[PATCH v2 00/18] libcamera: introduce Software ISP and Software IPA

[Hans de Goede]

Hi All,

So during our last video conference about the Software ISP Laurent
requested some docs on how the performance of the Software ISP has
been measured so far.

I have written the following patch (to be included in v3) for this:
https://github.com/jwrdegoede/libcamera/commit/94377ed500d8bb031d312c0f88cfa5d0905fd222

And since reading documentation in patch format sucks I have also
just put the full restructured-text below.

If you have time please take a look and provide feedback before
v3 eventually gets posted with this included.

Regards,

Hans


.. SPDX-License-Identifier: CC-BY-SA-4.0

.. _software-isp-benchmarking:

Software ISP benchmarking
=========================

The Software ISP is paricular sensitive to performance regressions
therefor it is a good idea to always benchmark the Software ISP
before and after making changes to it and ensure that there are
no performance regressions.

DebayerCpu class builtin benchmark
----------------------------------

The DebayerCpu class has a builtin benchmark. This benchmark
measures the time spend on processing (collecting statistics
and debayering) only, it does not measure the time spend on
capturing or outputting the frames.

The builtin benchmark always runs. So this can be used by simply
running "cam" or "qcam" with a pipeline using the Software ISP.

When it runs it will skip measuring the first 30 frames to
allow the caches and the CPU temperature (turbo-ing) to warm-up
and then it measures 30 fps and shows the total and per frame
processing time using an info level log message:

.. code-block::

   INFO Debayer debayer_cpu.cpp:907 Processed 30 frames in 244317us, 8143 us/frame

To get stable measurements it is advised to disable any other processes which
may cause significant CPU usage (e.g. disable wifi, bluetooth and browsers).
When possible it is also advisable to disable CPU turbo-ing and
frequency-scaling.

For example when benchmarking on a Lenovo ThinkPad X1 Yoga Gen 8, with
the charger plugged in, the CPU can be fixed to run at 2 GHz using:

.. code-block::

   sudo x86_energy_perf_policy --turbo-enable 0
   sudo cpupower frequency-set -d 2GHz -u 2GHz

with these settings the builtin bench reports a processing time of ~7.8ms/frame
on this laptop for FHD SGRBG10 (unpacked) bayer data.

Measuring power consumption
---------------------------

Since the Software ISP is often used on mobile devices it is also
important to measure power consumption and ensure that that does
not regress.

For example to measure power consumption on a Lenovo ThinkPad X1 Yoga Gen 8
it needs to be running on battery and it should be configured with its
platform-profile (/sys/firmware/acpi/platform_profile) set to balanced and
with its default turbo and frequency-scaling behavior to match real world usage.

Then start qcam to capture a FHD picture at 30 fps and position the qcam window
so that it is fully visible. After this run the following command to monitor
the power consumption:

.. code-block::

   watch -n 10 cat /sys/class/power_supply/BAT0/power_now /sys/class/hwmon/hwmon6/fan?_input

Note this not only measures the power consumption in ųW it also monitors
the speed of this laptop's 2 fans. This is important because depending on
the ambient temperature the 2 fans may spin up while testing and this
will cause an additional power consumption of approx. 0.5W messing up
the measurement.

After starting qcam + the watch command let the laptop sit without using
it for 2 minutes for the readings to stabilize. Then check that the fans
have not turned on and manually take a couple of consecutive power readings
and avarage these.

On the example Lenovo ThinkPad X1 Yoga Gen 8 laptop this results in
a measured power consumption of approx. 13W while running qcam versus
approx. 4-5W while setting idle with its OLED panel on.