Purpose
Optimize slow logging calls, especially in dependencies that might violate
logging best practices.
Current best practice
When a Logger.debug call, for instance, is passed an input that's expensive
to compute the best practice is to wrap it in a thunk (zero argument
function). That way, if the application's current log level is higher than
:debug the computation can be skipped. This approach works regardless of
whether the logger call was purged at compile-time using
:compile_time_purge_level.
Example
The following may be slow due to inspect.
Logger.debug "my_fun/1: input: #{inspect input}"
An optimized version:
Logger.debug fn ->
"my_fun/1: input: #{inspect input}"
end
Proposal
Add a new Logger application configuration setting, say :lazy, which makes
all the Logger macros wrap their chardata_or_fun input in a thunk. We could
use an is_function guard to avoid wrapping inputs that are already thunks,
but it's not strictly necessary; nested thunks already repeatedly evaluate
until they reach a non-thunk.
To maintain current behavior this new parameter would be disabled by
default. However, it's worth considering whether the potential cost of
hidden, suboptimal logging calls are greater than the cost of calling one
thunk per logging call. Before I learned the best practice the problem
apparent in my own package which calls Logger.debug thousands of times in
an expensive recursive function (see "Logging: a silent performance killer"
<https://elixirforum.com/t/logging-a-silent-performance-killer/3258>). It's
conceivable that many packages aren't following the best practice, and that
the performance degradation is more often death by a thousand cuts than a
centralized and noticeable issue like mine was.
If, for whatever reason, this proposal isn't accepted another avenue for
improvement is detecting suboptimal Logger calls through static analysis.
This could mean having elixirc emit warnings or adding checks to a 3rd
party linter. I already opened a proposal
<https://github.com/rrrene/credo/issues/299>with Credo.
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