Re: [PATCH v2 0/5] Add movablecore_map boot option

[Mel Gorman <mgorman@suse.de>]

On Thu, Nov 29, 2012 at 07:38:26PM +0900, Yasuaki Ishimatsu wrote:
> Hi Tony,
> 
> 2012/11/29 6:34, Luck, Tony wrote:
> >>1. use firmware information
> >>   According to ACPI spec 5.0, SRAT table has memory affinity structure
> >>   and the structure has Hot Pluggable Filed. See "5.2.16.2 Memory
> >>   Affinity Structure". If we use the information, we might be able to
> >>   specify movable memory by firmware. For example, if Hot Pluggable
> >>   Filed is enabled, Linux sets the memory as movable memory.
> >>
> >>2. use boot option
> >>   This is our proposal. New boot option can specify memory range to use
> >>   as movable memory.
> >
> >Isn't this just moving the work to the user? To pick good values for the
> 
> Yes.
> 
> >movable areas, they need to know how the memory lines up across
> >node boundaries ... because they need to make sure to allow some
> >non-movable memory allocations on each node so that the kernel can
> >take advantage of node locality.
> 
> There is no problem.
> Linux has already two boot options, kernelcore= and movablecore=.
> So if we use them, non-movable memory is divided into each node evenly.
> 

The motivation for those options was to reserve a percentage of memory
to be used for hugepage allocation. If hugepages were not being used at
a particular time then they could be used for other purposes. While the
system could in theory face lowmem/highmem style problems, in practice
it did not happen because the memory would be allocated as hugetlbfs
pages and unavailable anyway. The same does not really apply to a general
purpose system that you want to support memory hot-remove on so be wary of
lowmem/highmem style problems caused by relying too heavily on ZONE_MOVABLE.

-- 
Mel Gorman
SUSE Labs

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