[Ovmsdev] Duktape JSON vs. CBOR serialization performance

[Michael Balzer]

Everyone,

when debugging some lost events due to Duktape queue overflows, I 
stumbled across a major Duktape performance issue in the JSON 
serialization support: encoding the AuxBatMon history data object of 5 
arrays with 1440 numbers each (JSON size ~ 30 K) took ~ 12 seconds.

I searched the Duktape repository for this and found a "fastpath" 
optimization we can use but didn't enable, so I tried that: that took 
down the encoding time to ~ 8 seconds. Unfortunately, that's as good as 
it gets with JSON in Duktape.

So I checked the CBOR support 
(https://duktape.org/guide.html#builtin-cbor), and that works much 
better. Encoding the AuxBatMon history to CBOR takes ~ 500 ms. That's 
still not really good, but acceptable.

To enable using CBOR for data exchange with the web frontend as well, 
I've included the CBOR-JS library by Patrick Gansterer, and added a 
binary output option for Javascript commands. I've changed the AuxBatMon 
and PwrMon plugins to use CBOR instead of JSON, and both work and load 
much faster now.

I strongly suggest updating the plugins if you use them.

If you implement a plugin with some kind of data storage or transfer of 
larger objects to the web frontend, do not use JSON, use CBOR instead. 
I've added a section to our manual on this:

https://docs.openvehicles.com/en/latest/userguide/scripting.html#cbor

Regards,
Michael

-- 
Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
Fon 02333 / 833 5735 * Handy 0176 / 206 989 26


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