Hello
I've just pushed a series of commits that contain my latest updates on the
move to CBOR. The commits (see list at the bottom of this email) implement the
following:
* modify several functions and structures dealing with payloads to keep the
payload size in an extra variable, instead of using strlen
* import the "libcbor" library [1]
* add two cbor source files to the build
* convert several payloads in the base resources from JSON to CBOR [2]
Missing:
* there are still a couple more places where JSON is used and I haven't fixed
* a Cereal sink for the C++ API
* modifying the CoAP Content Format PDU header from 50 (application/json) to
60 (application/cbor) -- I'd have done this, but I couldn't find where it's
done, as COAP_MEDIATYPE_APPLICATION_JSON isn't used anywhere in our code
* any code above the C++ layer that assumes that the payload is JSON, instead
of using OCRepresentation like it should
Other improvements:
* the CBOR encoder does not allocate memory, unlike the cJSON one. This
should shrink dynamic memory usage on all devices and open up the
for the possibility of zero copy
* this gets rid of manual JSON encoders and decoders. I found one in the
source that is not even compliant
I was asked for some benchmarks. First off, code sizes:
$ find cjson libcbor -name \*.o | xargs size
text data bss dec hex filename
7787 16 8 7811 1e83 cjson/cJSON.o
1077 0 0 1077 435 libcbor/src/cborencoder.o
2870 0 0 2870 b36 libcbor/src/cborparser.o
Second, I prepared some performance benchmarks:
JSON CBOR Improvement
CPU ticks 14025 5100 2,75
CPU cycles 17127 6167 2,78
Branch misses 34,04 0,02 2057
Instructions 34555 5980 5,78
L1 data cache loads 10436 2017 5,17
L1 data cache misses 13,90 0,11 127
[see attached image for a graph]
Notes:
[1] the library will exist in an upstream on GitHub. I am in the process of
obtaining the necessary repository from the Intel GitHub account, so this
doesn't live on my personal GitHub account
[2] the conversion is unconditional. I am not keeping the old JSON code.
Relevant commits:
https://gerrit.iotivity.org/gerrit/969
https://gerrit.iotivity.org/gerrit/968
https://gerrit.iotivity.org/gerrit/975
https://gerrit.iotivity.org/gerrit/991
https://gerrit.iotivity.org/gerrit/905
https://gerrit.iotivity.org/gerrit/992
https://gerrit.iotivity.org/gerrit/967
https://gerrit.iotivity.org/gerrit/1102
--
Thiago Macieira - thiago.macieira (AT) intel.com
Software Architect - Intel Open Source Technology Center
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