Re: [Lhns-devel] Re: Who's doing what with cpu/memory/node hotplug?

Ashok Raj
2004-05-18T20:16:30Z

On Tue, May 18, 2004 at 09:58:38AM -0500, Russ Anderson wrote:
> Matthias Fouquet-Lapar wrote:
> >
> > [ Just as a site-note, I always wondered if hotplug would be a useful feature
> > for larger configurations to reduce boot-time. For example, on a 64P system
> > you start the kernel on some "root" CPU with a relativly small amount of
> > memory, so it should boot relativly fast. While the system is going into
> > the desired run-level, hotplug/add now adds in parallel all available CPUs,
> > main memory, I/O etc. The idea is that you probably don't need 64P and
> > 1 Terabyte to start system services
> > ]
>
> I can certainly think of cases (customers) where minimizing the total
> system down time is a huge issue. Getting a subset of the system
> back into production as quickly as possible would be very desirable.
>
> Another advantage of that type boot process is that it would catch
> any bugs (or regressions) in the hot add code. At a minumum, it
> would be a very good test of the hot add functionality.
>
> I think a boot option to allow that type of booting process
> would be a good idea.

The boot already follows the same methodology, the boot cpu comes up, and the rest of the cpu's are brought up. for e.g. smp_init() does call cpu_up() which is the same path for hotplug.

Not sure if cpu_up is a time consuming process, i think if you have more cpu's
the startup could be faster as you have now more resources to complete the
statup of kernel.

Often what i have seen is that the IO part of the probe is the most time
consuming process. Splitting the discovery of controllers and boot resources
first, and then deferring the rest if the io discovery process would
speedup boot. Also its your firmware that would do the same work as OS
in identifying all the controllers and disks upfront that contributes to
slow boot.

bringup all cpu() and then defer part of the probe/discovery (especially
disks) in parallel would provide real benefit i think.

Cheers,
ashok