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

[Chris Wilson]

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).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre