Split log_sys mutex in MySQL 5.5

One important bottleneck in the MySQL Server is the log_sys mutex in InnoDB. Experiments using mutex statistics showed that this mutex was accessed about 250k times per second and that about 75% of those accesses had to queue up to get the mutex. One particular nuisance is that while holding the log_sys mutex it is necessary to grab the buffer pool mutex to put the changed pages to the start of the flush list indicating it is now the youngest dirty page in the buffer pool (this happens as part of the mini commit functionality in InnoDB). To some extent this contention point is decreased by splitting out the buffer flush list from the buffer pool mutex.

We found a simple improvement of this particular problem. The simple solution is to introduce a new mutex, log_flush_order mutex, this mutex is acquired while still holding the log_sys mutex, as soon as it is acquired we can release the log_sys mutex. This gives us the property that the log_sys mutex is available for other operations such as starting a new log write while we still serialise the input of the dirty pages into the buffer pool flush list.

As can be easily seen this solution decrease the hold time of the log_sys mutex while not decreasing the frequency it is acquired.
In our experiments we saw that this very simple solution improved a Sysbench RW test by a few percent.