jm + via:twitter   2

Physical spam using cheap Bluetooth beacons spotted in the wild
Buzz Andersen on Twitter: "Crazy thing: my friends who run a high traffic coffee shop just found this thing under their bar. It turns out to be a super cheap Chinese Bluetooth LE beacon.…"
bluetooth  spam  via:twitter  alibaba  eddystone  ads 
15 days ago by jm
Striped (Guava: Google Core Libraries for Java 13.0.1 API)
Nice piece of Guava concurrency infrastructure in the latest release:
A striped Lock/Semaphore/ReadWriteLock. This offers the underlying lock striping similar to that of ConcurrentHashMap in a reusable form, and extends it for semaphores and read-write locks. Conceptually, lock striping is the technique of dividing a lock into many stripes, increasing the granularity of a single lock and allowing independent operations to lock different stripes and proceed concurrently, instead of creating contention for a single lock.<br>

The guarantee provided by this class is that equal keys lead to the same lock (or semaphore), i.e. if (key1.equals(key2)) then striped.get(key1) == striped.get(key2) (assuming Object.hashCode() is correctly implemented for the keys). Note that if key1 is not equal to key2, it is not guaranteed that striped.get(key1) != striped.get(key2); the elements might nevertheless be mapped to the same lock. The lower the number of stripes, the higher the probability of this happening.<br>

Prior to this class, one might be tempted to use Map<K, Lock>, where K represents the task. This maximizes concurrency by having each unique key mapped to a unique lock, but also maximizes memory footprint. On the other extreme, one could use a single lock for all tasks, which minimizes memory footprint but also minimizes concurrency. Instead of choosing either of these extremes, Striped allows the user to trade between required concurrency and memory footprint. For example, if a set of tasks are CPU-bound, one could easily create a very compact Striped<Lock> of availableProcessors() * 4 stripes, instead of possibly thousands of locks which could be created in a Map<K, Lock> structure.
locking  concurrency  java  guava  semaphores  coding  via:twitter 
september 2012 by jm

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