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

[Pavel Tatashin]

On Thu, Apr 30, 2020 at 5:31 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, 30 Apr 2020 16:11:18 -0400 Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> 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?