[Mesa-dev] Interesting i965 behavior: more fast color clears not helping dota2

[Chad Versace]

On 10/30/2013 06:43 AM, Paul Berry wrote:
> On 30 October 2013 00:00, Eric Anholt <eric at anholt.net> wrote:
>
>> One of the things I was wondering about for dota2 performance was
>> whether missing the fast clears was a big performance hit --
>> particularly with the fips numbers indicating a lot of time spent in
>> clears.  However, applying this patch:
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_blorp_clear.cpp
>> b/src/mesa/drivers/dr
>> index d1933ce..2843ed6 100644
>> --- a/src/mesa/drivers/dri/i965/brw_blorp_clear.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_blorp_clear.cpp
>> @@ -156,7 +156,11 @@ is_color_fast_clear_compatible(struct brw_context
>> *brw,
>>         if (color->f[i] != 0.0 && color->f[i] != 1.0) {
>>            perf_debug("Clear color unsupported by fast color clear.  "
>>                       "Falling back to slow clear.\n");
>> -         return false;
>> +         printf("SMASHING FAST CLEAR %f %f %f %f %dx%d\n",
>> +                color->f[0], color->f[1], color->f[2], color->f[3],
>> +                brw->ctx.DrawBuffer->_Xmax - brw->ctx.DrawBuffer->_Xmin,
>> +                brw->ctx.DrawBuffer->_Ymax - brw->ctx.DrawBuffer->_Ymin);
>> +         return true;
>>         }
>>      }
>>      return true;
>>
>> produced a lot of output:
>>
>> SMASHING FAST CLEAR 0.501961 0.501961 0.501961 1.000000 1440x900
>> SMASHING FAST CLEAR 0.901961 0.901961 0.901961 1.000000 64x64
>> SMASHING FAST CLEAR 0.501961 0.501961 0.501961 1.000000 1440x900
>> SMASHING FAST CLEAR 0.901961 0.901961 0.901961 1.000000 64x64
>>
>> but no performance difference:
>>
>> anholt at yvain:/home/anholt% ministat before after
>> x before
>> + after
>>
>> +--------------------------------------------------------------------------+
>> |   x       x       x   +    +           +   +   *       x
>>   +|
>> ||__________________M___A_|_________________AM__|____________|
>> |
>>
>> +--------------------------------------------------------------------------+
>>      N           Min           Max        Median           Avg        Stddev
>> x   6         13.51         13.64         13.55         13.56   0.056568542
>> +   6         13.56         13.68         13.61     13.606667   0.042739521
>> No difference proven at 95.0% confidence
>>
>> (test was done with the printf run only a single time)
>>
>
> Hmm, interesting.  A few thoughts:
>

> - Chad told me on Monday about some counterintuitive stuff he was seeing
> where fast clear appeared to be slower than slow clear.  I don't recall the
> details.

Disregard the counterintuitive behavior I observed until further notice.
Last night, Eero informed me that the microbenchmark's javascript was
written badly, which produced false framerate results due to synchronization
issues between Chrome's Webkit process and GPU process. I plan to fix the
javascript issues and re-test.