The concept lies on logging slow performing application or scripts, to know which task are the most difficult to be completed on time. From there, you will know which task taking too much time and most probably resources, and need to be optimize to make sure it doesn’t affect the other tasks to be completed in timely manner

Mysql implemented slow log, which will log queries taking too much time to be completed. Mysql do have features where you can set global configuration on the fly, and will be in effect until the next mysql restart. If you want to make it permanent, it need to be placed in my.cnf configuration file.

To make enable slow log:

SET GLOBAL slow_query_log := 1;
SET GLOBAL long_query_time=10;
SET GLOBAL LOG_QUERIES_NOT_USING_INDEXES = ON;
SET GLOBAL slow_query_log_file = ‘path’;

slow_query_log (1 = enable, 0 = disable)
long_query_time (the treshhold to consider it is slow, in seconds)
LOG_QUERIES_NOT_USING_INDEXES (if turned ON, it will consider queries not using indexes as slow)
slow_query_log_file (the path to the log file)

If you want to make the settings permanent in my.cnf file:
log_slow_queries=”path”
long_query_time=10
log-queries-not-using-indexes =1

For more information, please refer to official documentation on slow query log from mysql.com.

Besides mysql, your web server is a crucial part in processing your web-application as well, and for that, you might want to consider using this apache module, modlogslow, to log scripts which takes a long time to be completed.

It is an apache module, you need to compile it using apxs. Add some configurations to load the module file, and configure how will it behave, and you’re ready to go. The documentation provided in the archive, and also in the Google Code page, enough to get it running.

the code below will be enough to enable the module. Find httpd.conf, and put these few lines to enable it
LoadModule log_slow_module modules/mod_log_slow.so
LogSlowEnabled On
LogSlowLongRequestTime 8000
LogSlowFileName /usr/local/apache/logs/slow_log

Hope this would help you in monitoring your server, get to know who is sick and need attention.