a.k.a an argument for Shared Nothing. Continuing my !random post, I decided to see how long it would take to build up a cumulative normal distribution using the values generated by System.Random.Next(). I used and timed the following configurations:
1) Four threads on four cores sharing a Random (with thread synchronisation): 180 seconds.
2) One thread on one core with its own Random: 63 seconds
3) Four threads on four cores, each with their own Random (sharing nothing): 17 seconds.
The lesson here is that synchronisation is slow. So slow in this case, that it was actually faster to single-thread the application. However, when each thread was given its own object that it didn't have to share with the others, the speed increase was dramatic. Not only was it massively faster, but I'd put my money on it scaling pretty well with an 8, 16, 32 or even 64 way server.
Out of curiosity, I also tried this configuration:
4) Four threads on four cores, each with their own Random, locking it unnecessarily: 39 seconds.
I found it interesting that even uncontended locks could halve the speed of my algorithm.
The moral of the story is to think about your locking strategy (hint: avoid putting yourself in the position where you need one) when looking to parallelize tasks.