Java Concurrency (&c)

In brief: We've open sourced a tool that allows you to provide a callback every time your program performs an allocation. The Java Allocation Instrumenter can be found here. Give it a whirl, if you are interested. One thing that crops up a lot at my employer is the need to take an action on every allocation. This can happen in a lot of different contexts: The programmer has a task, and wants to know how much memory the task allocates, so wants to increment a counter on every allocation. The programmer wants to keep a histogram of most frequently accessed call sites. The programmer wants to prevent a task from allocating too much memory, so it keeps a counter on every allocation and throws an exception when the counter reaches a certain value. Because of the demand for this, a few of us put together a tool that instruments your code and invokes a callback on every allocation. The Allocation Instrumenter is a Java agent written using the java.lang.instrument API and ASM . Each al...

Cliff Click has a terrific post on Java-vs-native-code performance . Recommended. Cliff is the Chief JVM Architect at Azul Systems, was the lead designer behind Hotspot's server JIT compiler, and is an all-around smart guy. His main drawback is that he (apparently) gets dragged into flamewars on YouTube's comment section.

I got asked about this twice in one day, and I didn't know the answer, so I sat down and puzzled it out a bit. A SoftReference is a reference that the garbage collector can decide to clear if it is the only reference left to an object, and the GC decides (through some undefined process) that there is enough memory pressure to warrant clearing it. SoftReferences are generally used to implement memory-sensitive caches. A mistake many people make is to confuse it with a WeakReference , which is a reference that, if it is the only reference left to the object, the garbage collector will aggressively clear. I got asked how Hotspot's GC actually decides whether to clear SoftReferences. Twice. On the same day. (Hotspot is the Sun / OpenJDK JVM). The answer was that I had no idea, so I had to go look it up. Here is what I found out; I hope it is a useful reference for someone (pun intended). First, there is a global clock variable that is set with the current time (in millis)...

I get asked a lot about how the volatile keyword interacts with arrays, so it is probably worth a blog post on the subject. Those of you who have read my posts on volatile ( Volatile Fields and Synchronization , Volatile Does Not Mean Atomic and, most importantly, What Volatile Means in Java ) will have a pretty good idea of what volatile means, but it is probably worth it to provide a reminder. Basically, if you write to a volatile field, and then you have a later read that sees that write, then the actions that happened before that write are guaranteed to be ordered before and visible to the actions that happen after the read. In practice, what this means is that the compiler and the processor can't do any sneaky reordering to move actions that come before the write to after it, or actions that come after the write to before it. See my post on What Volatile Means in Java for more detail. With that out of the way, let's go through some examples of what you can do with vol...

Mailinator's Paul Tyma linked to me . If you are following from that link, the relevant blog entry you are looking for is probably this one, specifically, the entry labeled "visibility" .

Today, I'll discuss a little tweak I made to java.util.Logging that made my logging throughput double. I want to use it as an illustration that it often isn't very difficult to improve the performance of concurrent code by doing things that are actually pretty easy to do. So, "I" have an application that is running a couple of hundred threads on an 8-core machine, and it wants to log about 2MB a second using java.util.Logger . When I say "I have", I actually mean "someone else has", because if "I" had to log a megabyte a second, there is no way I would use java.util.Logger to do it. Still, we all make our choices. When I came to this code, it was already doing sensible things like buffering its output. It wasn't doing something more complicated, like using a dedicated logging thread. It could log about 1MB a second, and was chewing through CPU pretty rapidly. I just decided to run our profiler on the code and see where the bot...

For those who haven't been paying attention, there is a new project from Sun to attract proposals for small language changes to JDK7 . Even in the first day of open calls for proposals , the entries have been very interesting: Strings in Switch, a proposal from Joe Darcy to be able to use Strings in switch statements. Automated Resource Blocks, a proposal from Josh Bloch to be able to say things like: try (BufferedReader br = new BufferedReader(new FileReader(path)) { return br.readLine(); } instead of: BufferedReader br = new BufferedReader(new FileReader(path)); try { return br.readLine(); } finally { br.close(); } based on having BufferedReader implement a Disposable interface. Block expressions, a proposal from Neal Gafter to be allow things like: double pi2 = (double pi = Math.PI ; pi*pi)**; // pi squared* Basically, the principle here is that the (parenthesis-delimited) block would return a value. Exception handling improvements, another proposal from Neal Gaft...