Re: [PATCH 2/2] mm: Update NUMA counter threshold size

[kemi <kemi.wang@intel.com>]

On 2017a1'08ae??16ae?JPY 00:55, Tim Chen wrote:
> On 08/15/2017 02:58 AM, Mel Gorman wrote:
>> On Tue, Aug 15, 2017 at 04:45:36PM +0800, Kemi Wang wrote:

>> I'm fairly sure this pushes the size of that structure into the next
>> cache line which is not welcome.
>>>> vm_numa_stat_diff is an always incrementing field. How much do you gain
>> if this becomes a u8 code and remove any code that deals with negative
>> values? That would double the threshold without consuming another cache line.
> 
> Doubling the threshold and counter size will help, but not as much
> as making them above u8 limit as seen in Kemi's data:
> 
>       125         537         358906028 <==> system by default (base)
>       256         468         412397590
>       32765       394(-26.6%) 488932078(+36.2%) <==> with this patchset
> 
> For small system making them u8 makes sense.  For larger ones the
> frequent local counter overflow into the global counter still
> causes a lot of cache bounce.  Kemi can perhaps collect some data
> to see what is the gain from making the counters u8. 
> 
Tim, thanks for your answer. That is what I want to clarify.

Also, pls notice that the negative threshold/2 is set to cpu local counter
(e.g. vm_numa_stat_diff[]) once per-zone counter is updated in current code
path. This weakens the benefit of changing s8 to u8 in this case. 
>>
>> Furthermore, the stats in question are only ever incremented by one.
>> That means that any calcluation related to overlap can be removed and
>> special cased that it'll never overlap by more than 1. That potentially
>> removes code that is required for other stats but not locality stats.
>> This may give enough savings to avoid moving to s16.
>>
>> Very broadly speaking, I like what you're doing but I would like to see
>> more work on reducing any unnecessary code in that path (such as dealing
>> with overlaps for single increments) and treat incrasing the cache footprint
>> only as a very last resort.
>>
Agree. I will think about it more. 

>>>  #endif
>>>  #ifdef CONFIG_SMP
>>>  	s8 stat_threshold;
>>> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
>>> index 1e19379..d97cc34 100644
>>> --- a/include/linux/vmstat.h
>>> +++ b/include/linux/vmstat.h
>>> @@ -125,10 +125,14 @@ static inline unsigned long global_numa_state(enum zone_numa_stat_item item)
>>>  	return x;
>>>  }
>>>  

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