Testing some singleton implementations

[Andrej Mitrovic]

On 1/31/14, Andrej Mitrovic <andrej.mitrovich at gmail.com> wrote:
> Hmm yeah, but I was expecting better numbers. Even after the 'static'
> fix in the bug as noted by Stanislav the atomic version is slower.

Actually, I think I understand why this happens. Logically, the atomic
version will do an atomic read for *every* access, whereas the TLS
implementation only checks a thread-local boolean flag. Even though
the TLS implementation forces each new thread to enter the
synchronized block *on the first read for that thread*, on subsequent
reads that thread will not enter the synchronized block anymore.

After the very first call of every thread, the cost of the read
operation for the TLS version is a TLS read, whereas for the atomic
version it is an atomic read. I guess TLS read operations simply beat
atomic read operations.

The atomic implementation probably beats the TLS version when a lot of
new threads are being spawned at once and they only retrieve the
singleton which has already been initialized. E.g., say a 1000 threads
are spawned. In the atomic version, the 1000 threads will all do an
atomic read and not enter the synchronized block, whereas in the TLS
version the 1000 threads will all need to enter a synchronized block
on the very first read.