Re: [PATCH v4] drm/i915: Execlists small cleanups and micro-optimisations

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 29/02/16 10:53, Chris Wilson wrote:
> On Mon, Feb 29, 2016 at 10:45:34AM +0000, Tvrtko Ursulin wrote:
>> This ok?
>>
>> """
>> One unexplained result is with "gem_latency -n 0" (dispatching
>> empty batches) which shows 5% more throughput, 8% less CPU time,
>> 25% better producer and consumer latencies, but 15% higher
>> dispatch latency which looks like a possible measuring artifact.
>> """
>
> I doubt it is a measuring artefact since throughput = 1/(dispatch +
> latency + test overhead), and the dispatch latency here is larger than
> the wakeup latency and so has greater impact on throughput in this
> scenario.

I don't follow you, if dispatch latency has larger effect on throughput 
how to explain the increase and still better throughput?

I see in gem_latency this block:

	measure_latency(p, &p->latency);
	igt_stats_push(&p->dispatch, *p->last_timestamp - start);

measure_latency waits for the batch to complete and then dispatch 
latency uses p->last_timestamp which is something written by the GPU and 
not a CPU view of the latency ?

Regards,

Tvrtko

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