Changes between Version 232 and Version 233 of doc/app/runtime_parameters

Timestamp:

Nov 6, 2012 11:59:05 AM (13 years ago)

Author:

raasch

Comment:

--

doc/app/runtime_parameters

@@ -1366 +1366 @@
 {{{#!td
 Number of processors along x-direction of the virtual processor net.\\\\
-For parallel runs, the total number of processors to be used is given by the '''mrun'''-option -X. By default, depending on the type of the parallel computer, PALM generates a 1d processor net (domain decomposition along x, [#npey npey] = ''1'') or a 2d-net (this is favored on machines with fast communication network and/or large number of processors (>256)). In case of a 2d-net, it is tried to make it more or less square-shaped. If, for example, 16 processors are assigned (-X 16), a 4 * 4 processor net is generated ('''npex''' = 4, '''npey''' = 4). This choice is optimal for square total domains ([../inipar#nx nx] = [../inipar#ny ny]), since then the number of ghost points at the lateral boundarys of the subdomains reaches a minimum. If nx and ny differ extremely, the processor net should be manually adjusted using adequate values for '''npex''' and '''npey'''.\\\\
+For parallel runs, the total number of processors to be used is given by the '''mrun'''-option -X. By default, PALM tries to generate a 2d processor net (domain decomposition along x and y), which is more or less square-shaped. If, for example, 16 processors are assigned (-X 16), a 4 * 4 processor net is generated ('''npex''' = 4, '''npey''' = 4). This choice is optimal for square total domains ([../inipar#nx nx] = [../inipar#ny ny]), because it minimizes the number of ghost points at the lateral boundarys of the subdomains. If nx and ny differ extremely, the processor net should be manually adjusted using adequate values for '''npex''' and '''npey'''.\\\\
 '''Important:'''\\
 The value of '''npex''' * '''npey''' must exactly match the value assigned by the '''mrun'''-option -X. Otherwise the model run will abort with a corresponding error message.\\\\ 
-Additionally, the specification of '''npex''' and '''npey''' may of course override the default setting for the domain decomposition (1d or 2d) which may have a significant (negative) effect on the code performance. 
+A specification of '''npex''' = 1 or '''npey''' = 1 overrides the default setting for the domain decomposition, i.e. it switches to a 1d decomposition, which might have a significant effect on the code performance. On machines with very slow communication network, the performance may improve, but usually it will degrade.
 }}}
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