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

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

On 29/02/16 11:13, Chris Wilson wrote:
> On Mon, Feb 29, 2016 at 11:01:49AM +0000, Tvrtko Ursulin wrote:
>>
>> 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 ?
>
> Exactly, measurements are entirely made from the running engine clock
> (which is ~80ns clock, and should be verified during init). The register
> is read before dispatch, inside the batch and then at wakeup, but the
> information is presented as dispatch = batch - start and
> wakeup = end - batch, so to get the duration (end - start) we need
> to add the two together. Throughput will also include some overhead from
> the test iteration (that will mainly be scheduler interference).
>
> My comment about dispatch having greater effect, is in terms of
> its higher absolute value (so the relative % means a larger change wrt
> throughput).

Change to this then?

"""
     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 an amplified effect of test
     overhead.
"""

Regards,

Tvrtko
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