linux-kernel - Re: [PATCH 6/7] mm: parallelize deferred_init

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Date:   Mon, 04 May 2020 16:38:03 -0700
From:   Josh Triplett <josh@...htriplett.org>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Michal Hocko <mhocko@...nel.org>, Pavel Machek <pavel@....cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Shile Zhang <shile.zhang@...ux.alibaba.com>,
        Tejun Heo <tj@...nel.org>, Zi Yan <ziy@...dia.com>,
        linux-crypto@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/7] mm: parallelize deferred_init_memmap()

On May 4, 2020 3:33:58 PM PDT, Alexander Duyck <alexander.duyck@...il.com> wrote:
>On Thu, Apr 30, 2020 at 1:12 PM Daniel Jordan
><daniel.m.jordan@...cle.com> wrote:
>>         /*
>> -        * Initialize and free pages in MAX_ORDER sized increments so
>> -        * that we can avoid introducing any issues with the buddy
>> -        * allocator.
>> +        * More CPUs always led to greater speedups on tested
>systems, up to
>> +        * all the nodes' CPUs.  Use all since the system is
>otherwise idle now.
>>          */
>
>I would be curious about your data. That isn't what I have seen in the
>past. Typically only up to about 8 or 10 CPUs gives you any benefit,
>beyond that I was usually cache/memory bandwidth bound.

I've found pretty much linear performance up to memory bandwidth, and on the systems I was testing, I didn't saturate memory bandwidth until about the full number of physical cores. From number of cores up to number of threads, the performance stayed about flat; it didn't get any better or worse.

- Josh