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Grok speed improvements

The benchmark for testing speed improvements was to process 3000 lines for '%SYSLOGBASE .*? %IPORHOST%'. Profiler was Devel::Profile.

Before:

time running program:  11.9738  (85.07%)
number of calls:       409362

%Time    Sec.     #calls   sec/call  F  name
64.52    7.6336     2870   0.002660     main::meta2string
10.00    1.1838     5740   0.000206     main::filter
 8.88    1.0507     3000   0.000350     main::handle
 3.14    0.3710    54828   0.000007     main::handle_capture
 0.77    0.0908     2870   0.000032     main::react
After
time running program:  2.5216  (82.73%)
number of calls:       105152

%Time    Sec.     #calls   sec/call  F  name
40.56    1.0228     3000   0.000341     main::handle
15.22    0.3838    54828   0.000007     main::handle_capture
 4.47    0.1128     2870   0.000039     main::meta2string
 3.31    0.0834     2870   0.000029     main::react
 2.61    0.0658    14747   0.000004     main::debug
 1.81    0.0456      237   0.000192     main::readlog
Primary changes were mostly to pregenerate a few regular expressions. Previously, I was generating the same regex every time filter() or meta2string() was being called. These small changes gave grok a serious boost in speed: what was taking 12 seconds now takes 2.5 seconds.

One example of a simple optimization is this:

before: my $re = join("|", map { qr/(?:$_)/ } keys(%$filters));
after:  our $re ||= join("|", map { qr/(?:$_)/ } keys(%$filters));
This may decrease readability, but it only sets $re once no matter how many times that line of code is executed. This line of code change was in the filter() function. Just doing this one simple change reduced the runtime of each filter() call by 97%; which reduces its runtime to something trivially small by comparison.

At this point, this is where I was very happy to have written tests for grok. To verify that grok still behaved properly after making these speed improvements, I just ran the tests suite. It passed, giving me some confidence in the stability of the changes.