Cache coherence and Java

You all know my good old friend Dough Lea ? A moron who instead of putting any sensible locking strategy into Java has been putting the most nonsensical thing in it ? No ? Well, let’s dig into his latest bullshit. Somewhere along the lines he decided that cache coherence is really something you want the programmer to care about.

http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html gives a demonstration of the thing most people would expect to work that does not work. Basically, if you have a synchronized write to a variable, the (non synchronized) reads from other threads can retrieve a stale state version from their cache. That means, that if you have to synchronize to write a variable, you likely have to synchronize all reads as well. If you don’t want to do that, or if you cannot do that, then you should  use the volatile keyword.

That means two things:

  1. synchronizing treemaps/hashmaps through a facade basically means also locking on read accesses. Huge performance penalty.
  2. if you have an executor array of threads that will work their ways through an ordered set of thunks to be executed, then each of these thunks has to have all their variables marked as volatile. Otherwise no cache coherence can be guaranteed and a thunk that would work on thread 1, might just not work when executed on thread 2.

It really hurts me to have to think through bullshit like this. You expect your cache to be coherent. That’s it. If it is not then why the hell am I using a virtual machine ? I could as well start programming kernels.

Leave a Reply

Your email address will not be published. Required fields are marked *