minor CPU wake-up question

minor CPU wake-up question

[Linda Walsh]

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...


  63.2% (  6.0)           xfsbufd : schedule_timeout (process_timeout)
  21.1% (  2.0)     <kernel core> : clocksource_register 
(clocksource_watchdog)
   2.1% (  0.2)     <kernel core> : __netdev_watchdog_up (dev_watchdog)
   2.1% (  0.2)     <kernel core> : page_writeback_init (wb_timer_fn)
   2.1% (  0.2)     <kernel core> : neigh_table_init_no_netlink 
(neigh_periodic_
   2.1% (  0.2)              init : schedule_timeout (process_timeout)
   1.1% (  0.1)          xfssyncd : schedule_timeout (process_timeout)
   1.1% (  0.1)     <kernel core> : init_nonfatal_mce_checker 
(delayed_work_time
   1.1% (  0.1)              cron : do_nanosleep (hrtimer_wakeup)
   1.1% (  0.1)              nscd : schedule_timeout (process_timeout)
   1.1% (  0.1)        irqbalance : do_nanosleep (hrtimer_wakeup)
   1.1% (  0.1)     <kernel core> : __neigh_event_send (neigh_timer_handler)
   1.1% (  0.1)     <kernel core> : dst_run_gc (dst_run_gc)
----

Re: minor CPU wake-up question

[Eric Sandeen]

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