[Intel-gfx] [PATCH] drm/i915: initialize ring frequency scaling table on SNB

[Jesse Barnes]

On Wed, 22 Jun 2011 11:42:04 -0700
Eric Anholt <eric at anholt.net> wrote:

> On Tue, 21 Jun 2011 15:24:24 -0700, Jesse Barnes <jbarnes at virtuousgeek.org> wrote:
> > The ring frequency scaling table tells the PCU to treat certain GPU
> > frequencies as if they were a given CPU frequency for purposes of
> > scaling the ring frequency.  Normally the PCU will scale the ring
> > frequency based on the CPU P-state, but with the table present, it will
> > also take the GPU frequency into account.  The scaling_factor used in
> > this patch may not be ideal, but is enough to increase performance in
> > nexuiz on a 1366x768 panel by about 20%.
> > 
> > The main downside of keeping the ring frequency high while the CPU is
> > at a low frequency (or asleep altogether) is increased power
> > consumption.  But then if you're keeping your GPU busy, you probably
> > want the extra performance.
> 
> The intent of the patch sounds good, but it doesn't seem to be doing
> anything here (graphs below).  If I run a "while true; do; done" loop to
> keep a CPU busy during measurement, though, OA gets a 15.9% +/- 0.6%
> win, and nexuiz around 20%.  There are no complaints in dmesg.
> 
> x /home/anholt/nexuiz-before
> + /home/anholt/nexuiz-after
> +-------------------------------------------------------------------------------+
> | x       +                +    x                                x             +|
> ||_|_______________________M____MA_____A_________________________|_________|    |
> +-------------------------------------------------------------------------------+
>     N           Min           Max        Median           Avg        Stddev
> x   3     59.217461     59.775295     59.479322     59.490693    0.27909057
> +   3     59.287311     59.893383     59.440451     59.540382    0.31515189
> No difference proven at 95.0% confidence
> 
> x /home/anholt/oa-before
> + /home/anholt/oa-after
> +-------------------------------------------------------------------------------+
> |x  x                           +  +    +               x      *          +    x|
> |    |________________________|_________MA_______A______M__________|________|   |
> +-------------------------------------------------------------------------------+
>     N           Min           Max        Median           Avg        Stddev
> x   5         184.2         187.2         186.3        185.72     1.3809417
> +   5         185.4           187         185.7        186.04    0.71624018
> No difference proven at 95.0% confidence
> 
> x /home/anholt/taiji-before
> + /home/anholt/taiji-after
> +-------------------------------------------------------------------------------+
> |x      x                  +     xx       +    x          + ++     +           x|
> |    |____________________________A___|______________A______M_|_____|           |
> +-------------------------------------------------------------------------------+
>     N           Min           Max        Median           Avg        Stddev
> x   6       132.486       139.931       135.608       135.596     2.6966051
> +   6       134.974       138.832       138.111     137.41183      1.434609
> No difference proven at 95.0% confidence