Date:	Tue, 05 May 2015 23:39:09 -0400
From:	Waiman Long <waiman.long@...com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Nathan Zimmer <nzimmer@....com>,
	Dave Hansen <dave.hansen@...el.com>,
	Scott Norton <scott.norton@...com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/13] Parallel struct page initialisation v4

On 05/05/2015 11:01 AM, Waiman Long wrote:
> On 05/05/2015 10:31 AM, Mel Gorman wrote:
>> On Tue, May 05, 2015 at 09:55:52AM -0400, Waiman Long wrote:
>>> On 05/05/2015 06:45 AM, Mel Gorman wrote:
>>>> On Mon, May 04, 2015 at 02:30:46PM -0700, Andrew Morton wrote:
>>>>>> Before the patch, the boot time from elilo prompt to ssh login 
>>>>>> was 694s.
>>>>>> After the patch, the boot up time was 346s, a saving of 348s 
>>>>>> (about 50%).
>>>>> Having to guesstimate the amount of memory which is needed for a
>>>>> successful boot will be painful.  Any number we choose will be wrong
>>>>> 99% of the time.
>>>>>
>>>>> If the kswapd threads have started, all we need to do is to wait: 
>>>>> take
>>>>> a little nap in the allocator's page==NULL slowpath.
>>>>>
>>>>> I'm not seeing any reason why we can't start kswapd much earlier -
>>>>> right at the start of do_basic_setup()?
>>>> It doesn't even have to be kswapd, it just should be a thread 
>>>> pinned to
>>>> a done. The difficulty is that dealing with the system hashes means 
>>>> the
>>>> initialisation has to happen before vfs_caches_init_early() when 
>>>> there is
>>>> no scheduler. Those allocations could be delayed further but then 
>>>> there is
>>>> the possibility that the allocations would not be contiguous and 
>>>> they'd
>>>> have to rely on CMA to make the attempt. That potentially alters the
>>>> performance of the large system hashes at run time.
>>>>
>>>> We can scale the amount initialised with memory sizes relatively easy.
>>>> This boots on the same 1TB machine I was testing before but that is
>>>> hardly a surprise.
>>>>
>>>> ---8<---
>>>> mm: meminit: Take into account that large system caches scale 
>>>> linearly with memory
>>>>
>>>> Waiman Long reported a 24TB machine triggered an OOM as parallel 
>>>> memory
>>>> initialisation deferred too much memory for initialisation. The likely
>>>> consumer of this memory was large system hashes that scale with memory
>>>> size. This patch initialises at least 2G per node but scales the 
>>>> amount
>>>> initialised for larger systems.
>>>>
>>>> Signed-off-by: Mel Gorman<mgorman@...e.de>
>>>> ---
>>>>   mm/page_alloc.c | 15 +++++++++++++--
>>>>   1 file changed, 13 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 598f78d6544c..f7cc6c9fb909 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -266,15 +266,16 @@ static inline bool 
>>>> early_page_nid_uninitialised(unsigned long pfn, int nid)
>>>>    */
>>>>   static inline bool update_defer_init(pg_data_t *pgdat,
>>>>                   unsigned long pfn, unsigned long zone_end,
>>>> +                unsigned long max_initialise,
>>>>                   unsigned long *nr_initialised)
>>>>   {
>>>>       /* Always populate low zones for address-contrained 
>>>> allocations */
>>>>       if (zone_end<   pgdat_end_pfn(pgdat))
>>>>           return true;
>>>>
>>>> -    /* Initialise at least 2G of the highest zone */
>>>> +    /* Initialise at least the requested amount in the highest 
>>>> zone */
>>>>       (*nr_initialised)++;
>>>> -    if (*nr_initialised>   (2UL<<   (30 - PAGE_SHIFT))&&
>>>> +    if ((*nr_initialised>   max_initialise)&&
>>>>       (pfn&   (PAGES_PER_SECTION - 1)) == 0) {
>>>>           pgdat->first_deferred_pfn = pfn;
>>>>           return false;
>>>> @@ -299,6 +300,7 @@ static inline bool 
>>>> early_page_nid_uninitialised(unsigned long pfn, int nid)
>>>>
>>>>   static inline bool update_defer_init(pg_data_t *pgdat,
>>>>                   unsigned long pfn, unsigned long zone_end,
>>>> +                unsigned long max_initialise,
>>>>                   unsigned long *nr_initialised)
>>>>   {
>>>>       return true;
>>>> @@ -4457,11 +4459,19 @@ void __meminit memmap_init_zone(unsigned 
>>>> long size, int nid, unsigned long zone,
>>>>       unsigned long end_pfn = start_pfn + size;
>>>>       unsigned long pfn;
>>>>       struct zone *z;
>>>> +    unsigned long max_initialise;
>>>>       unsigned long nr_initialised = 0;
>>>>
>>>>       if (highest_memmap_pfn<   end_pfn - 1)
>>>>           highest_memmap_pfn = end_pfn - 1;
>>>>
>>>> +    /*
>>>> +     * Initialise at least 2G of a node but also take into account 
>>>> that
>>>> +     * two large system hashes that can take up an 8th of memory.
>>>> +     */
>>>> +    max_initialise = min(2UL<<   (30 - PAGE_SHIFT),
>>>> +            (pgdat->node_spanned_pages>>   3));
>>>> +
>>> I think you may be pre-allocating too much memory here. On the 24-TB
>>> machine, the size of the dentry and inode hash tables were 16G each.
>>> So the ratio is about is about 32G/24T = 0.13%. I think a shift
>>> factor of (>>  8) which is about 0.39% should be more than enough.
>> I was taking the most pessimistic value possible to match where those
>> hashes currently get allocated from so that the locality does not change
>> after the series is applied. Can you try both (>>  3) and (>>  8) and 
>> see
>> do both work and if so, what the timing is?
>
> Sure. I will try both and get you the results, hopefully by tomorrow 
> at the latest.
>

With the modified patch, both (>>3) and (>>8) worked without any 
problem. The bootup times are:

1. Unpatch 4.0 kernel - 694s
2. Patch kernel with 4G/node - 346s
3. Patch kernel with (>>3) - 389s
4. Patch kernel with (>>8) - 353s

Cheers,
Longman
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