On Fri, Jul 30, 2010 at 9:38 AM, Dave Hansen <dave@xxxxxxxxxxxxxxxxxx> wrote: > 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. That's the normal SPARSEMEM case. > > SPARSEMEM_EXTREME would be a bit different. It'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. If 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. The 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: > > 512k@0x00000000 > 512k@0x10000000 > 512k@0x20000000 > ... > 512k@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. But, I haven't heard you say that you have this kind > of configuration, yet. :) > > SPARSEMEM_EXTREME is really easy to test. You just have to set it in > your .config. To 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 it. 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 > > -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href