[MITgcm-support] building with MPI on a dual-core mac

[Constantinos Evangelinos]

On Tuesday 21 July 2009 5:22:00 pm Klymak Jody wrote:

> While I suppose I can guess, what is the technical difference between
> "user", "system" and "wallclock"?  I suppose a large difference
> between "wallclock" and "system" means lots of MPI overhead?

user time "u" is processor time spent on behalf of a process in user 
(non-privileged) code. 
system time "s" is processor time spent on behalf of a process in the 
operating system kernel (or equivalent, depending on the operating system) 
wallclock time "w" is self explanatory.

Essentially all of your computational code should be user time. Part of your 
I/O will count as user and part as system time (how big each part is depends 
on the O/S). Communication time is treated similarly to I/O (communicating 
over Ethernet has a significant system component - and corresponding overhead 
of switching to kernel mode - communicating over a high speed interconnect 
like Myrinet, Infiniband etc. should be mainly user time).  Moreover time 
spent waiting for data (with the process not relinquishing the cpu busy 
spinning away) counts as user or system time (e.g. time spent waiting for 
data from main memory or time spent waiting for data from disk). If however 
the O/S switches control of the cpu away from an idling process, that time 
only counts as wallclock time.

Given the above, u+s <= w (to within precision - u and s usually are to 0.01s 
while w is to 1us.) and ideally u+s=w (that would indicate a process that 
spends no time idling waiting for data). A large discrepancy between u+s and 
w indicates either load imbalance or significant network or disk I/O issues.

Constantinos
-- 
Dr. Constantinos Evangelinos
Department of Earth, Atmospheric and Planetary Sciences
Massachusetts Institute of Technology