json: Improve string parsing.

To parse a json string prior to this change, json_lex_input is called
with each character of the string. If the character needs to be copied
to the buffer, it is copied individually. This is an expensive
operation, as often there are multiple characters in a row
that need to be copied, and copying memory in blocks is more efficient
than byte by byte. To improve this, the string is now copied
in blocks with an offset counter. A copy is performed when the parser
state equals done.

Functions that are called for each character use a lot of CPU cycles.
Making these functions inline greatly reduces the cycles used and
improves overall performance. Since json_lex_input was only needed in
one place, it doesn't have to be its own function.

There is also a conditional that checks if the current character is a
new line, which is quite unlikely. When this was examined with perf, the
comparison had a very high CPU cycle usage. To improve this, the
OVS_UNLIKELY macro was used, which forces the compiler to switch the
order of the instructions.

Here is the improvement seen in the json-string-benchmark test:

  SIZE      Q  S         BEFORE       AFTER      CHANGE
--------------------------------------------------------
100000      0  0 :      0.842 ms     0.489 ms   -41.9 %
100000      2  1 :      0.917 ms     0.535 ms   -41.7 %
100000      10 1 :      1.063 ms     0.656 ms   -38.3 %
10000000    0  0 :     85.328 ms    49.878 ms   -41.5 %
10000000    2  1 :     92.555 ms    54.778 ms   -40.8 %
10000000    10 1 :    106.728 ms    66.735 ms   -37.5 %
100000000   0  0 :    955.375 ms   621.950 ms   -34.9 %
100000000   2  1 :   1031.700 ms   665.200 ms   -35.5 %
100000000   10 1 :   1189.300 ms   796.050 ms   -33.0 %

Here Q is probability (%) for a character to be a '\"' and
S is probability (%) to be a special character ( < 32).

Signed-off-by: Rosemarie O'Riorden <roriorden@redhat.com>
Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

0 files changed, 0 insertions, 0 deletions