Why idle_freq is set to RPn and not RPe

Why idle_freq is set to RPn and not RPe

[Kamble, Sagar A]

Hi Chris,

With below commit, idle frequency is made RPn (HW Min).
Why are we not keeping it at RPe (Efficient Frequency)?
My understanding was to set Rpe on idle so that when GPU is out of RC6 
it can start operating at efficient frequency.

commit aed242ff7ebb697e4dff912bd4dc7ec7192f7581
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Mar 18 09:48:21 2015 +0000

     drm/i915: Relax RPS contraints to allows setting minfreq on idle


Thanks
Sagar

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Re: Why idle_freq is set to RPn and not RPe

[Chris Wilson]

On Wed, Dec 30, 2015 at 02:51:27PM +0530, Kamble, Sagar A wrote:
> Hi Chris,
> 
> With below commit, idle frequency is made RPn (HW Min).
> Why are we not keeping it at RPe (Efficient Frequency)?
> My understanding was to set Rpe on idle so that when GPU is out of
> RC6 it can start operating at efficient frequency.

The driver is *idle*. When there is work to be submitted to the GPU,
then we go back to RPe (though we wait for it to wake up first). RPe is
just an inflexion point on the power curve.
-Chris

-- 
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Re: Why idle_freq is set to RPn and not RPe

[Szwichtenberg, Radoslaw]

Hello Chris!

The question is: why this change in behavior was made? 

On previous platforms Gfx Turbo frequency selection range in driver was in between Rpe & Rp0. Since Rpe is the possible Fmax at Vmin, it was used as the starting frequency once driver booted and any value lower than that was not requested.

Thanks!
Radek

> -----Original Message-----
> From: Chris Wilson [mailto:chris@chris-wilson.co.uk]
> Sent: Wednesday, December 30, 2015 10:31 AM
> To: Kamble, Sagar A
> Cc: S, Deepak; Szwichtenberg, Radoslaw; Intel Graphics Development; Goel,
> Akash
> Subject: Re: Why idle_freq is set to RPn and not RPe
> 
> On Wed, Dec 30, 2015 at 02:51:27PM +0530, Kamble, Sagar A wrote:
> > Hi Chris,
> >
> > With below commit, idle frequency is made RPn (HW Min).
> > Why are we not keeping it at RPe (Efficient Frequency)?
> > My understanding was to set Rpe on idle so that when GPU is out of
> > RC6 it can start operating at efficient frequency.
> 
> The driver is *idle*. When there is work to be submitted to the GPU, then
> we go back to RPe (though we wait for it to wake up first). RPe is just an
> inflexion point on the power curve.
> -Chris
> 
> --
> Chris Wilson, Intel Open Source Technology Centre
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Re: Why idle_freq is set to RPn and not RPe

[Chris Wilson]

On Wed, Dec 30, 2015 at 09:50:15AM +0000, Szwichtenberg, Radoslaw wrote:
> Hello Chris!
> 
> The question is: why this change in behavior was made? 

To take a defensive position to ensure minimum power consumption.
The GPU is idle, the driver has no work for it to do, it should be power
gated, but on the off-chance that fails pick the lowest power mode we
have available for it to idle in.
-Chris

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Re: Why idle_freq is set to RPn and not RPe

[Kamble, Sagar A]

[-- Attachment #1.1: Type: text/plain, Size: 1704 bytes --]

Turbo frequency range is Rpe to Rp0 when GPU is active as, on workload 
submission frequency is taken to Rpe.

Does the HW require us to drop to RPn before entering RC6?
If we can enter RC6 even with other frequencies I think we can keep 
running at Rpe on Idle.

Only benefit of running at Rpn might be some additional power saving 
when workload is very lightweight. Is that correct Chris?


Thanks
Sagar



On 12/30/2015 3:20 PM, Szwichtenberg, Radoslaw wrote:
> Hello Chris!
>
> The question is: why this change in behavior was made?
>
> On previous platforms Gfx Turbo frequency selection range in driver was in between Rpe & Rp0. Since Rpe is the possible Fmax at Vmin, it was used as the starting frequency once driver booted and any value lower than that was not requested.
>
> Thanks!
> Radek
>
>> -----Original Message-----
>> From: Chris Wilson [mailto:chris@chris-wilson.co.uk]
>> Sent: Wednesday, December 30, 2015 10:31 AM
>> To: Kamble, Sagar A
>> Cc: S, Deepak; Szwichtenberg, Radoslaw; Intel Graphics Development; Goel,
>> Akash
>> Subject: Re: Why idle_freq is set to RPn and not RPe
>>
>> On Wed, Dec 30, 2015 at 02:51:27PM +0530, Kamble, Sagar A wrote:
>>> Hi Chris,
>>>
>>> With below commit, idle frequency is made RPn (HW Min).
>>> Why are we not keeping it at RPe (Efficient Frequency)?
>>> My understanding was to set Rpe on idle so that when GPU is out of
>>> RC6 it can start operating at efficient frequency.
>> The driver is *idle*. When there is work to be submitted to the GPU, then
>> we go back to RPe (though we wait for it to wake up first). RPe is just an
>> inflexion point on the power curve.
>> -Chris
>>
>> --
>> Chris Wilson, Intel Open Source Technology Centre


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Re: Why idle_freq is set to RPn and not RPe

[Chris Wilson]

On Wed, Dec 30, 2015 at 04:09:46PM +0530, Kamble, Sagar A wrote:
>    Turbo frequency range is Rpe to Rp0 when GPU is active as, on workload
>    submission frequency is taken to Rpe.
> 
>    Does the HW require us to drop to RPn before entering RC6?
>    If we can enter RC6 even with other frequencies I think we can keep
>    running at Rpe on Idle.

Remember that we quite frequently prevent the hardware going into RC6,
and that it has been known for the hardware to fail to enter RC6 itself
(through driver error or whatnot). Going to the extreme, why wouldn't
you set Rp0 on idle, since that will give the best restart latency?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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Re: Why idle_freq is set to RPn and not RPe

[Kamble, Sagar A]

On 12/30/2015 4:20 PM, Chris Wilson wrote:
> On Wed, Dec 30, 2015 at 04:09:46PM +0530, Kamble, Sagar A wrote:
>>     Turbo frequency range is Rpe to Rp0 when GPU is active as, on workload
>>     submission frequency is taken to Rpe.
>>
>>     Does the HW require us to drop to RPn before entering RC6?
>>     If we can enter RC6 even with other frequencies I think we can keep
>>     running at Rpe on Idle.
> Remember that we quite frequently prevent the hardware going into RC6,
I assume this is threshold times in TO/EI mode for which GT is idle but 
not power gated.
> and that it has been known for the hardware to fail to enter RC6 itself
> (through driver error or whatnot).
And assume this is because of forcewake/rc6 setup errors in driver paths 
which should not happen in best case :)
Agree that running at Rpn makes sense.
>   Going to the extreme, why wouldn't
> you set Rp0 on idle, since that will give the best restart latency?
True. We can have different logic that starts from Rp0 and comes down if 
perf is met.
> -Chris
>
Thanks for the inputs Chris.
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Re: Why idle_freq is set to RPn and not RPe

[Szwichtenberg, Radoslaw]

Hello Chris,

Happy New Year!
Thanks for answers so far. I have some additional questions.

You wrote that this change was made to take a defensive position to ensure minimum power consumption - did we do any power measurements to confirm the benefit? How does this change affect user experience and overall performance? Did we do any performance impact measurements? Is same change done in Windows implementation? Was this change in the algorithm discussed with HW team?

Thanks!
Radek

> -----Original Message-----
> From: Kamble, Sagar A
> Sent: Thursday, December 31, 2015 6:38 PM
> To: Chris Wilson; Szwichtenberg, Radoslaw; S, Deepak; Intel Graphics
> Development; Goel, Akash
> Subject: Re: Why idle_freq is set to RPn and not RPe
> 
> 
> 
> On 12/30/2015 4:20 PM, Chris Wilson wrote:
> > On Wed, Dec 30, 2015 at 04:09:46PM +0530, Kamble, Sagar A wrote:
> >>     Turbo frequency range is Rpe to Rp0 when GPU is active as, on
> workload
> >>     submission frequency is taken to Rpe.
> >>
> >>     Does the HW require us to drop to RPn before entering RC6?
> >>     If we can enter RC6 even with other frequencies I think we can keep
> >>     running at Rpe on Idle.
> > Remember that we quite frequently prevent the hardware going into RC6,
> I assume this is threshold times in TO/EI mode for which GT is idle but not
> power gated.
> > and that it has been known for the hardware to fail to enter RC6
> > itself (through driver error or whatnot).
> And assume this is because of forcewake/rc6 setup errors in driver paths
> which should not happen in best case :) Agree that running at Rpn makes
> sense.
> >   Going to the extreme, why wouldn't
> > you set Rp0 on idle, since that will give the best restart latency?
> True. We can have different logic that starts from Rp0 and comes down if
> perf is met.
> > -Chris
> >
> Thanks for the inputs Chris.
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Re: Why idle_freq is set to RPn and not RPe

[Chris Wilson]

On Tue, Jan 05, 2016 at 04:51:39PM +0000, Szwichtenberg, Radoslaw wrote:
> Hello Chris,
> 
> Happy New Year!
> Thanks for answers so far. I have some additional questions.
> 
> You wrote that this change was made to take a defensive position to ensure minimum power consumption - did we do any power measurements to confirm the benefit? How does this change affect user experience and overall performance? Did we do any performance impact measurements? Is same change done in Windows implementation? Was this change in the algorithm discussed with HW team?

None of the above. You can verify for yourself through code inspection
that it will not impact ux.
-Chris

-- 
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