Re: [PATCH] mm: page_alloc: Default to node-ordering on 64-bit NUMA machines

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2014/09/02 22:51), Johannes Weiner wrote:
> On Mon, Sep 01, 2014 at 01:55:51PM +0100, Mel Gorman wrote:
>> Zones are allocated by the page allocator in either node or zone order.
>> Node ordering is preferred in terms of locality and is applied automatically
>> in one of three cases.
>>
>>    1. If a node has only low memory
>>
>>    2. If DMA/DMA32 is a high percentage of memory
>>
>>    3. If low memory on a single node is greater than 70% of the node size
>>
>> Otherwise zone ordering is used to preserve low memory. Unfortunately
>> a consequence of this is that a machine with balanced NUMA nodes will
>> experience different performance characteristics depending on which node
>> they happen to start from.
>>
>> The point of zone ordering is to protect lower nodes for devices that require
>> DMA/DMA32 memory. When NUMA was first introduced, this was critical as 32-bit
>> NUMA machines commonly suffered from low memory exhaustion problems. On
>> 64-bit machines the primary concern is devices that are 32-bit only which
>> is less severe than the low memory exhaustion problem on 32-bit NUMA. It
>> seems there are really few devices that depends on it.
>>
>> AGP -- I assume this is getting more rare but even then I think the allocations
>> 	happen early in boot time where lowmem pressure is less of a problem
>>
>> DRM -- If the device is 32-bit only then there may be low pressure. I didn't
>> 	evaluate these in detail but it looks like some of these are mobile
>> 	graphics card. Not many NUMA laptops out there. DRM folk should know
>> 	better though.
>>
>> Some TV cards -- Much demand for 32-bit capable TV cards on NUMA machines?
>>
>> B43 wireless card -- again not really a NUMA thing.
>>
>> I cannot find a good reason to incur a performance penalty on all 64-bit NUMA
>> machines in case someone throws a brain damanged TV or graphics card in there.
>> This patch defaults to node-ordering on 64-bit NUMA machines. I was tempted
>> to make it default everywhere but I understand that some embedded arches may
>> be using 32-bit NUMA where I cannot predict the consequences.
>
> This patch is a step in the right direction, but I'm not too fond of
> further fragmenting this code and where it applies, while leaving all
> the complexity from the heuristics and the zonelist building in, just
> on spec.  Could we at least remove the heuristics too?  If anybody is
> affected by this, they can always override the default on the cmdline.
>
I'm okay with removing heuristics. There were a request to add "automatic detection"
at the time this feature was developped. But I'm not sure whether the logic is
still required. i.e. at that age, node-0 memory was small and default node order
can cause OOM easily.

Thanks,
-Kame

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