I have simple Lazarus/FPC application (with no explicit threads)
which does intensive calculations (local thresholding with big window
size) on image, which is stored into memory as 2D byte array.
(so only memory access is done and some integer calculations)
When I run this application and look at Task Manager or Resource
Monitor I see, that all 4 cores "are used" (at least performance
graph shows usage or in other words activity in graph increasees)
Total CPU usage is <= 25% (which points out to fact, that only 1/4
cores is used)
Why this ? As far as I expect, that single threaded application
should use only one core, so I would expect activity only on one core
not on all four cores
(I know that in theory CPU can switch single thread between cores,
but I doubt that this is case (as switching has extra cost)... or is ?)
Nailed it right there....
All modern CPU automatically swaps running threads (unless affinity is
set) to other cores in case there is a high load. This is done from a
heat perspective, since running on a single cores will make a local
heat spot on the dye - a things which is not prefered and also not really
cost effient.
Thanks. It gives sense to me.
But moving thread from one core to another has any nonzero cost, does not ?
(So I wonder that CPU does that also when time for thread completion is
not so much big ... so temperature of CPU does not increase so much)
-Laco.
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