LOGCXX-546] Prevent serialization of a multi-threaded application

[swebb2066]

when using disabled logging statements.

throughputtests output after applying these changes:

**************************************************************
Benchmarking Single threaded: 1000000 messages
**************************************************************
Log4cxx NoFormat pattern: %m%n Elapsed: 4.866 secs 205,516/sec
Log4cxx DateOnly pattern: [%d] %m%n Elapsed: 4.875 secs 205,130/sec
Log4cxx DateClassLevel pattern: [%d] [%c] [%p] %m%n Elapsed: 4.898 secs 204,167/sec
Log4cxx Logging with FMT Elapsed: 2.437 secs 410,320/sec
Log4cxx Logging static string Elapsed: 3.409 secs 293,331/sec
Log4cxx Logging static string with FMT Elapsed: 2.759 secs 362,386/sec
Log4cxx Logging disabled debug Elapsed: 0.1323 secs 7,558,764/sec
Log4cxx Logging disabled trace Elapsed: 0.1336 secs 7,486,832/sec
Log4cxx Logging enabled debug Elapsed: 3.463 secs 288,802/sec
Log4cxx Logging enabled trace Elapsed: 3.463 secs 288,729/sec
**************************************************************
Benchmarking multithreaded threaded: 1000000 messages/thread, 4 threads
**************************************************************
Log4cxx Logging with FMT MT Elapsed: 4.469 secs 223,767/sec
Log4cxx Logging with FMT MT Elapsed: 4.512 secs 221,653/sec
Log4cxx Logging with FMT MT Elapsed: 4.578 secs 218,420/sec
Log4cxx Logging with FMT MT Elapsed: 4.583 secs 218,199/sec
**************************************************************
Benchmarking multithreaded disabled: 1000000 messages/thread, 4 threads
**************************************************************
Log4cxx Logging disabled MT Elapsed: 0.1453 secs 6,883,621/sec
Log4cxx Logging disabled MT Elapsed: 0.1457 secs 6,862,385/sec
Log4cxx Logging disabled MT Elapsed: 0.1458 secs 6,860,196/sec
Log4cxx Logging disabled MT Elapsed: 0.1459 secs 6,855,386/sec

[swebb2066]

To determine the cost of Logger holding a weak pointer instead of a raw pointer as in this PR, I experimently changed m_priv->repository to a std::weak_ptr. The result shows calls to isDisabled() from multiple threads are some what serialized. The per thread throughput is less half the single thread throughput.

throughputtests output with Logger holding a weak pointer:

Benchmarking library only(no writing out):
**************************************************************
Benchmarking Single threaded: 1000000 messages
**************************************************************
Log4cxx NoFormat pattern: %m%n Elapsed: 5.001 secs 199,972/sec
Log4cxx DateOnly pattern: [%d] %m%n Elapsed: 5 secs 200,001/sec
Log4cxx DateClassLevel pattern: [%d] [%c] [%p] %m%n Elapsed: 5.01 secs 199,591/sec
Log4cxx Logging with FMT Elapsed: 2.576 secs 388,129/sec
Log4cxx Logging static string Elapsed: 3.279 secs 304,998/sec
Log4cxx Logging static string with FMT Elapsed: 2.807 secs 356,189/sec
Log4cxx Logging disabled debug Elapsed: 0.1941 secs 5,152,795/sec
Log4cxx Logging disabled trace Elapsed: 0.1938 secs 5,159,546/sec
Log4cxx Logging enabled debug Elapsed: 3.322 secs 301,064/sec
Log4cxx Logging enabled trace Elapsed: 3.295 secs 303,469/sec
**************************************************************
Benchmarking multithreaded threaded: 1000000 messages/thread, 4 threads
**************************************************************
Log4cxx Logging with FMT MT Elapsed: 4.526 secs 220,932/sec
Log4cxx Logging with FMT MT Elapsed: 4.553 secs 219,640/sec
Log4cxx Logging with FMT MT Elapsed: 4.708 secs 212,424/sec
Log4cxx Logging with FMT MT Elapsed: 4.727 secs 211,554/sec
**************************************************************
Benchmarking multithreaded disabled: 1000000 messages/thread, 4 threads
**************************************************************
Log4cxx Logging disabled MT Elapsed: 0.4405 secs 2,270,056/sec
Log4cxx Logging disabled MT Elapsed: 0.4408 secs 2,268,693/sec
Log4cxx Logging disabled MT Elapsed: 0.4505 secs 2,219,599/sec
Log4cxx Logging disabled MT Elapsed: 0.4569 secs 2,188,823/sec

And LOGCXX-322 is still a problem.

[swebb2066]

I believe this also fixes https://issues.apache.org/jira/browse/LOGCXX-532