Higer latency with dynamic tick (need for an io-ondemand govenor?)

[Woodruff, Richard]

Hi,

When capturing some traces with dynamic tick we were noticing the
interrupt latency seems to go up a good amount. If you look at the trace
the gpio IRQ is now offset a good amount.  Good news I guess is its
pretty predictable.

* If we couple this with progressively higher latency C-States we see
that IO speed can fall by a good amount, especially for PIO mixes.  Now
if QOS is maintained you may or may-not care.

I was wondering what thoughts of optimizing this might be.

One thought was if an io-ondemand of some sort was used.  It could track
interrupt statistics and be feed back into cpu-idle.  When there is a
high interrupt load period it could shrink the acceptable latency and
thus help choose a good a C-State which favors throughput.  Some moving
average window could be used to track it.

Perhaps a new interrupt attribute could be attached at irq request time
to allow the tracking of bandwidth important devices.

The attached is captured on a .22 kernel.  The same should be available
in a bit on a .24 kernel.

Regards,
Richard W.

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