CCSTM

The reference STM implementation for scala.concurrent.stm. CCSTM is a library-only STM based on the SwissTM algorithm, extended to reduce the overhead of non-transactional accesses, allow partial rollback, and include modular blocking and composition operators retry and orAtomic.

During construction the system property "ccstm.stats" is checked. If it is "true" or "1" (actually if it starts with any of the characters 't', 'T', 'y', 'Y', or '1') then statistics are recorded while the program runs and printed to Console during JVM shutdown.

Statistics are tracked separately for top-level transactions and true nested transactions. Many nested atomic blocks can be merged into the top-level transaction by CCSTM for efficiency; these are not reported as nested.

Reported statistics are either counts or exponential histograms. For histograms sum is the sum of the samples, count is the number of transactions for which the statistic was non-zero, avg is sum/count and the histogram reports in brackets the number of samples that had a value of 1, 2..3, 4..7, 8..15, and so on.

Counters:

• commits -- committed transactions
• alternatives -- alternatives provided to atomic, one sample per call to atomic
• retrySet -- memory locations watched while performing modular blocking, one sample per top-level blocking event
• retryWaitElapsed -- milliseconds elapsed during modular blocking, one sample per top-level blocking event
• explicitRetries -- explicit retries using retry, retryFor, Ref.View.await or Ref.View.tryAwait
• unrecordedTxns -- rollbacks that were to erase a successful use of atomic.unrecorded
• optimisticRetries -- rollbacks that were automatically retried, one line per OptimisticFailureCause.category
• failures -- rollbacks that were not retried, one line for each type of exception in UncaughtExceptionCause
• blockingAcquires -- internal locks that could not be acquired immediately
• commitReadSet -- optimistic Ref reads, one sample per committed top-level transaction
• commitBargeSet -- locations read pessimistically, one sample per committed top-level transaction
• commitWriteSet -- locations written, one sample per committed top-level transaction
• rollbackReadSet -- optimistic Ref reads, one sample per transaction that was rolled back
• rollbackBargeSet -- locations read pessimistically, one sample per transaction that was rolled back
• rollbackWriteSet -- locations written pessimistically, one sample per transaction that was rolled back

Read and write set counts for a nested transaction are merged into its parent if it commits, they are not counted separately during the nested commit.