After the performance test is done the analysis part starts. To get a really deep understanding of the test results you can’t continue without analyzing the log files.
Hopefully you have all the data you need in the log files. Ususally this means you should set the log level on DEBUG or similar. The problem with the log files is that they can get easily very big. A few hundred MByte are not uncommon. Manually analyzing very large log files is not feasible.
This doesn’t mean you should not inspect your log files manually. Far from it: Open the log files in your favorite editor and have a glance what you see in there. Any errors, exceptions or other useful information? This might give a hint what was going on in your tested application.
As soon as you know what you want to analyze further: grep is the next tool. Counting occurrences of errors is easy.
grep -c "Exception" application.log