Re: minor CPU wake-up question

[Eric Sandeen <sandeen@sandeen.net>]

Linda Walsh wrote:
> I recently ran into "powertop" (fr. lesswatts.org) that shows how often
> interrupts awaken a processor under a tickless kernel.
> 
> The display indicates the counts are over a 10 second period. 
> Barring any disk activity, why would xfsbufd wake up each copy of itself
> up when there doesn't seem like there would be anything to do.
> 
> Is a separate process really needed for each partition (that seems to be
> the case)?  I don't know if it is 1 interrupt/bufd or 6 on 1, but
> it is fairly constant with 6 interrupts each period.
> FWIW, dirty_writeback_centiseconds is set to 1500(1499) and makes
> no difference in the count.
> 
> It doesn't seem to be a big deal, other than it is at the top of the
> interrupt-chart with usually 60% or more of the ticks.  Might be nice
> to not have it on top if it isn't necessary...

I think by default, xfsbufd wakes up each second for each filesystem.
See fs.xfs.xfsbufd_centisecs, default 100, or 1s.  I think powertop is
reporting wakeups/second, so it looks you have 6 filesystems mounted?

Honestly if that's your most frequent entry, you're probably doing
pretty well...

-Eric