Rescheduling Interrupts, Tweaking the Scheduler, and bug #177895
 

Hi all,

Some apps on duo core machines seem to be generating a lot of kernel
wakeups (for example amarok during mp3 playback) which in turn generates
a lot of "Rescheduling Interrupts" as the scheduler tries to re-balance
core useage.

A lot of these rescheduling interrupts seem legitimate wake-ups, for
example amarok waking up the 2nd core to make X re-render on one core
while another core busy doing mp3 decode and playback. However the
rescheduling interrupts do look very high on an nearly idle system (e.g.
just running amarok!) when examining a running system with tools such as
powertop.

Alessio has been helpful to find an upstream patch that reduces the
heavy handed rescheduling interrupts when the system is idle on a multi
core system. namely upstream commit
33b0c4217dcd67b788318c3192a2912b530e4eef which tweaks the multi-core
scheduler intialiser flags in include/linux/topology.h to be less
aggressive for IDLE wake ups.

With the fix, we do see a reduction in rescheduling interrupts - that
is, sleeping cores are woken up less by kernel IPI events, however the
CPU C0..C3 residency does change:

"Fixed" Kernel Current Kernel
Rescheduling
Interrupts/sec ~210-220 ~240-250

C0 residency 37% 23%
C1 "" 0% 0%
C2 "" 0% 0%
C3 "" 63% 28%

Power (Watts) 30.1 27.4

Figures were obtained from powertop running for 8 minutes with average
of 10 second samples.

So... it appears that reducing the "Rescheduling Interrupts" by tweaking
the current scheduler reduces the overall time the processor core is in
the lowest C3 state, and increases the busy C0 state residency, hence
increasing the overall power usage.

With this in mind, I think I will no longer pursue any more fixes to the
scheduler to reduced the "Scheduling Interrupts" as it is detrimental to
power consumption for laptop users et al.

There are a heap of other more significant changes upstream in the
scheduler which may improve things generally but they are far too
intrusive to make so late in the Hardy release cycle.

I will therefore close bug 177895 as "Won't Fix" - and add some notes to
explain why.

Is this OK?

Colin



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Rescheduling Interrupts, Tweaking the Scheduler, and bug #177895
 

On Fri, 2008-04-04 at 12:32 +0100, Colin Ian King wrote:
> Hi all,
>
> Some apps on duo core machines seem to be generating a lot of kernel
> wakeups (for example amarok during mp3 playback) which in turn generates
> a lot of "Rescheduling Interrupts" as the scheduler tries to re-balance
> core useage.
>
> A lot of these rescheduling interrupts seem legitimate wake-ups, for
> example amarok waking up the 2nd core to make X re-render on one core
> while another core busy doing mp3 decode and playback. However the
> rescheduling interrupts do look very high on an nearly idle system (e.g.
> just running amarok!) when examining a running system with tools such as
> powertop.
>
> Alessio has been helpful to find an upstream patch that reduces the
> heavy handed rescheduling interrupts when the system is idle on a multi
> core system. namely upstream commit
> 33b0c4217dcd67b788318c3192a2912b530e4eef which tweaks the multi-core
> scheduler intialiser flags in include/linux/topology.h to be less
> aggressive for IDLE wake ups.
>
> With the fix, we do see a reduction in rescheduling interrupts - that
> is, sleeping cores are woken up less by kernel IPI events, however the
> CPU C0..C3 residency does change:
>
> "Fixed" Kernel Current Kernel
> Rescheduling
> Interrupts/sec ~210-220 ~240-250
>
> C0 residency 37% 23%
> C1 "" 0% 0%
> C2 "" 0% 0%
> C3 "" 63% 28%
>
> Power (Watts) 30.1 27.4
>
> Figures were obtained from powertop running for 8 minutes with average
> of 10 second samples.
>
> So... it appears that reducing the "Rescheduling Interrupts" by tweaking
> the current scheduler reduces the overall time the processor core is in
> the lowest C3 state, and increases the busy C0 state residency, hence
> increasing the overall power usage.
>
> With this in mind, I think I will no longer pursue any more fixes to the
> scheduler to reduced the "Scheduling Interrupts" as it is detrimental to
> power consumption for laptop users et al.
>
> There are a heap of other more significant changes upstream in the
> scheduler which may improve things generally but they are far too
> intrusive to make so late in the Hardy release cycle.
>
> I will therefore close bug 177895 as "Won't Fix" - and add some notes to
> explain why.
>
> Is this OK?

Excellent investigation into this bug. I'm in agreement with your
evaluation and suggested handling of the bug report. +1 from me.

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Rescheduling Interrupts, Tweaking the Scheduler, and bug #177895
 

On Fri, 2008-04-04 at 09:03 -0400, Ben Collins wrote:
> On Fri, 2008-04-04 at 12:32 +0100, Colin Ian King wrote:
> > Hi all,
> >
> > Some apps on duo core machines seem to be generating a lot of kernel
> > wakeups (for example amarok during mp3 playback) which in turn generates
> > a lot of "Rescheduling Interrupts" as the scheduler tries to re-balance
> > core useage.
> >
> > A lot of these rescheduling interrupts seem legitimate wake-ups, for
> > example amarok waking up the 2nd core to make X re-render on one core
> > while another core busy doing mp3 decode and playback. However the
> > rescheduling interrupts do look very high on an nearly idle system (e.g.
> > just running amarok!) when examining a running system with tools such as
> > powertop.
> >
> > Alessio has been helpful to find an upstream patch that reduces the
> > heavy handed rescheduling interrupts when the system is idle on a multi
> > core system. namely upstream commit
> > 33b0c4217dcd67b788318c3192a2912b530e4eef which tweaks the multi-core
> > scheduler intialiser flags in include/linux/topology.h to be less
> > aggressive for IDLE wake ups.
> >
> > With the fix, we do see a reduction in rescheduling interrupts - that
> > is, sleeping cores are woken up less by kernel IPI events, however the
> > CPU C0..C3 residency does change:
> >
> > "Fixed" Kernel Current Kernel
> > Rescheduling
> > Interrupts/sec ~210-220 ~240-250
> >
> > C0 residency 37% 23%
> > C1 "" 0% 0%
> > C2 "" 0% 0%
> > C3 "" 63% 28%
> >
> > Power (Watts) 30.1 27.4
> >
> > Figures were obtained from powertop running for 8 minutes with average
> > of 10 second samples.
> >
> > So... it appears that reducing the "Rescheduling Interrupts" by tweaking
> > the current scheduler reduces the overall time the processor core is in
> > the lowest C3 state, and increases the busy C0 state residency, hence
> > increasing the overall power usage.
> >
> > With this in mind, I think I will no longer pursue any more fixes to the
> > scheduler to reduced the "Scheduling Interrupts" as it is detrimental to
> > power consumption for laptop users et al.
> >
> > There are a heap of other more significant changes upstream in the
> > scheduler which may improve things generally but they are far too
> > intrusive to make so late in the Hardy release cycle.
> >
> > I will therefore close bug 177895 as "Won't Fix" - and add some notes to
> > explain why.
> >
> > Is this OK?
>
> Excellent investigation into this bug. I'm in agreement with your
> evaluation and suggested handling of the bug report. +1 from me.
>
just wanted to say as a user troubled by this question how great it is
to have such a helpful explanation of the issue -- maybe put the whole
email in the bug report? it's really thoughtful.

thanks,
matt


> --
> Ubuntu : http://www.ubuntu.com/
> Linux1394: http://wiki.linux1394.org/
> SwissDisk: http://www.swissdisk.com/
>
>
--
Matt Price
matt.price@utoronto.ca

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