[mpich-discuss] mpdboot beheviour

[Dave Goodell]

On Jan 15, 2010, at 10:57 AM, Cezary Śliwa wrote:

> Regarding mpdboot in mpich2-1.2.1. The default is to use --ncpus=1  
> rather than the value from  mpd.hosts. Does it make sense? Why not  
> use the value from mpd.hosts as the default?

This is a longstanding known user interface problem.  It trips people  
up all the time, including me on occasion.  Unfortunately, we are  
unlikely to change the behavior for two reasons: (1) it will break  
compatibility with the thousands of scripts out there that invoke  
mpdboot assuming the old behavior and (2) mpd is receiving a bare  
minimum of maintenance and development at this point because it is  
being replaced by hydra.

> In case of running a job under PBS or SGE, the correct number is in  
> the file. At present, one has to extract this information from the  
> file and put it on the mpdboot command line. This is cumbersome.

I agree wholeheartedly.

> Moreover, if the host running mpdboot is not in mpd.hosts, it makes  
> sense to use --ncpus=0 as the default.

Just FYI, this doesn't actually work the way you would expect.   
mpdboot still basically sets up an mpd as though you had specified -- 
ncpus=1.  You can't use mpd to have a "head node" in a straightforward  
fashion.  The best you can do is use the "-1" option to mpiexec to  
avoid placing processes locally first, but that is a pretty weak  
option too.

Hydra supports running mpiexec on a node that isn't in the hostfile  
(at least with all bootstrap servers that support remote process  
creation).

-Dave