Re: [PATCH (update 3)] timer: Run calc_load halfway through each round_jiffies second

Simon Arlott
2007-03-02T17:33:39Z

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