Re: boot-time slowdown for measure_migration_cost

[Prarit Bhargava]

Luck, Tony wrote:
> > The boot-time migration cost auto-tuning stuff seems to have
> > been merged to Linus' tree since 2.6.15.  On little one- or
> > two-processor systems, the time required to measure the
> > migration costs isn't very noticeable, but by the time we
> > get to even a four-processor ia64 box, it adds about
> > 30 seconds to the boot time, which seems like a lot.
>
>
> I only see about 16 seconds for a 4-way tiger (not that 16 seconds
> is good ... but it not as bad as 30).  This was with a build
> from tiger_defconfig that sets CONFIG_NR_CPUS=4 ... so I wonder
> what's causing the factor of two.  I measured with a printk
> each side of build_sched_domains() and booted with the "time"
> command line arg to get:

I've noticed the delay on a 16p and 64p.  At first I thought it was a 
system hang but have since learned to live with the delay.

 What happens on a 512 cpu
> Altix (if it's quadratic, they may be still waiting for the
> boot to finish :-)


Not quadratic.  This is a 64p Altix ...

[    9.942253] Brought up 64 CPUs
[    9.942904] Total of 64 processors activated (143654.91 BogoMIPS).
[    9.943995] build_sched_domains: start
[   32.108439] migration_cost=0,32232,39021
[   37.894391] build_sched_domains: end

P.

> -Tony
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