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.
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no longer maintained; cache performance can be better, load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.
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.
In certain C libraries (e.g uClibc), isnan() and related functions are
implemented in libm, so json-c needs to link against it. This commit
therefore adds an AC_TRY_LINK() test to check whether a program
calling isnan() can be properly linked with no special flags. If not,
we assume linking against libm is needed.
The json-c.pc.in file is also adjusted so that in the case of static
linking against json-c, -lm is also used.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
