[R-SIG-Mac] Multi-thread R processes performance on 8-core Mac-Pro

[elw at stderr.org]

> I am running R 2.4.1 on the new 8-core Mac Pro with the parSapply 
> function from the Snow package. Tests using 2,4, and 8 threads with 
> makeCluster() yield somewhat disappointing results. The 4 thread process 
> is fastest. With 8 threads, all the cores max out at about 70% power, 
> and even then it is slower than the 4 thread process which maxes out the 
> 4 threads at about 90-95%. This suggests the additional 4 cores on the 
> Mac Pro do not improve performance in an embarrassingly parallel R/Snow 
> environment...

> The function I tested in parSapply runs a regression (a call to "lm"). I 
> am using Snow/rpvm package combination. There is no issue of limited 
> memory. There are over 3 gigs of free RAM in the tests.


There has been some discussion (elsewhere) of the mac pro motherboard's 
inability to keep all eight cores supplied with data from memory at an 
adequate rate.

Your processor cores may simply be starved for data.  The amount of memory 
available doesn't matter, much, if there's no IO bandwidth left between 
the CPUs and the DIMMs....................

--e