Sensor Value PropertiesChanged Events

[Bills, Jason M]

On 2/1/2021 5:26 PM, Ed Tanous wrote:
> On Mon, Feb 1, 2021 at 4:44 PM Bills, Jason M
> <jason.m.bills at linux.intel.com> wrote:
>>
>> Hi All,
>>
>> There is an issue and idea that James Feist and I chatted about to maybe
>> relieve some of our D-Bus traffic.
>>
>> A major contributor to our D-Bus traffic (as seen in dbus-monitor) is
>> the polling sensors updating the xyz.openbmc_project.Sensor.Value.Value
>> property on each polling loop, which generates a PropertiesChanged
>> signal for every sensor on every polling loop (once per second?).
>>
>> The concern is that more important D-Bus messages could be getting
>> delayed as D-Bus processes these Sensor Value signals.
>>
>> The idea to fix this is to change the sensors with a custom getter on
>> the Value property, so the last read can be pulled from D-Bus using a
>> get-property call, but it would no longer signal a PropertiesChanged event.
> 
> Doesn't this break..... like... everything that relies on sensor
> values changing over time?

I think this was my incorrect assumption that the PropertiesChanged 
signal for sensor value updates was not used and could be removed 
without significant impact.  I will abandon my proposed change, but I'm 
glad there are other thoughts and discussion around this issue.

Thanks!
-Jason

> 
>>
>> I pushed a proposed change here:
>> https://gerrit.openbmc-project.xyz/c/openbmc/dbus-sensors/+/40199.
>>
>> Our original assumption was that nobody was matching on this
>> PropertiesChanged signal for the Value property; however, it was pointed
>> out to me today, that PID control has a match for it and may be using it.
> 
snip...