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

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Date:   Thu, 21 Dec 2017 09:17:06 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     kemi <kemi.wang@...el.com>
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 Thu 21-12-17 16:06:50, kemi wrote:
> 
> 
> On 2017年12月20日 18:12, Michal Hocko wrote:
> > On Wed 20-12-17 13:52:14, kemi wrote:
> >>
> >>
> >> On 2017年12月19日 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%)

Please describe the numbers some more. Is this an average? What is the
std? Can you see any difference with a more generic workload?

> 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?

> 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.
-- 
Michal Hocko
SUSE Labs