excel - Auto-fill uses half the cores -

i'm autofilling columns in excel (one @ time). these column use udf wrote in exceldna. using taskmanager, notice half of cores being used. excel settings set "use processors on computer." can't figure out why half cores in use. thoughts?

this page contains insightful explainations multi-processing in excel 2007+ http://msdn.microsoft.com/en-us/library/office/aa730921(v=office.12).aspx#office2007excelperf_excelperformanceimprovements

*ref: "multithreaded calculations" "some excel features not use multithreaded calculation, example: data table calculation (but structured references tables use mtc). user-defined functions (but xll functions can multithread-enabled). xlm functions. indirect, cell functions use either format2 or address options. getpivotdata , other functions referring pivottables or cubes. range.calculate , range.calculaterowmajororder. cells in circular reference loops. "

since doesn't describe "autofill" - i'll take crack @ concurrency perspective. please forgive if of speculation..

certain functions require "in-order" task cannot split across processors, might suspect fill 1 of them. (it requires sequential operation in of it's modes.. example: stepping 1,1.2,1.4,etc). in example, processor 2 can't start middle of page without performing new/custom independant calculation. special functions have designed. perhaps decided not code these kinds of scenerios. formulas can't see being possible, because creating formula tree.

other operations indepentant based on formula trees (see link). implies excel won't multi-process complext formula tree either .. if have numerous formulas connected, (that tree) processsed, in order, 1 processor.

granted, there's kinds of complex work-arounds these situations.. (perhaps similiar compilers, sql servers, etc) documentation above highly suggestive microsoft designed streamline independant tasks only.