please verify the code example for app_config.xml
there are [] in the app config file and we don't use it anymore... so it cause error

Please read the note at the head of the configuration page:

Note: Wherever you see square brackets [ ] used in the examples, these are not used. They show additional options. When using the specific piece of code, delete the square brackets, or the option you try to use will be ignored.

This applies to the app_config.xml file, just as it does to the cc_config.xml file documented in the first two major sections of the page - and indeed to all XML files documented in the Wiki.

[...] is a conventional way of indicating optional sections when documenting computer code, and is widely used in other contexts as well.

ow okey sorry didn't know that we used it for comment. I was used to # or & this kind of signs.
so thanks for the precision

ow okey sorry didn't know that we used it for comment. I was used to # or & this kind of signs.
so thanks for the precision

That's OK, it makes sense when you get used to it.

Note that optional sections in app_config.xml may span several lines, and also be nested, like

   [<app_version>
       <app_name>uppercase</app_name>
       [<plan_class>mt</plan_class>]
       [<avg_ncpus>x</avg_ncpus>]
       [<ngpus>x</ngpus>]
       [<cmdline>--nthreads 7</cmdline>]
   </app_version>]

The whole <app_version> structure is optional, but if you want to use it, you must include the opening tag, an <app_name> tag, and the closing tag. You may want to use one or more of the optional lines as well.

It's hard to convey all that detail with #, &, ;, ', //, or any of the myriad other ways of indicating a comment line: the [ ] technique is both concise and precise.