Logging in Rails, is there any performance hit?

Question

Rails comes bundled with Ruby's logger class in the standard library. The available log levels are: :debug, :info, :warn, :error, and :fatal.

I was wondering if I add extensive logging in my Rails application with log level set to :debug for development and testing, will there be a performance impact when running in production with logging turned-off or set at higher level, such as config.log_level = :fatal?

Answer 1

The short answer is that logging will always have a performance impact, particularly when logging to disk. However, there are a few subtleties.

Firstly, using the :debug level will have a greater performance penalty than :fatal, as a far greater number of strings are being evaluated and written to the log output (e.g. disk).

Another potential pitfall is that if you have many calls like this in your code:

logger.debug = "my string to debug with a #{variable}"

There will be a performance impact even if the allowed output level doesn't include debug. The reason is that Ruby has to evaluate these strings, which includes instantiating the somewhat heavy String object and interpolating the variables, and that takes time.

Therefore, it's recommended to pass blocks to the logger methods, as these are only evaluated if the output level is the same or included in the allowed level (i.e. lazy loading). The same code rewritten would be:

logger.debug { "my string to debug with a #{variable}" }

The contents of the block, and therefore the string interpolation, is only evaluated if debug is enabled. This performance savings is only really noticeable with large amounts of logging, but it's a good practice to employ.

You can read more about this in the Logger docs.