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

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Date:   Tue, 15 Aug 2017 10:51:21 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Kemi Wang <kemi.wang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dave <dave.hansen@...ux.intel.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Ying Huang <ying.huang@...el.com>,
        Aaron Lu <aaron.lu@...el.com>, Tim Chen <tim.c.chen@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm: Update NUMA counter threshold size

On 08/15/2017 10:30 AM, Mel Gorman wrote:
> On Tue, Aug 15, 2017 at 09:55:39AM -0700, Tim Chen wrote:

>>
>> 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. 
>>
> 
> The same comments hold. The increase of a cache line is undesirable but
> there are other places where the overall cost can be reduced by special
> casing based on how this counter is used (always incrementing by one).

Can you be more explicit of what optimization you suggest here and changes
to inc/dec_zone_page_state?  Seems to me like we will still overflow
the local counter with the same frequency unless the threshold and
counter size is changed.

Thanks.

Tim

> It would be preferred if those were addressed to see how close that gets
> to the same performance of doubling the necessary storage for a counter.
> doubling the storage 
>