One important question with a logging library is: is it fast enough? While Log4cxx may not be the fastest logging implementation, it is more than fast enough for the vast majority of cases.

In performance testing done on a developer's system(utilising an Intel i5-8400 processor and a virtual machine), it is possible for Log4cxx to handle over 2,000,000 messages/second in a single-threaded application. Since Log4cxx is designed to be multithread-safe, logging from multiple threads makes this throughput much lower. Delays in writing messages to disk can also greatly decrease performance, depending on how much data is being logged.

If the logging of a particular level is disabled, performance can also be a lot better. While Log4cxx can generate 2,000,000 log messages/second, skipping a disabled logging request requires only a few nano-seconds, so application performance is not affected when logging requests are not removed from the release build.

For the best performance, use the LOG4CXX_[level]_FMT series of macros, as the {fmt} library they use is significantly faster (up to twice as fast as operator<<). Note that you must include the headers from {fmt} manually.

These two pieces of logging code are logically equivalent(printing out the same values), however the one utilizing fmt is close to 2x as fast on some systems.

for( int x = 0; x < howmany; x++ ){
         LOG4CXX_INFO( logger, "Hello logger: msg number " << x);
}

for( int x = 0; x < howmany; x++ ){
    LOG4CXX_INFO_FMT( logger, "Hello logger: msg number {}", x);
}

If you wish to benchmark Log4cxx on your own system, have a look at the tools under the src/test/cpp/throughput and src/test/cpp/benchmark directories. The throughput tests may be built by specifying BUILD_THROUGHPUT with CMake when building Log4cxx. The benckmark tests require Google's Benchmark library and may be built by specifying BUILD_BENCHMARK_CHECKS with CMake when building Log4cxx.