =A0 =A0512k@0x20000000 > =A0 =A0 =A0 =A0... > =A0 =A0 =A0 =A0512k@0xF0000000 > > If you use SPARSEMEM_EXTREME on that it will degenerate to having the > same memory consumption as classic SPARSEMEM, along with the extra > lookup of EXTREME. =A0But, I haven't heard you say that you have this kin= d > of configuration, yet. :) > > SPARSEMEM_EXTREME is really easy to test. =A0You just have to set it in > your .config. =A0To get much use out of it, you'd also need to make the > SECTION_SIZE, like the 512k we were talking about. > Thanks for good explanation. When this problem happened, I suggested to use section size 16M. The space isn't a big cost but failed since Russell doesn't like it. So I tried to enhance sparsemem to support hole but you guys doesn't like i= t. Frankly speaking myself don't like this approach but I think whoever have to care of the problem. Hmm, Is it better to give up Samsung's good embedded board? It depends on Russell's opinion. I will hold this patch until reaching the conclusion of controversial discussion. Thanks, Dave. > -- Dave > > --=20 Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org