Re: [PATCH 3/3] drm/i915: Tweak RPS thresholds to more aggressively downclock

[Jesse Barnes <jbarnes@virtuousgeek.org>]

On Wed, 25 Sep 2013 17:34:57 +0100
Chris Wilson <chris@chris-wilson.co.uk> wrote:

> After applying wait-boost we often find ourselves stuck at higher clocks
> than required. The current threshold value requires the GPU to be
> continuously and completely idle for 313ms before it is dropped by one
> bin. Conversely, we require the GPU to be busy for an average of 90% over
> a 84ms period before we upclock. So the current thresholds almost never
> downclock the GPU, and respond very slowly to sudden demands for more
> power. It is easy to observe that we currently lock into the wrong bin
> and both underperform in benchmarks and consume more power than optimal
> (just by repeating the task and measuring the different results).
> 
> An alternative approach, as discussed in the bspec, is to use a
> continuous threshold for upclocking, and an average value for downclocking.
> This is good for quickly detecting and reacting to state changes within a
> frame, however it fails with the common throttling method of waiting
> upon the outstanding frame - at least it is difficult to choose a
> threshold that works well at 15,000fps and at 60fps. So continue to use
> average busy/idle loads to determine frequency change.
> 
> v2: Use 3 power zones to keep frequencies low in steady-state mostly
> idle (e.g. scrolling, interactive 2D drawing), and frequencies high
> for demanding games. In between those end-states, we use a
> fast-reclocking algorithm to converge more quickly on the desired bin.
> 
> v3: Bug fixes - make sure we reset adj after switching power zones.
> 
> v4: Tune - drop the continuous busy thresholds as it prevents us from
> choosing the right frequency for glxgears style swap benchmarks. Instead
> the goal is to be able to find the right clocks irrespective of the
> wait-boost.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Kenneth Graunke <kenneth@whitecape.org>
> Cc: Stéphane Marchesin <stephane.marchesin@gmail.com>
> Cc: Owen Taylor <otaylor@redhat.com>
> Cc: "Meng, Mengmeng" <mengmeng.meng@intel.com>
> Cc: "Zhuang, Lena" <lena.zhuang@intel.com>
> ---

It's a little scary to mess with these, but we've gotten some good
numbers so far so I guess it's ok.

As a follow up, it might be nice to expose the power, balanced,
performance profiles to userspace via sysfs.  Since we can't solve this
problem for all users and all needs, we can just punt it out to
userspace. :)

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>

Jesse
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