Biased locking design decision

Question

I am trying understand a rationale behind biased locking and making it a default. Since reading this blog post, namely:

"Since most objects are locked by at most one thread during their lifetime, we allow that thread to bias an object toward itself"

I am perplexed... Why would anyone design a synchronized set of methods to be accessed by one thread only? In most cases, people devise certain building blocks specifically for the multi-threaded use-case, and not a single-threaded one. In such cases, EVERY lock aquisition by a thread which is not biased is at the cost of a safepoint, which is a huge overhead! Could someone please help me understand what I am missing in this picture?

Answer 1

The reason is probably that there are a decent number of libraries and classes that are designed to be thread safe but that are still useful outside of such circumstances. This is especially true of a number of classes that predate the Collections framework. Vector and it's subclasses is a good example. If you also consider that most java programs are not multi threaded it is in most cases an overall improvement to use a biased locking scheme, this is especially true of legacy code where the use of such Classes is all to common.

Answer 2

You are correct in a way, but there are cases when this is needed, as Holger very correctly points in his comment. There is so-called, the grace period when no biased-locking is attempted at all, so it's not like this will happen all the time. As I last remember looking at the code, it was 5 seconds. To prove this you would need a library that could inspect Java Object's header (jol comes to my mind), since biased locking is hold inside mark word. So only after 5 seconds will the object that held a lock before will be biased towards the same lock.

EDIT

I wanted to write a test for this, but seems like there is one already! Here is the link for it