Messages in this thread | | | Date | Fri, 02 Mar 2007 17:32:51 +0000 | From | Simon Arlott <> | Subject | Re: [PATCH (update 3)] timer: Run calc_load halfway through each round_jiffies second |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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