Choosing number of map/reduce slots (with hyperthreading)

[Adam Phelps]

By scouring various web pages and lists via google I've found some 
general recommendations when it comes to setting the number of map and 
reduce slots for a cluster.  It seems to come down to setting them to 
roughly the number of cores on the machine, minus some if there will be 
other processes active (such as HBase region servers), and to set the 
per-task memory usage so that the total will stay below that of the 
system.  Is this a reasonably general heuristic?

One thing I haven't been able to find advice on is whether this 
heuristic should be adjusted for machines that have hyperthreading 
enabled.  My thought is that it wouldn't be beneficial to increase the 
number of slots (especially in a CPU-bound application) as slots equal 
to the # of cores would already be fully utilizing the CPU.  Are there 
alternative thoughts regarding that?

- Adam