Messages in this thread Patch in this message | | | Date | Thu, 01 Mar 2007 09:11:59 +0000 | From | Simon Arlott <> | Subject | Re: round_jiffies and load average |
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On 24/02/07 15:19, Simon Arlott wrote: > I've modified the driver of an USB device (cxacru) to schedule the next poll > for status every 1s using round_jiffies_relative instead of just waiting 1s > since the last poll was completed. This process takes on average 11ms to > complete and while it is waiting for a response it's considered running. > The load average is calculated every 5 seconds equivalent to round_jiffies, > so I have a problem with the load average always tending towards 1.00. > Perhaps the load average count could be run just before or in the middle of > a second instead?
I've added some printks to show exactly what happens (see below).
Since calc_load() appears to be run as often as possible, would it be ok to move it to run half a second out of step from rounded jiffies? Its count value is initialised to LOAD_FREQ so it would be easy to add HZ/2.
Otherwise every process using round_jiffies to 1s (1.00), 2s (0.50), 3s (0.33), 4s (0.25), 5s (0.00 or 1.00) could affect the load average in a constant way. (Beyond 10s the effect is unlikely to be noticed).
After a while of this (~3min): [ 192.147049] cxacru_poll_status(..), jiffies=4294832000 [ 192.151319] cxacru_poll_status(..), jiffies=4294832004, next=996 [ 192.463459] calc_load(27), count=-20, jiffies=4294832316 [ 192.463472] calc_load(27), count=4980, jiffies=4294832316
Jiffies wraps: [ 327.121469] cxacru_poll_status(..), jiffies=4294967000 [ 327.139428] cxacru_poll_status(..), jiffies=4294967018, next=1000 [ 327.437739] calc_load(27), count=-20, jiffies=20 [ 327.437750] calc_load(27), count=4980, jiffies=20
The next run occurs during cxacru_poll_status: [ 332.416288] cxacru_poll_status(..), jiffies=5000 [ 332.417312] calc_load(1), count=-1, jiffies=5001 [ 332.417322] calc_load(1), count=4999, jiffies=5001 [ 332.423382] cxacru_poll_status(..), jiffies=5007, next=993
This happens without fail until I reboot: [ 672.350970] cxacru_poll_status(..), jiffies=345000 [ 672.351913] calc_load(1), count=-1, jiffies=345001 [ 672.351926] calc_load(1), count=4999, jiffies=345001 [ 672.367737] cxacru_poll_status(..), jiffies=345016, next=984
It's interesting that calc_load() is run most of the time 19ms later (count=-20) than usual until jiffies wraps, then it runs on time or 1ms late (count=-2). Full log available on request (75K), HZ=1000 with NO_HZ.
(diff below of cxacru.c based on http://lkml.org/lkml/2007/2/23/328) --- diff --git a/drivers/usb/atm/cxacru.c b/drivers/usb/atm/cxacru.c index c8b69bf..4717fa4 100644 --- a/drivers/usb/atm/cxacru.c +++ b/drivers/usb/atm/cxacru.c @@ -535,6 +535,7 @@ static void cxacru_poll_status(struct work_struct *work) struct atm_dev *atm_dev = usbatm->atm_dev; int ret; + printk(KERN_DEBUG "cxacru_poll_status(..), jiffies=%lu\n", jiffies); ret = cxacru_cm_get_array(instance, CM_REQUEST_CARD_INFO_GET, buf, CXINF_MAX); if (ret < 0) { atm_warn(usbatm, "poll status: error %d\n", ret); @@ -599,6 +600,8 @@ static void cxacru_poll_status(struct work_struct *work) reschedule: schedule_delayed_work(&instance->poll_work, round_jiffies_relative(msecs_to_jiffies(POLL_INTERVAL*1000))); + printk(KERN_DEBUG "cxacru_poll_status(..), jiffies=%lu, next=%lu\n", jiffies, + round_jiffies_relative(msecs_to_jiffies(POLL_INTERVAL*1000))); } static int cxacru_fw(struct usb_device *usb_dev, enum cxacru_fw_request fw, diff --git a/kernel/timer.c b/kernel/timer.c index cb1b86a..9c2b816 100644 --- a/kernel/timer.c +++ b/kernel/timer.c @@ -1230,12 +1230,14 @@ static inline void calc_load(unsigned long ticks) count -= ticks; if (unlikely(count < 0)) { active_tasks = count_active_tasks(); + printk(KERN_DEBUG "calc_load(%lu), count=%d, jiffies=%lu\n", ticks, count, jiffies); do { CALC_LOAD(avenrun[0], EXP_1, active_tasks); CALC_LOAD(avenrun[1], EXP_5, active_tasks); CALC_LOAD(avenrun[2], EXP_15, active_tasks); count += LOAD_FREQ; } while (count < 0); + printk(KERN_DEBUG "calc_load(%lu), count=%d, jiffies=%lu\n", ticks, count, jiffies); } }
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