Re: [PATCH (update 3)] timer: Run calc_load halfway through eachround_jiffies second
From: Simon Arlott
Date: Fri Mar 02 2007 - 12:34:00 EST
On 02/03/07 16:35, Eric Dumazet wrote:
On Friday 02 March 2007 16:15, Simon Arlott wrote:
Whenever jiffies is started at a multiple of 5*HZ or wraps, calc_load is
run exactly on the second which is when tasks using round_jiffies will
be scheduled to run. This has a bad effect on the load average, making
it tend towards 1.00 if a task happens to run every time the load is
being calculated.
This changes calc_load so that it updates load half a second after any
tasks scheduled using round_jiffies.
I believe this patch is too complex/hazardous and may break exp decay
computation.
Only for a single calculation whenever it has to adjust, which should only
happen every 49.7 days (on 32-bit archs). (Or 5 minutes after booting...
I always wondered why that happened and now I see it's initialised so it
always wraps early). Whilst it is in sync with jiffies it will not affect
the process - count is just set to the current value every time. Even with
NO_HZ because jiffies will be correct when calc_load is called.
(Even if nobody care about avenrun[] those days :), do you ? )
You could just change LOAD_FREQ from (5*HZ) to (5*HZ+1)
You can see that 5.01 instead of 5.00 second gives the same EXP_xx values.
So (5*HZ + 1) is safe. (because HZ >= 100)
On HZ=1000, this would cause the load average to be pushed towards +1.00
for up to 2 minutes every ~83 minutes with no obvious cause. (If a task
takes ~10-20ms to run, so 20 runs are needed at HZ=1000 before it passes
it again).
On HZ=100 it would happen every ~8 minutes for up to 10 seconds and never
be noticed.
Using 5*HZ+2 would move this to ~167 and ~17 minutes which would mitigate
the effect further still without changing the exp values.
1884.25 -> 1883.62
2014.15 -> 2014.02
2036.65 -> 2036.61
Will anyone notice if the load is adjusted slightly less frequently?
If this is considered preferable to adjusting calc_load to avoid almost all
round_jiffies scheduled tasks (some of which may take longer than ~15ms to
run), then I have no problems with it - I just needed something to stop my
driver changes doing odd things to the load average for other people. I'll
continue to run with this version, is it possible to add a Kconfig option
for it somewhere?
--
Simon Arlott
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