Does std.log have a performance impact

Question

In Varnish, does the std.log subroutine have a performance impact I should be concerned with? For example, if I call it 3-4 times a request, will that have a cumulative effect when dealing with a large number of requests?

From what I can tell, std.log logs to shared memory by requesting a lock, writing the message, and releasing the lock. This should be pretty fast, but if it happens during every single request wouldn't that affect concurrent requests?

Answer 1

Varnish uses a shared memory log (shm-log) for all logging. This works as a circular buffer and stores a small amount of log data - 80MB by default. It is fast. Other tools are provided for analysing and generating output from the shm-log area. These tools are relatively slow since they must output data either to screen or disk, but they don't interfere with the performance of Varnish itself.

I'd be surprised if adding an additional 3 or 4 log entries per request has any measurable performance impact at all, seeing as each request already generates far more than that (one for every request header, for example). I'd say you are far more likely to encounter performance problems with your backend/s.