Re: boot-time slowdown for measure_migration_cost

[John Hawkes]

From: "Luck, Tony" <tony.luck@xxxxxxxxx>
...
> So the variation in the computed value of migration_cost was at worst
> 2% with these modifications to the algorithm.  Do you really need to know
> the value to this accuracy?  What 2nd order bad effects would occur from
> using an off-by-2% value for scheduling decisions?
>
> On the plus side Prarit's results show that this time isn't scaling with
> NR_CPUS ... apparently just cache size and number of domains are significant
> in the time to compute.

Yes, the calculation is done just once per domain level, and a desire to
achieve great accuracy for the calculation presupposes that the cpuM-to-cpuN
migration cost for a given domain level is identical (or very close) across
all the CPU pairs.  That is, for a given domain level, only one CPU pair are
chosen for the calculation.  For the ia64/sn2 NUMA Altix, and I suspect for
other NUMA platforms, this just isn't true for the middle domain level (i.e.,
the level that appears when the CPU count is >32p) -- i.e., some CPU pairs are
"closer" than other pairs.  The variation for other CPU pairs in this domain
level is certainly much greater than 2%.

John Hawkes

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