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SubjectRe: [PATCH] Document huge memory/cache overhead of memory controller in Kconfig
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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