linux-kernel - Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem

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Date:	Fri, 11 Jul 2014 10:55:59 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Tejun Heo <tj@...nel.org>
cc:	Jiang Liu <jiang.liu@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Jianyu Zhan <nasa4836@...il.com>, malc <av1474@...tv.ru>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Fabian Frederick <fabf@...net.be>,
	Tony Luck <tony.luck@...el.com>, linux-mm@...ck.org,
	linux-hotplug@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem_id() to
 support memoryless node

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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