Re: [PATCH v2 3/5] mm: enlarge NUMA counters threshold size

[kemi]

On 2017å12æ20æ 18:12, Michal Hocko wrote:
> On Wed 20-12-17 13:52:14, kemi wrote:
>>
>>
>> On 2017å12æ19æ 20:40, Michal Hocko wrote:
>>> On Tue 19-12-17 14:39:24, Kemi Wang wrote:
>>>> We have seen significant overhead in cache bouncing caused by NUMA counters
>>>> update in multi-threaded page allocation. See 'commit 1d90ca897cb0 ("mm:
>>>> update NUMA counter threshold size")' for more details.
>>>>
>>>> This patch updates NUMA counters to a fixed size of (MAX_S16 - 2) and deals
>>>> with global counter update using different threshold size for node page
>>>> stats.
>>>
>>> Again, no numbers.
>>
>> Compare to vanilla kernel, I don't think it has performance improvement, so
>> I didn't post performance data here.
>> But, if you would like to see performance gain from enlarging threshold size
>> for NUMA stats (compare to the first patch), I will do that later. 
> 
> Please do. I would also like to hear _why_ all counters cannot simply
> behave same. In other words why we cannot simply increase
> stat_threshold? Maybe calculate_normal_threshold needs a better scaling
> for larger machines.
> 

I will add this performance data to changelog in V3 patch series.

Test machine: 2-sockets skylake platform (112 CPUs, 62G RAM)
Benchmark: page_bench03
Description: 112 threads do single page allocation/deallocation in parallel.
               before                           after
                                       (enlarge threshold size)       
CPU cycles     722                              379(-47.5%)

Some thinking about that:
a) the overhead due to cache bouncing caused by NUMA counter update in fast path 
severely increase with more and more CPUs cores
b) AFAIK, the typical usage scenario (similar at least)for which this optimization can 
benefit is 10/40G NIC used in high-speed data center network of cloud service providers.