100% CPU usage, so why any wakeups at all?

[Jan Ceuleers]

All,

I apologise for what is probably another naive question (but it's not in 
the FAQ).

One of my machines is currently transcoding some video. The job is 
running at the lowest priority, and hardly anything else is running. 
This leads to 100% CPU usage, in the P-state that corresponds with the 
lowest clock speed the system is capable of (ondemand cpufreq governor).

Nevertheless, "nice powertop" still shows several hundred 
wakeups-from-idle per second.

My question is not about the wakeups but rather why the system is idling 
in the first place.

I suspect I know the reason (waiting for IO); am I on the right track?

Thx, Jan



===================
root at fe2:~# nice powertop -d
PowerTOP 1.9    (C) 2007 Intel Corporation

Collecting data for 15 seconds
< Detailed C-state information is only available on Mobile CPUs (laptops) >
P-states (frequencies)
   1500 Mhz     0.0%
    800 Mhz   100.0%
Wakeups-from-idle per second : 817.9    interval: 15.0s
no ACPI power usage estimate available
Top causes for wakeups:
   65.8% (373.6)       <interrupt> : eth0
   13.1% ( 74.7)   mythfrontend.re : schedule_timeout (process_timeout)
    5.6% ( 31.8)       <interrupt> : uhci_hcd:usb1, uhci_hcd:usb2, 
uhci_hcd:usb3, ehci_hcd:usb4
    2.9% ( 16.5)       mythbackend : futex_wait (hrtimer_wakeup)
    2.2% ( 12.5)       cpufreq-set : queue_delayed_work_on 
(delayed_work_timer_fn)
    2.1% ( 12.0)   mythfrontend.re : futex_wait (hrtimer_wakeup)
    1.4% (  7.9)   <kernel module> : usb_hcd_poll_rh_status (rh_timer_func)
    1.2% (  6.6)   <kernel module> : dvb_usb_remote_init 
(delayed_work_timer_fn)
    1.2% (  6.6)     <kernel core> : ehci_work (ehci_watchdog)
    0.7% (  4.0)       <interrupt> : ide0
    0.4% (  2.1)              Xorg : do_setitimer (it_real_fn)
    0.4% (  2.0)   xfce-mcs-manage : schedule_timeout (process_timeout)
    0.3% (  1.5)               mtd : schedule_timeout (process_timeout)
    0.3% (  1.5)               mtd : futex_wait (hrtimer_wakeup)
    0.3% (  1.5)           apcupsd : do_nanosleep (hrtimer_wakeup)
    0.2% (  1.0)              ntpd : do_setitimer (it_real_fn)
    0.2% (  1.0)           apache2 : schedule_timeout (process_timeout)
    0.2% (  1.0)        gam_server : schedule_timeout (process_timeout)
    0.2% (  1.0)         nm-applet : schedule_timeout (process_timeout)
    0.2% (  1.0)     <kernel core> : queue_delayed_work_on 
(delayed_work_timer_fn)
    0.2% (  1.0)            dhcdbd : schedule_timeout (process_timeout)
    0.2% (  1.0)       mythbackend : schedule_timeout (process_timeout)
    0.2% (  1.0)   xfce4-mixer-plu : schedule_timeout (process_timeout)
    0.2% (  1.0)       xfce4-panel : schedule_timeout (process_timeout)
    0.2% (  0.9)          events/0 : sk_reset_timer (tcp_write_timer)
    0.1% (  0.8)     <kernel core> : sk_reset_timer (tcp_delack_timer)
    0.1% (  0.6)     <kernel core> : neigh_table_init_no_netlink 
(neigh_periodic_timer)
    0.1% (  0.3)         kdvb-fe-0 : schedule_timeout (process_timeout)
    0.0% (  0.2)   update-notifier : schedule_timeout (process_timeout)
    0.0% (  0.2)   <kernel module> : neigh_table_init_no_netlink 
(neigh_periodic_timer)
    0.0% (  0.1)   xfce4-menu-plug : schedule_timeout (process_timeout)
    0.0% (  0.1)         ssh-agent : schedule_timeout (process_timeout)
    0.0% (  0.1)         ssh-agent : do_setitimer (it_real_fn)
    0.0% (  0.1)         xfdesktop : schedule_timeout (process_timeout)
    0.0% (  0.1)              sshd : sk_reset_timer (tcp_write_timer)
    0.0% (  0.1)   gnome-volume-ma : schedule_timeout (process_timeout)
    0.0% (  0.1)     <kernel core> : ip_rt_init (rt_check_expire)
    0.0% (  0.1)         transcode : addrconf_verify (addrconf_verify)
    0.0% (  0.1)          gconfd-2 : schedule_timeout (process_timeout)
    0.0% (  0.1)               mtd : inet_twsk_schedule (inet_twdr_hangman)
    0.0% (  0.1)   gnome-power-man : schedule_timeout (process_timeout)
    0.0% (  0.1)     <kernel core> : page_writeback_init (wb_timer_fn)
    0.0% (  0.1)       miniserv.pl : schedule_timeout (process_timeout)
    0.0% (  0.1)   <kernel module> : i2c_start (delayed_work_timer_fn)
    0.0% (  0.1)           apcupsd : schedule_timeout (process_timeout)