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

[Josh Triplett]

On Thu, Apr 30, 2020 at 02:31:31PM -0700, Andrew Morton 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 :)

Some test results on a system with 96 CPUs and 192GB of memory:

Without this patch series:
[    0.487132] node 0 initialised, 23398907 pages in 292ms
[    0.499132] node 1 initialised, 24189223 pages in 304ms
...
[    0.629376] Run /sbin/init as init process

With this patch series:
[    0.227868] node 0 initialised, 23398907 pages in 28ms
[    0.230019] node 1 initialised, 24189223 pages in 28ms
...
[    0.361069] Run /sbin/init as init process

That makes a huge difference; memory initialization is the largest
remaining component of boot time.

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

Cloud systems and other virtual machines may have a huge amount of
memory but not necessarily run for a long time; on such systems, boot
time becomes critically important. Reducing boot time on cloud systems
and VMs makes the difference between "leave running to reduce latency"
and "just boot up when needed".

- Josh Triplett