linux-kernel - Re: [RFC PATCH 0/14] Parallel memory initialisation

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Date:	Wed, 15 Apr 2015 10:50:45 -0400
From:	Waiman Long <waiman.long@...com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Linux-MM <linux-mm@...ck.org>, Robin Holt <holt@....com>,
	Nathan Zimmer <nzimmer@....com>, Daniel Rahn <drahn@...e.com>,
	Davidlohr Bueso <dbueso@...e.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Tom Vaden <tom.vaden@...com>,
	Scott Norton <scott.norton@...com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/14] Parallel memory initialisation

On 04/15/2015 09:38 AM, Mel Gorman wrote:
> On Wed, Apr 15, 2015 at 09:15:50AM -0400, Waiman Long wrote:
>>> <SNIP>
>>> Patches are against 4.0-rc7.
>>>
>>>   Documentation/kernel-parameters.txt |   8 +
>>>   arch/ia64/mm/numa.c                 |  19 +-
>>>   arch/x86/Kconfig                    |   2 +
>>>   include/linux/memblock.h            |  18 ++
>>>   include/linux/mm.h                  |   8 +-
>>>   include/linux/mmzone.h              |  37 +++-
>>>   init/main.c                         |   1 +
>>>   mm/Kconfig                          |  29 +++
>>>   mm/bootmem.c                        |   6 +-
>>>   mm/internal.h                       |  23 ++-
>>>   mm/memblock.c                       |  34 ++-
>>>   mm/mm_init.c                        |   9 +-
>>>   mm/nobootmem.c                      |   7 +-
>>>   mm/page_alloc.c                     | 398 +++++++++++++++++++++++++++++++-----
>>>   mm/vmscan.c                         |   6 +-
>>>   15 files changed, 507 insertions(+), 98 deletions(-)
>>>
>> I had included your patch with the 4.0 kernel and booted up a
>> 16-socket 12-TB machine. I measured the elapsed time from the elilo
>> prompt to the availability of ssh login. Without the patch, the
>> bootup time was 404s. It was reduced to 298s with the patch. So
>> there was about 100s reduction in bootup time (1/4 of the total).
>>
> Cool, thanks for testing. Would you be able to state if this is really
> important or not? Does booting 100s second faster on a 12TB machine really
> matter? I can then add that justification to the changelog to avoid a
> conversation with Andrew that goes something like
>
> Andrew: Why are we doing this?
> Mel:    Because we can and apparently people might want it.
> Andrew: What's the maintenance cost of this?
> Mel:    Magic beans
>
> I prefer talking to Andrew when it's harder to predict what he'll say.

Booting 100s faster is certainly something that is nice to have. Right 
now, more time is spent in the firmware POST portion of the bootup 
process than in the OS boot. So I would say this patch isn't really 
critical right now as machines with that much memory are relatively 
rare. However, if we look forward to the near future, some new memory 
technology like persistent memory is coming and machines with large 
amount of memory (whether persistent or not) will become more common. 
This patch will certainly be useful if we look forward into the future.

>> However, there were 2 bootup problems in the dmesg log that needed
>> to be addressed.
>> 1. There were 2 vmalloc allocation failures:
>> [    2.284686] vmalloc: allocation failure, allocated 16578404352 of
>> 17179873280 bytes
>> [   10.399938] vmalloc: allocation failure, allocated 7970922496 of
>> 8589938688 bytes
>>
>> 2. There were 2 soft lockup warnings:
>> [   57.319453] NMI watchdog: BUG: soft lockup - CPU#1 stuck for 23s!
>> [swapper/0:1]
>> [   85.409263] NMI watchdog: BUG: soft lockup - CPU#1 stuck for 22s!
>> [swapper/0:1]
>>
>> Once those problems are fixed, the patch should be in a pretty good
>> shape. I have attached the dmesg log for your reference.
>>
> The obvious conclusion is that initialising 1G per node is not enough for
> really large machines. Can you try this on top? It's untested but should
> work. The low value was chosen because it happened to work and I wanted
> to get test coverage on common hardware but broke is broke.
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index f2c96d02662f..6b3bec304e35 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -276,9 +276,9 @@ static inline bool update_defer_init(pg_data_t *pgdat,
>   	if (pgdat->first_deferred_pfn != ULONG_MAX)
>   		return false;
>
> -	/* Initialise at least 1G per zone */
> +	/* Initialise at least 32G per node */
>   	(*nr_initialised)++;
> -	if (*nr_initialised>  (1UL<<  (30 - PAGE_SHIFT))&&
> +	if (*nr_initialised>  (32UL<<  (30 - PAGE_SHIFT))&&
>   	(pfn&  (PAGES_PER_SECTION - 1)) == 0) {
>   		pgdat->first_deferred_pfn = pfn;
>   		return false;

I will try this out when I can get hold of the 12-TB machine again.

The vmalloc allocation failures were for the following hash tables:
- Dentry cache hash table entries
- Inode-cache hash table entries

Those hash tables scale linearly with the amount of memory available in 
the system. So instead of hardcoding a certain value, why don't we make 
it a certain % of the total memory but bottomed out to 1G at the low end?

Cheers,
Longman
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