[R] snow documentation comments

[Ross Boylan]

On Wed, 2004-03-24 at 08:03, Luke Tierney wrote:
> On Tue, 23 Mar 2004, Ross Boylan wrote:
> 
> > There are a few points I found unclear or unmentioned in the snow
> > documentation (mostly I looked at the cluster.html web page).  I thought
> > I'd mention them here.
> > 
> > What is the start up environment for the children?
> > --------------------------------------------------
> > My best guess at the answer is in parentheses
> > Do they inherit shell variables? (no)
> > Do they inherit variables set in R or other aspects of the R
> > environment? (no)
> > What directory does it start in? (the directory you are running in)
> > What user are you? (same as original)
> > 
> > I realize some of these answers might depend on the parallelization
> > layer you are using, or whether you select the homogenous option.
> 
> They do
I've since realized they may also depend on the exact version of the
package used (e.g., newer lams apparently have the ability to export
environment variables) and the way it is invoked.

Personally, I'd even find a single sentence saying explicitly that "it
depends" clarifying.  It would probably also be good to discuss the
behavior of the socket-based communication, since that it supplied by
the snow package.  Information on the other interfaces would be nice
too, but probably hazardous to provide accurately.

I'll not for the record that my answers above are using Rmpi with
lam/mpi 6.5.8-2 on Debian, running just on my local computer.  And some
of the answers are speculative!
> 
> > Number of nodes or number of children?
> > --------------------------------------
> > I thought all the counts of nodes (e.g., in makeCluster(5)) were of the
> > total nodes in the cluster, i.e., children + 1.  However, I did a
> > makeCluster(2) and I got 2 additional R processes running.  Have I
> > misunderstood the semantics, or is it essentially an implementation
> > detail that the master node starts a new R process?
> 
> makeCluster(2) creates a cluster of two processes that the master
> process uses.  So there are a total of three processes.
So work that is distributed via snow goes to the two processes only?