linux-kernel - Re: [PATCH v2 1/3] Documentation: admin-guide: pm: Add efficiency vs. latency tradeoff to uncore documentation

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Message-ID: <e1d7028e69cb226acf30ed5c316e5fea20546bc4.camel@linux.intel.com>
Date: Thu, 29 Aug 2024 14:39:10 +0300
From: Tero Kristo <tero.kristo@...ux.intel.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Hans de Goede <hdegoede@...hat.com>,
 srinivas.pandruvada@...ux.intel.com,  platform-driver-x86@...r.kernel.org,
 LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] Documentation: admin-guide: pm: Add efficiency
 vs. latency tradeoff to uncore documentation

On Thu, 2024-08-29 at 12:18 +0300, Ilpo Järvinen wrote:
> On Wed, 28 Aug 2024, Tero Kristo wrote:
> 
> > Added documentation about the functionality of efficiency vs.
> > latency tradeoff
> > control in intel Xeon processors, and how this is configured via
> > sysfs.
> > 
> > Signed-off-by: Tero Kristo <tero.kristo@...ux.intel.com>
> > ---
> > v2:
> >   * Largely re-wrote the documentation
> > 
> >  .../pm/intel_uncore_frequency_scaling.rst     | 59
> > +++++++++++++++++++
> >  1 file changed, 59 insertions(+)
> > 
> > diff --git a/Documentation/admin-
> > guide/pm/intel_uncore_frequency_scaling.rst b/Documentation/admin-
> > guide/pm/intel_uncore_frequency_scaling.rst
> > index 5ab3440e6cee..26ded32b06f5 100644
> > --- a/Documentation/admin-
> > guide/pm/intel_uncore_frequency_scaling.rst
> > +++ b/Documentation/admin-
> > guide/pm/intel_uncore_frequency_scaling.rst
> > @@ -113,3 +113,62 @@ to apply at each uncore* level.
> >  
> >  Support for "current_freq_khz" is available only at each fabric
> > cluster
> >  level (i.e., in uncore* directory).
> > +
> > +Efficiency vs. Latency Tradeoff
> > +-------------------------------
> > +
> > +The Efficiency Latency Control (ELC) feature improves performance
> > +per watt. With this feature hardware power management algorithms
> > +optimize trade-off between latency and power consumption. For some
> > +latency sensitive workloads further tuning can be done by SW to
> > +get desired performance.
> > +
> > +The hardware monitors the average CPU utilization across all cores
> > +in a power domain at regular intervals and decides an uncore
> > frequency.
> > +While this may result in the best performance per watt, workload
> > may be
> > +expecting higher performance at the expense of power. Consider an
> > +application that intermittently wakes up to perform memory reads
> > on an
> > +otherwise idle system. In such cases, if hardware lowers uncore
> > +frequency, then there may be delay in ramp up of frequency to meet
> > +target performance.
> > +
> > +The ELC control defines some parameters which can be changed from
> > SW.
> > +If the average CPU utilization is below a user defined threshold
> > +(elc_low_threshold_percent attribute below), the user defined
> > uncore
> > +frequency floor frequency will be used (elc_floor_freq_khz
> > attribute
> 
> Consider the following simplification:
> 
> "the user defined uncore frequency floor frequency" ->
> "the user-defined uncore floor frequency"
> 
> I think it tells the same even without that first "frequency".
> 
> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> 

Yeah, it looks kind of silly. I think that's just a typo from my side,
thanks for catching.

-Tero