Performance improvements #196
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This can be a very considerable performance saver.
This provides more control over some detail aspects, many of which are performance related.
This also adds a new API json_global_set_string_hash() which permits to select the hash function. The default one is the only one that was previously present. So there are no changes to existing apps, and the new hash function needs to be explicitely be opted in. Especially for smaller strings, the perllike functions seems to be around twice as fast as the other one, with similarly good results in value distribution.
These items were used for statistics tracking, but no code at all exists to consume them. By removing them we save a) space because they counters required space, and did so in each and every json object b) performance because calloc() needs to write less data and the counters are no longer maintained; cache performance can be better, load on OS main memory is lighter We could conditionally enable/disable these counters, but I have not done this they were really nowhere used and it looked more like a left-over from the import of hashtable code.
smalls strings inside json_objects had a high overhead because dynamic memory allocation was needed for each of them. This also meant that the pointer needed to be updated. This is now changed so that small strings can directly be stored inside the json_object. Note that on the regular 64 bit machines a pointer takes 8 bytes. So even without increasing memory, we could store string up to 7 bytes directly inside the object. The max size is configurable. I have selected up to 31 bytes (which means a buffer of 32 including the NUL byte). This brings a 24-bytes memory overhead, but I consider that still useful because the memory allocator usually also has quite some overhead (16 bytes) for dyn alloced memory blocks. In any case, the max buffer size can be tweaked via #define.
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This PR contains performance improvements which are especially useful for high performance applications (I use json-c in rsyslog, among others). This PR will probably be amended by more improvements (depending on when it is merged). Please let me know any concerns you may have.