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

[Eric Anholt]

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
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