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

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Date:   Thu, 21 Dec 2017 18:31:19 +0800
From:   kemi <kemi.wang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Johannes Weiner <hannes@...xchg.org>,
        Christopher Lameter <cl@...ux.com>,
        YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Nikolay Borisov <nborisov@...e.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Dave <dave.hansen@...ux.intel.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Ying Huang <ying.huang@...el.com>,
        Aaron Lu <aaron.lu@...el.com>, Aubrey Li <aubrey.li@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/5] mm: enlarge NUMA counters threshold size



On 2017年12月21日 16:59, Michal Hocko wrote:
> On Thu 21-12-17 16:23:23, kemi wrote:
>>
>>
>> On 2017年12月21日 16:17, Michal Hocko wrote:
> [...]
>>> Can you see any difference with a more generic workload?
>>>
>>
>> I didn't see obvious improvement for will-it-scale.page_fault1
>> Two reasons for that:
>> 1) too long code path
>> 2) server zone lock and lru lock contention (access to buddy system frequently) 
> 
> OK. So does the patch helps for anything other than a microbenchmark?
> 
>>>> 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
>>>
>>> What is an effect on a smaller system with fewer CPUs?
>>>
>>
>> Several CPU cycles can be saved using single thread for that.
>>
>>>> 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.
>>>
>>> I would expect those would disable the numa accounting altogether.
>>>
>>
>> Yes, but it is still worthy to do some optimization, isn't?
> 
> Ohh, I am not opposing optimizations but you should make sure that they
> are worth the additional code and special casing. As I've said I am not
> convinced special casing numa counters is good. You can play with the
> threshold scaling for larger CPU count but let's make sure that the
> benefit is really measurable for normal workloads. Special ones will
> disable the numa accounting anyway.
> 

I understood. Could you give me some suggestion for those normal workloads, Thanks.
I will have a try and post the data ASAP.