Contention and scalability

[chenhao-ye]

For now, there are some unknown scalability bottlenecks:

1. When trying to commit to the global timeline log, multiple processes might result in contention of the log tail.
2. In a strict mode when every process might apply the newest log entries detected in every read operation: once a process publishes its transaction by committing it to the global timeline log, other processes would see it and try to read this log and apply it. This would cause a huge amount of traffic to read the cacheline that holding the log entries.

1 is yet an open question. One possible solution is to introduce partial ordering, but might be too complicated and out of the scope.
2 is possible to alleviate: once a process commits its log entries to the global timeline, it prefers to use another cacheline to write its next log entries. This will leave the previous cacheline holding the log entries untouched and shared-read by other processes.