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Mv spin locks
- merged to master September 3, 2013
- development started August 21, 2013
For performance discussions, it is best to assume Erlang's schedulers run continuous busy wait loops. The continuing thought is that performance is lost anytime leveldb gives up a its time slice to a spinning Erlang scheduler. This branch begins the conversion of tight threading synchronization points from pthread_mutex_t to pthread_spinlock_t. pthread_spinlock_t keeps the time slice with the current thread instead of going to sleep and causing a longer spin within Erlang.
Not all platforms support pthread_spinlock_t. Google established good rules for platform specific implementation. The new Spin and SpinLock classes build upon the Google code. Where pthread_spinlock_t is not available, the new Spin and SpinLock classes default to using the established Mutex class under the covers. Example: Mac OSX does not support pthread_spinlock_t and therefore continues to use pthread_mutex_t.
This branch only changes Mutex to Spin within util/cache.cc. This alone has demonstrated performance improvements for leveldb iterators used by Riak AAE and 2i features.
port/port_posix.h is where the implementation support / logical switch occurs. The new class Spin only gets defined if the proper C feature defines exist. If they do not exist, class Spin becomes a synonym for Google's Mutex class.
util/mutexlock.h and util/mutexlock.cc contain the declaration and implementation respectively for the new SpinLock class. This new class ultimately uses either the Spin or Mutex class based upon the compile time decisions of port/port_posix.h. SpinLock is a straight copy of the MutexLock class with name changes. There is no logic difference.
util/cache.cc is the first user of the new Spin and SpinLock classes. Every Mutex and MutexLock in this file was globally changed to Spin and SpinLock. The id_mutex_ (now id_spin_) usage should really be replaced with an atomic increment. But that is a different, future branch.