[gst-devel] possible optimisations in pad_push mutex handling

[Marco Ballesio]

Hi,

continuing a discussion begun from Felipe this week, I completed my homework
to retrieve a testing patch which disables a few mutexes from gst_pad_push
(attached) and profile its effects on both ARM and X86 architectures.

The first positive surprise is that, even after applying the patch, the
following command runs properly on both the architectures:

time gst-launch --gst-disable-registry-update audiotestsrc num-buffers=60000
blocksize=128 ! "audio/x-raw-int, rate=8000, width=16"  ! audioconvert !
audioconvert ! audioconvert ! fakesink

the second effect is that, running the command a few times and getting a
statistic set of results, with both optimisation enabled and disabled, I
found that the execution time for the command has a mean improvement of
about 2.4% on ARM and about 2% on x86 (Intel Atom processor). Here a few
outputs:

On x86, mutexes disabled:
real  0m9.385s user  0m8.773s sys 0m0.076s
real  0m9.206s user  0m8.529s sys 0m0.068s

mutexes enabled:
real  0m9.553s user  0m8.949s sys 0m0.028s
real  0m9.669s user  0m9.017s sys 0m0.092s

On ARM (omap3430), mutexes disabled:
9.57user 0.07system 0:09.68elapsed
9.63user 0.03system 0:09.71elapsed

mutexes enabled:
9.88user 0.07system 0:09.98elapsed
9.80user 0.07system 0:09.90elapsed


Considering a mean VoIP pipeline has a number of elements more than double
than the ones in my example, the penalty on a real-life stream-engine
pipeline should be proportional.

Some months ago I was studying a way to detect thread boundaries from within
a pipeline and then, in case the application "promises" never to interact
with it, to choose an optimised path for gst_path_push (then I dropped the
work because of other tasks). Would it be interesting to resume such an
approach?

P.S. it appears the penalty decreases when buffers have bigger sizes,
as already
shown from Felipe<http://felipec.wordpress.com/2010/10/07/gstreamer-embedded-and-low-latency-are-a-bad-combination/>
.

Regards
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