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

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Date:   Wed, 6 May 2020 19:01:48 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        Josh Triplett <josh@...htriplett.org>,
        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 Wed, May 06, 2020 at 06:43:35PM -0400, Daniel Jordan wrote:
> On Wed, May 06, 2020 at 03:36:54PM -0700, Alexander Duyck wrote:
> > On Wed, May 6, 2020 at 3:21 PM Daniel Jordan <daniel.m.jordan@...cle.com> wrote:
> > >
> > > On Tue, May 05, 2020 at 07:55:43AM -0700, Alexander Duyck wrote:
> > > > One question about this data. What is the power management
> > > > configuration on the systems when you are running these tests? I'm
> > > > just curious if CPU frequency scaling, C states, and turbo are
> > > > enabled?
> > >
> > > Yes, intel_pstate is loaded in active mode without hwp and with turbo enabled
> > > (those power management docs are great by the way!) and intel_idle is in use
> > > too.
> > >
> > > > I ask because that is what I have seen usually make the
> > > > difference in these kind of workloads as the throughput starts
> > > > dropping off as you start seeing the core frequency lower and more
> > > > cores become active.
> > >
> > > If I follow, you're saying there's a chance performance would improve with the
> > > above disabled, but how often would a system be configured that way?  Even if
> > > it were faster, the machine is configured how it's configured, or am I missing
> > > your point?
> > 
> > I think you might be missing my point. What I was getting at is that I
> > know for performance testing sometimes C states and P states get
> > disabled in order to get consistent results between runs, it sounds
> > like you have them enabled though. I was just wondering if you had
> > disabled them or not. If they were disabled then you wouldn't get the
> > benefits of turbo and as such adding more cores wouldn't come at a
> > penalty, while with it enabled the first few cores should start to
> > slow down as they fell out of turbo mode. So it may be part of the
> > reason why you are only hitting about 10x at full core count.

I checked the memory bandwidth of the biggest system, the Skylake.  Couldn't
find official specs for it, all I could quickly find were stream results from a
blog post of ours that quoted a range of about 123-145 GB/s over both nodes
when compiling with gcc.  That's with all CPUs.

Again using all CPUs, multithreaded page init is doing 41 GiB/s per node
assuming it's just touching the 64 bytes of each page struct, so assuming
there's more memory traffic than just struct page, it seems another part of the
reason for only 10x is we're bottlenecked on memory bandwidth.