Date: Fri, 11 Jul 2014 10:55:59 -0500 (CDT)
From: Christoph Lameter <cl@gentwo.org>
To: Tejun Heo <tj@kernel.org>
cc: Jiang Liu <jiang.liu@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>, David Rientjes <rientjes@google.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Vladimir Davydov <vdavydov@parallels.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Rik van Riel <riel@redhat.com>, Wanpeng Li <liwanp@linux.vnet.ibm.com>,
        Zhang Yanfei <zhangyanfei@cn.fujitsu.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jianyu Zhan <nasa4836@gmail.com>, malc <av1474@comtv.ru>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Fabian Frederick <fabf@skynet.be>, Tony Luck <tony.luck@intel.com>,
        linux-mm@kvack.org, linux-hotplug@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem_id() to
 support memoryless node
In-Reply-To: <20140711153302.GA30865@htj.dyndns.org>
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References: <1405064267-11678-1-git-send-email-jiang.liu@linux.intel.com> <1405064267-11678-8-git-send-email-jiang.liu@linux.intel.com> <20140711144205.GA27706@htj.dyndns.org> <alpine.DEB.2.11.1407111012210.25527@gentwo.org> <20140711152156.GB29137@htj.dyndns.org>
 <20140711153302.GA30865@htj.dyndns.org>
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Sender: linux-kernel-owner@vger.kernel.org

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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