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

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Date:   Thu, 30 Apr 2020 17:40:59 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
        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>,
        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 0/7] padata: parallelize deferred page init

On Thu, Apr 30, 2020 at 5:31 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> 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?

Large machines indeed have a lengthy uptime, but they also can host a
large number of VMs meaning that downtime of the host increases the
downtime of VMs in cloud environments. Some VMs might be very sensible
to downtime: game servers, traders, etc.

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