Follow-up Analysis of Split Rollback Segment Mutex

I performed a new set of tests of the patch to split the
rollback segment mutex on Linux. All these tests gave
positive results with improvements in the order of 2%.

One could also derive from the results some conclusions.
The first conclusion is that this split mainly improves
things when the number of threads is high and thus
contention of mutexes is higher. At 256 threads a number
of results improved up to 15%.

The numbers on lower number of threads were more timid
although in many cases an improvement was still seen.

What was also noticeable was that the sysbench read-write
with less reads which makes the transactions much shorter
the positive impact was much greater and the positive
impact on long transactions was much smaller (+0.4%
versus +2.5%). The impact on the short transaction test
with less reads was very positive also on lower number
of threads, the result on 32 threads improved 7%.

So the conclusion is that this patch is a useful contribution
to improvements and in particular improves matters on high
number of threads and with short transactions. According to
a comment on the previous blog it is also very positive in
insert benchmarks.