boot-time slowdown for measure_migration_cost

[Bjorn Helgaas]

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.

Is that expected?  Is the information we get really worth
that much?  Could the measurement be done at run-time
instead?  Is there a smaller hammer we could use, e.g.,
flushing just the buffer rather than the *entire* cache?
Did we just implement sched_cacheflush() incorrectly for
ia64?

Only ia64, x86, and x86_64 currently have a non-empty
sched_cacheflush(), and the x86* ones contain only "wbinvd()".
So I suspect that only ia64 sees this slowdown.  But I would
guess that other arches will implement it in the future.

Bjorn
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/