I have done a long series of tests to verify that
the impact of the LOCK_alarm removal, removing two
variables from LOCK_threadcount protection to being
atomic increments instead and decreasing hold time
of LOCK_open is positive in most if not all cases.
There are a number of test cases needed:
1) With and Without cpuspeed activated
2) With sysbench local and with sysbench on another
3) With MySQL Server limited to 2,4,6,8,12,16 cores.
4) With number of threads going from 1 to 256 threads
in fair sized steps.
I've done most of those tests for Sysbench Readonly
and Sysbench Readwrite. The results are positive in
almost all cases towards the baseline which is based
off the MySQL 5.4.3 tree (not exactly 5.4.3 but close
In the case of networked benchmark with cpuspeed
activated the gain is biggest, top performance goes up
about 10% and also top performance moves from 64 to 128
threads, for 256 threads the performance increases
by about 50%.
When cpuspeed is deactivated and we use networked
benchmarks the network handling becomes a bottleneck, so
the numbers here are less interesting since we need to
resolve the network bottleneck first.
With cpuspeed activated and local communication the top
performance increase by about 8% and there is gain
for all number of threads. The gain is a bit higher
on sysbench readonly than on sysbench readwrite.
With cpuspeed deactivated and local communication we
naturally get the best numbers but also the
smallest gains. Top performance of Sysbench
Readonly increased by 2.5% and for Readwrite
it increased 4%, the top performance for
sysbench readwrite also moved from 16 threads
to 32 threads. The improvement is slightly
better on more threads.