linux-kernel - Re: [PATCH 10/13] x86: mm: Enable deferred struct page initialisation on x86-64

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Date:	Fri, 24 Apr 2015 15:04:27 -0400
From:	Waiman Long <waiman.long@...com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Nathan Zimmer <nzimmer@....com>,
	Dave Hansen <dave.hansen@...el.com>,
	Scott Norton <scott.norton@...com>,
	Daniel J Blueman <daniel@...ascale.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/13] x86: mm: Enable deferred struct page initialisation
 on x86-64

On 04/24/2015 11:20 AM, Mel Gorman wrote:
> On Fri, Apr 24, 2015 at 10:35:49AM -0400, Waiman Long wrote:
>> On 04/23/2015 05:23 AM, Mel Gorman wrote:
>>> On Wed, Apr 22, 2015 at 04:45:00PM -0700, Andrew Morton wrote:
>>>> On Wed, 22 Apr 2015 18:07:50 +0100 Mel Gorman<mgorman@...e.de>   wrote:
>>>>
>>>>> --- a/arch/x86/Kconfig
>>>>> +++ b/arch/x86/Kconfig
>>>>> @@ -32,6 +32,7 @@ config X86
>>>>>   	select HAVE_UNSTABLE_SCHED_CLOCK
>>>>>   	select ARCH_SUPPORTS_NUMA_BALANCING if X86_64
>>>>>   	select ARCH_SUPPORTS_INT128 if X86_64
>>>>> +	select ARCH_SUPPORTS_DEFERRED_STRUCT_PAGE_INIT if X86_64&&   NUMA
>>>> Put this in the "config X86_64" section and skip the "X86_64&&"?
>>>>
>>> Done.
>>>
>>>> Can we omit the whole defer_meminit= thing and permanently enable the
>>>> feature?  That's simpler, provides better test coverage and is, we
>>>> hope, faster.
>>>>
>>> Yes. The intent was to have a workaround if there were any failures like
>>> Waiman's vmalloc failures in an earlier version but they are bugs that
>>> should be fixed.
>>>
>>>> And can this be used on non-NUMA?  Presumably that won't speed things
>>>> up any if we're bandwidth limited but again it's simpler and provides
>>>> better coverage.
>>> Nothing prevents it. There is less opportunity for parallelism but
>>> improving coverage is desirable.
>>>
>> Memory access latency can be more than double for local vs. remote
>> node memory. Bandwidth can also be much lower depending on what kind
>> of interconnect is between the 2 nodes. So it is better to do it in
>> a NUMA-aware way.
> I do not believe that is what he was asking. He was asking if we could
> defer memory initialisation even when there is only one node. It does not
> gain much in terms of boot times but it improves testing coverage.

Thanks for the clarification.

>> Within a NUMA node, however, we can split the
>> memory initialization to 2 or more local CPUs if the memory size is
>> big enough.
>>
> I considered it but discarded the idea. It'd be more complex to setup and
> the two CPUs could simply end up contending on the same memory bus as
> well as contending on zone->lock.
>

I don't think we need that now. However, we may have to consider this 
when one day even a single node can have TBs of memory unless we move to 
a page size larger than 4k.

Cheers,
Longman
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