linux-kernel - Re: [PATCH 0/7] padata: parallelize deferred page init

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Date:   Thu, 30 Apr 2020 14:31:31 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     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>,
        Josh Triplett <josh@...htriplett.org>,
        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@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] padata: parallelize deferred page init

On Thu, 30 Apr 2020 16:11:18 -0400 Daniel Jordan <daniel.m.jordan@...cle.com> wrote:

> Sometimes the kernel doesn't take full advantage of system memory
> bandwidth, leading to a single CPU spending excessive time in
> initialization paths where the data scales with memory size.
> 
> Multithreading naturally addresses this problem, and this series is the
> first step.
> 
> It extends padata, a framework that handles many parallel singlethreaded
> jobs, to handle multithreaded jobs as well by adding support for
> splitting up the work evenly, specifying a minimum amount of work that's
> appropriate for one helper thread to do, load balancing between helpers,
> and coordinating them.  More documentation in patches 4 and 7.
> 
> The first user is deferred struct page init, a large bottleneck in
> kernel boot--actually the largest for us and likely others too.  This
> path doesn't require concurrency limits, resource control, or priority
> adjustments like future users will (vfio, hugetlb fallocate, munmap)
> because it happens during boot when the system is otherwise idle and
> waiting on page init to finish.
> 
> This has been tested on a variety of x86 systems and speeds up kernel
> boot by 6% to 49% by making deferred init 63% to 91% faster.

How long is this up-to-91% in seconds?  If it's 91% of a millisecond
then not impressed.  If it's 91% of two weeks then better :)

Relatedly, how important is boot time on these large machines anyway? 
They presumably have lengthy uptimes so boot time is relatively
unimportant?

IOW, can you please explain more fully why this patchset is valuable to
our users?