Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem_id() to support memoryless node

[Christoph Lameter]

On Fri, 11 Jul 2014, Tejun Heo wrote:

> On Fri, Jul 11, 2014 at 11:21:56AM -0400, Tejun Heo wrote:
> > Even if that's the case, there's no reason to burden everyone with
> > this distinction.  Most users just wanna say "I'm on this node.
> > Please allocate considering that".  There's nothing wrong with using
> > numa_node_id() for that.
>
> Also, this is minor but don't we also lose fallback information by
> doing this from the caller?  Please consider the following topology
> where each hop is the same distance.
>
>    A - B - X - C - D
>
> Where X is the memless node.  num_mem_id() on X would return either B
> or C, right?  If B or C can't satisfy the allocation, the allocator
> would fallback to A from B and D for C, both of which aren't optimal.
> It should first fall back to C or B respectively, which the allocator
> can't do anymoe because the information is lost when the caller side
> performs numa_mem_id().

True but the advantage is that the numa_mem_id() allows the use of a
consitent sort of "local" node which increases allocator performance due
to the abillity to cache objects from that node.

> Seems pretty misguided to me.

IMHO the whole concept of a memoryless node looks pretty misguided to me.

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