Ocean modellers considering the NCOM will be interested in its timing requirements and cost comparisons with other global ocean-circulation models. This document provides a comparison of the timings for various configuations of three such models presently used in NCAR research:
The most straightforward, basic measure of model timing comparison is the number of CPU seconds per timestep per gridpoint required by each model. The table below summarizes this information for a small subset of NCOM, POCM, and POP experiments.
From early timing studies, we know that the additional physical and numerical features added to the NCOM cause it to run approximately a factor of 1.9 more slowly than the GFDL MOM 1.1. From data in the table, we can directly compare the timing costs of two models based upon MOM 1.1: the x2 NCOM and the 2x2 POCM. The ratio of these timing costs, 1.21e-5 : 0.70e-5 , is approximately 1.7, and is comparable to the NCOM/MOM 1.1 timing ratio.
Note that the POCM timing information was obtained from a single- processor run. For a more accurate comparison between POCM and NCOM, the 5-10% improvement in timing for the NCOM on a single processor should be included.
|Model||Seconds/ Timestep/ Gridpoint||Seconds/ Timestep||Number of Processors||Computer Hardware||Model Physics||Momentum Timestep (seconds)||Acceleration Factors 1|
|x2(NCOM)||1.24e-5||9.42||8||YMP||4||1800.0||[ 1,1 ]|
|23M(POP)||7.23e-5||256.0||64||T3D 3||6||7200.0||[ 1,1 ]|
In synchronous integrations, the tracer and barotropic stream
function timesteps are equal to the momentum timestep value.
For asynchronous (accelerated) integrations, the numbers in
square brackets indicate the accelerated integration timestep factors.
For example, "3504.0 [10,100]" should be interpreted as follows:
|momentum timestep||= 3504* 1 =||3504 seconds|
|upper-ocean tracer timestep||= 3504* 10 =||35040 seconds|
|abyssal-ocean tracer timestep||= 3504* 100 =||350400 seconds|
The computational speed of a Cray-T3D
is approximately one-tenth that of a Cray-YMP.
Table 2. Model Resolution
|Model||Number of Zonal Gridpoints||Number of Meridional Gridpoints||Number of Vertical Gridpoints||Total Number of Gridpoints||Number of Islands|
|2x2 (POCM)||182||85||20||309400||4 (big)|
|Model||Reference Number from Table 1.||Restoring Boundary Conditions||Bulk Forcing||Isopycnal Mixing||K-Profile Parameter- ization (KPP)||Upwinding||Coupled with Flux- Coupler||Free Surface|
Note: the number of cpu hours for an accelerated integration is obtained by dividing the number of CPU hours listed above by the surface tracer acceleration factor. The model calendar follows the surface tracer time step.
|x2(NCOM)/2x2(POCM)||grid factor||= 2.45|
|2x2(POCM)/x2(NCOM)||timestep factor||= 0.90|
|x2(NCOM)/2x2(POCM)||physics factor||= 1.90|
grid factor * timestep factor * physics factor = 4.2
Therefore, the scaled x2 NCOM cpu time is 33.62 / 4.2= 8 hours.
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Last modified on 11 March 1997