[Mesa-dev] [PATCH 10/18] i965: Speculatively flush the batch after transform feedback

[Martin Peres]

On 06/07/15 22:36, Kenneth Graunke wrote:
> On Monday, July 06, 2015 11:33:15 AM Chris Wilson wrote:
>> Since the purpose of transform feedback tends to be for the client to
>> act upon the results to change the geometry in the scene, it is likely
>> that the client will soon be waiting upon the results. Flush the batch
>> early so that we don't build up a long queue of commands afterwards that
>> could delay the readback.
>> ---
>>   src/mesa/drivers/dri/i965/gen7_sol_state.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/src/mesa/drivers/dri/i965/gen7_sol_state.c b/src/mesa/drivers/dri/i965/gen7_sol_state.c
>> index 857ebe5..13dbe5b 100644
>> --- a/src/mesa/drivers/dri/i965/gen7_sol_state.c
>> +++ b/src/mesa/drivers/dri/i965/gen7_sol_state.c
>> @@ -494,6 +494,12 @@ gen7_end_transform_feedback(struct gl_context *ctx,
>>   
>>      brw_batch_end(&brw->batch);
>>   
>> +   /* We will likely want to read the results in the very near future, so
>> +    * push this primitive to hardware if it is currently idle.
>> +    */
>> +   if (!brw_batch_busy(&brw->batch))
>> +      brw_batch_flush(&brw->batch);
>> +
>>      /* EndTransformFeedback() means that we need to update the number of
>>       * vertices written.  Since it's only necessary if DrawTransformFeedback()
>>       * is called and it means mapping a buffer object, we delay computing it
>>
> We need some data to justify this change.

I actually get a negative perf improvement out of this one, -0.9% on a 
customer benchmark.