Messages in this thread | | | Date | Fri, 22 Feb 2008 10:11:33 +0530 | From | Balbir Singh <> | Subject | Re: [PATCH] Document huge memory/cache overhead of memory controller in Kconfig |
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Andi Kleen wrote: >> 1. We could create something similar to mem_map, we would need to handle 4 > > 4? At least x86 mainline only has two ways now. flatmem and vmemmap. > >> different ways of creating mem_map. > > Well it would be only a single way to create the "aux memory controller > map" (or however it will be called). Basically just a call to single > function from a few different places. > >> 2. On x86 with 64 GB ram, > > First i386 with 64GB just doesn't work, at least not with default 3:1 > split. Just calculate it yourself how much of the lowmem area is left > after the 64GB mem_map is allocated. Typical rule of thumb is that 16GB > is the realistic limit for 32bit x86 kernels. Worrying about > anything more does not make much sense. >
I understand what you say Andi, but nothing in the kernel stops us from supporting 64GB. Should a framework like memory controller make an assumption that not more than 16GB will be configured on an x86 box?
>> if we decided to use vmalloc space, we would need 64 >> MB of vmalloc'ed memory > > Yes and if you increase mem_map you need exactly the same space > in lowmem too. So increasing the vmalloc reservation for this is > equivalent. Just make sure you use highmem backed vmalloc. >
I see two problems with using vmalloc. One, the reservation needs to be done across architectures. Two, a big vmalloc chunk is not node aware, if all the pages come from the same node, we have a penalty to pay in a NUMA system.
-- Warm Regards, Balbir Singh Linux Technology Center IBM, ISTL
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