Re: switching to top frequency too frequent with ondemand governorand no_hz

[Markus Trippelsdorf]

On 2011.06.01 at 20:00 +0200, Markus Trippelsdorf wrote:
> On 2011.06.01 at 13:34 -0400, David C Niemi wrote:
> > On 06/01/2011 12:08 PM, Markus Trippelsdorf wrote:
> > > There seems to be a major difference in the behavior of the ondemand
> > > governor depending on whether CONFIG_NO_HZ is set or not in the kernel
> > > .config.
> > >
> > > In the NO_HZ case the ondemand governor spends too much time at the
> > > highest frequency and is also very trigger happy.
> > >
> > > I have compared the two cases on my system:
> > > powernow-k8: Found 1 AMD Phenom(tm) II X4 955 Processor (4 cpu cores) (version 2.20.00)
> > > powernow-k8:    0 : pstate 0 (3200 MHz)
> > > powernow-k8:    1 : pstate 1 (2500 MHz)
> > > powernow-k8:    2 : pstate 2 (2100 MHz)
> > > powernow-k8:    3 : pstate 3 (800 MHz)
> > >
> > > When I run:
> > > watch -n.1 'cat /proc/cpuinfo|grep MHz'
> > > on an otherwise idle system, I can see that the frequency always stays
> > > at 800 MHz in the "CONFIG_NO_HZ not set" case. But it will very
> > > frequently switch to 3200 MHz in the CONFIG_NO_HZ=y case under the same
> > > conditions.
> > >
> > > This also manifests itself in the cpufreq/stats/time_in_state
> > > statistics (again on a mostly idle system):
> > >
> > > First taken with:
> > > echo 200 > /sys/devices/system/cpu/cpufreq/ondemand/sampling_down_factor
> > > (BTW wouldn't it make sense to use something like this as the default
> > > value?)
> > >
> > > cat /sys/devices/system/cpu/cpu0/cpufreq/stats/time_in_state
> > >
> > > CONFIG_NO_HZ not set: 
> > > 3200000 5845
> > > 2500000 0
> > > 2100000 5
> > > 800000 31552
> > >
> > > CONFIG_NO_HZ=y:
> > > 3200000 17650
> > > 2500000 0
> > > 2100000 0
> > > 800000 31129
> > >
> > >
> > > And with the default sampling_down_factor=1
> > >
> > > CONFIG_NO_HZ not set: 
> > > 3200000 140
> > > 2500000 2
> > > 2100000 29
> > > 800000 16614
> > >
> > > CONFIG_NO_HZ=y:
> > > 3200000 538
> > > 2500000 9
> > > 2100000 77
> > > 800000 16287
> > >
> > > Now my question is, is this expected? And what could be done to make the
> > > NO_HZ behavior more like the "CONFIG_NO_HZ not set" behavior.
> > 
> > A very interesting bit of information.  What do you have set for
> > up_threshold?  You may have to set it higher for CONFIG_NO_HZ than
> > without, based on your symptoms.  Another thing to look at is your
> > sampling_rate.  I'm guessing it differs between CONFIG_NO_HZ being set
> > or not.
> 
> I've played with all those parameters, but unfortunately it didn't make
> any difference.
> 
> > And perhaps you need to set sampling_down_factor a bit lower.  I
> > consider 100 a reasonable default, but a default of "1" was put in
> > initially to make the behavior of the patch that enabled the factor
> > identical with not having the patch.  If you are more concerned with
> > saving power than maximizing throughput, you might consider a much
> > lower value like 5 or 10.
> 
> Yes, I've tried different values and 200 turned out to be the best based
> on my preferences (throughput over power saving). It makes a big
> difference in the compile time of bigger projects, especially during the
> configuration phase.
> 
> But I have found the root cause of symptoms described above by
> bisection. It turned out that 2.6.39 is also affected, so I've bisected
> down to 2.6.38. 
> This is the result:
> 
>  5cb2c3bd0c5e0f3ced63f250ec2ad59d7c5c626a is the first bad commit
>  commit 5cb2c3bd0c5e0f3ced63f250ec2ad59d7c5c626a
>  Author: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>  Date:   Mon Feb 7 17:14:25 2011 +0100
> 
>      [CPUFREQ] calculate delay after dbs_check_cpu
> 
> When I revert the above in 3.0-rc1 the CONFIG_NO_HZ=y symptoms vanish.

Here are some numbers to back this claim:

cat /sys/devices/system/cpu/cpu0/cpufreq/stats/time_in_state
(with sampling_down_factor=200)

CONFIG_NO_HZ not set:
3200000 1766
2500000 0
2100000 1479
800000 30787

CONFIG_NO_HZ=y:
3200000 922
2500000 0
2100000 2313
800000 31217

So the behavior in both cases is (roughly) the same again.

-- 
Markus
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